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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>11 Iterations and Comprehensions</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="icons.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">▼</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">The Racket Guide</a></td></tr></table></div><div class="tocviewsublisttop" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="intro.html" class="tocviewlink" data-pltdoc="x">Welcome to Racket</a></td></tr><tr><td align="right">2 </td><td><a href="to-scheme.html" class="tocviewlink" data-pltdoc="x">Racket Essentials</a></td></tr><tr><td align="right">3 </td><td><a href="datatypes.html" class="tocviewlink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr><tr><td align="right">4 </td><td><a href="scheme-forms.html" class="tocviewlink" data-pltdoc="x">Expressions and Definitions</a></td></tr><tr><td align="right">5 </td><td><a href="define-struct.html" class="tocviewlink" data-pltdoc="x">Programmer-<wbr></wbr>Defined Datatypes</a></td></tr><tr><td align="right">6 </td><td><a href="modules.html" class="tocviewlink" data-pltdoc="x">Modules</a></td></tr><tr><td align="right">7 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">8 </td><td><a href="i_o.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">9 </td><td><a href="regexp.html" class="tocviewlink" data-pltdoc="x">Regular Expressions</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Exceptions and Control</a></td></tr><tr><td align="right">11 </td><td><a href="for.html" class="tocviewselflink" data-pltdoc="x">Iterations and Comprehensions</a></td></tr><tr><td align="right">12 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">13 </td><td><a href="classes.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">14 </td><td><a href="units.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">15 </td><td><a href="reflection.html" class="tocviewlink" data-pltdoc="x">Reflection and Dynamic Evaluation</a></td></tr><tr><td align="right">16 </td><td><a href="macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">17 </td><td><a href="languages.html" class="tocviewlink" data-pltdoc="x">Creating Languages</a></td><
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<span style="font-style: italic">sequences</span>. Lists, vectors, strings, byte strings, input
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ports, and hash tables can all be used as sequences, and constructors
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like <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span> offer even more kinds of sequences.</p><p>Variants of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> accumulate iteration results in different
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ways, but they all have the same syntactic shape. Simplifying for
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now, the syntax of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> is</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">sequence-expr</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></blockquote><p>A <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> loop iterates through the sequence produced by the
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<span class="RktVar">sequence-expr</span>. For each element of the sequence,
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<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> binds the element to <span class="RktVar">id</span>, and then it evaluates
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the <span class="RktVar">body</span>s for side effects.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">123</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">"abc"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a..."</span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">a...b...c...</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">display</a></spa
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accumulates <span class="RktVar">body</span> results into a list, instead of
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evaluating <span class="RktVar">body</span> only for side effects. In more
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technical terms, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span> implements a <span style="font-style: italic">list
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comprehension</span>.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252A%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">*</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'(1 4 9)</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">"abc"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'(#\a #\b #\c)</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'(0 1 2 3)</span></p></td></tr></table></blockquote></div></p><p>The full syntax of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> accommodates multiple sequences to
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iterate in parallel, and the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span> variant nests the
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iterations instead of running them in parallel. More variants of
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<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span> accumulate <span class="RktVar">body</span> results
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in different ways. In all of these variants, predicates that prune
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iterations can be included along with bindings.</p><p>Before details on the variations of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>, though, it’s best to
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see the kinds of sequence generators that make interesting examples.</p><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""sequences"">11.1<tt> </tt><a name="(part._sequences)"></a>Sequence Constructors</h4><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span> function generates a sequence of numbers, given
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an optional starting number (which defaults to <span class="RktVal">0</span>), a number
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before which the sequence ends, and an optional step (which defaults
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to <span class="RktVal">1</span>). Using a non-negative integer <span class="RktVar">k</span> directly as
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a sequence is a shorthand for <span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span><span class="stt"> </span><span class="RktVar">k</span><span class="RktPn">)</span>.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">012</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">012</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span
|
||
|
|
starting number must be an exact non-negative integer (which defaults
|
||
|
|
to <span class="RktVal">0</span>), the step is always <span class="RktVal">1</span>, and there is no upper
|
||
|
|
limit. A <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> loop using just <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span> will never
|
||
|
|
terminate unless a body expression raises an exception or otherwise
|
||
|
|
escapes.</p><p><div class="SIntrapara">Example:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._if%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">if</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%7E3d%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">=</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktVal">10</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._error%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">error</a></span><span class="hspace"> </span><span class="RktVal">"too much!"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">0123456789</span></p></td></tr><tr><td><p><span class="RktErr">too much!</span></p></td></tr></table></blockquote></div></p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._stop-before%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">stop-before</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._stop-after%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">stop-after</a></span> functions construct
|
||
|
|
a new sequence given a sequence and a predicate. The new sequence is
|
||
|
|
like the given sequence, but truncated either immediately before or
|
||
|
|
immediately after the first element for which the predicate returns
|
||
|
|
true.</p><p><div class="SIntrapara">Example:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._stop-before%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">stop-before</a></span><span class="hspace"> </span><span class="RktVal">"abc def"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=characters.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._char-whitespace%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">char-whitespace?</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">abc</span></p></td></tr></table></blockquote></div></p><p>Sequence constructors like <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-list%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-list</a></span>, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-vector%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-vector</a></span> and
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-string%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-string</a></span> simply make explicit the use of a list, vector, or
|
||
|
|
string as a sequence. Along with <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span>,
|
||
|
|
these constructors raise an exception when given the
|
||
|
|
wrong kind of value, and since they otherwise avoid a run-time
|
||
|
|
dispatch to determine the sequence type, they enable more efficient
|
||
|
|
code generation; see <a href="for.html#%28part._for-performance%29" data-pltdoc="x">Iteration Performance</a> for more information.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-string%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-string</a></span><span class="hspace"> </span><span class="RktVal">"abc"</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">abc</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-string%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-string</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktVal">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._display%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktErr">in-string: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: string</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: '(1
|
||
|
|
in a <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> form, the corresponding sequences are traversed in
|
||
|
|
parallel:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Chapter ~a. ~a\n"</span><span class="hspace"> </span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td><p><span class="RktOut">Chapter 1. Intro</span></p></td></tr><tr><td><p><span class="RktOut">Chapter 2. Details</span></p></td></tr><tr><td><p><span class="RktOut">Chapter 3. Conclusion</span></p></td></tr></table></td></tr></table></blockquote><p>With parallel sequences, the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> expression stops iterating
|
||
|
|
when any sequence ends. This behavior allows <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span>,
|
||
|
|
which creates an infinite sequence of numbers, to be used for
|
||
|
|
indexing:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"Chapter ~a. ~a\n"</span><span class="hspace"> </span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td><p><span class="RktOut">Chapter 1. Intro</span></p></td></tr><tr><td><p><span class="RktOut">Chapter 2. Details</span></p></td></tr><tr><td><p><span class="RktOut">Chapter 3. Conclusion</span></p></td></tr></table></td></tr></table></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span> form, which has the same syntax as <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>,
|
||
|
|
nests multiple sequences instead of running them in parallel:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Reference"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a ~a\n"</span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td><p><span class="RktOut">Guide Intro</span></p></td></tr><tr><td><p><span class="RktOut">Guide Details</span></p></td></tr><tr><td><p><span class="RktOut">Guide Conclusion</span></p></td></tr><tr><td><p><span class="RktOut">Reference Intro</span></p></td></tr><tr><td><p><span class="RktOut">Reference Details</span></p></td></tr><tr><td><p><span class="RktOut">Reference Conclusion</span></p></td></tr></table></td></tr></table></blockquote><p>Thus, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span> is a shorthand for nested <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>s in the
|
||
|
|
same way that <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">let*</a></span> is a shorthand for nested <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">let</a></span>s.</p><p>The <span class="RktPn">#:when</span><span class="stt"> </span><span class="RktVar">boolean-expr</span> form of a <span class="RktVar">clause</span> is
|
||
|
|
another shorthand. It allows the <span class="RktVar">body</span>s to evaluate only
|
||
|
|
when the <span class="RktVar">boolean-expr</span> produces a true value:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Reference"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:when</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a ~a\n"</span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td><p><span class="RktOut">Guide Intro</span></p></td></tr><tr><td><p><span class="RktOut">Guide Conclusion</span></p></td></tr><tr><td><p><span class="RktOut">Reference Intro</span></p></td></tr><tr><td><p><span class="RktOut">Reference Conclusion</span></p></td></tr></table></td></tr></table></blockquote><p>A <span class="RktVar">boolean-expr</span> with <span class="RktPn">#:when</span> can refer to any of the
|
||
|
|
preceding iteration bindings. In a <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> form, this scoping
|
||
|
|
makes sense only if the test is nested in the iteration of the
|
||
|
|
preceding bindings; thus, bindings separated by <span class="RktPn">#:when</span> are
|
||
|
|
mutually nested, instead of in parallel, even with <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Reference"</span><span class="hspace"> </span><span class="RktVal">"Notes"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:when</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">"Notes"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="hspace"> </span><span class="RktVal">"Index"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:when</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> 
|
||
|
|
the <span class="RktVar">body</span>s evaluate only when the <span class="RktVar">boolean-expr</span>
|
||
|
|
produces a false value.</p><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""for_list_and_for__list"">11.3<tt> </tt><a name="(part._for_list_and_for__list)"></a><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/list</a></span></h4><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span> form, which has the same syntax as <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>,
|
||
|
|
evaluates the <span class="RktVar">body</span>s to obtain values that go into a
|
||
|
|
newly constructed list:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-append%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">string-append</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._number-%7E3estring%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">number->string</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">". "</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("1. Intro" "2. Details" "3. Conclusion")</span></p></td></tr></table></blockquote><p>A <span class="RktPn">#:when</span> clause in a <span class="RktSym">for-list</span> form prunes the result
|
||
|
|
list along with evaluations of the <span class="RktVar">body</span>s:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:when</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=number-types.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._odd%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">odd?</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("Intro" "Conclusion")</span></p></td></tr></table></blockquote><p>This pruning behavior of <span class="RktPn">#:when</span> is more useful with
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span> than <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>. Whereas a plain <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=when_unless.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._when%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">when</a></span>
|
||
|
|
form normally suffices with <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>, a <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=when_unless.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._when%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">when</a></span> expression
|
||
|
|
form in a <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span> would cause the result list to contain
|
||
|
|
<a href="void_undefined.html" class="RktModLink" data-pltdoc="x"><span class="nobreak"><span class="RktRes">#<void></span></span></a>s instead of omitting list elements.</p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/list</a></span> form is like <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span>, nesting multiple
|
||
|
|
iterations:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Ref."</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-append%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">string-append</a></span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">" "</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("Guide Intro" "Guide Details" "Ref. Intro" "Ref. Details")</span></p></td></tr></table></blockquote><p>A <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/list</a></span> form is not quite the same thing as nested
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span> forms. Nested <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span>s would produce a
|
||
|
|
list of lists, instead of one flattened list. Much like
|
||
|
|
<span class="RktPn">#:when</span>, then, the nesting of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/list</a></span> is more
|
||
|
|
useful than the nesting of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span>.</p><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""for_vector_and_for__vector"">11.4<tt> </tt><a name="(part._for_vector_and_for__vector)"></a><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/vector</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/vector</a></span></h4><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/vector</a></span> form can be used with the same syntax as the
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/list</a></span> form, but the evaluated <span class="RktVar">body</span>s go into a
|
||
|
|
newly-constructed vector instead of a list:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/vector</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-append%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">string-append</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._number-%7E3estring%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">number->string</a></span><span class="hspace"> </span><span class="RktSym">i</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">". "</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'#("1. Intro" "2. Details" "3. Conclusion")</span></p></td></tr></table></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/vector</a></span> form behaves similarly, but the iterations are
|
||
|
|
nested as in <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span>.</p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/vector</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/vector</a></span> forms also allow the
|
||
|
|
length of the vector to be constructed to be supplied in advance. The
|
||
|
|
resulting iteration can be performed more efficiently than plain
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/vector</a></span> or <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/vector</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">chapters</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fvector%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/vector</a></span><span class="hspace"> </span><span class="RktPn">#:length</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._length%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">length</a></span><span class="hspace"> </span><span class="RktSym">chapters</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktSym">chapters</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-append%2529%2529&version=8.6" class="RktValLink Sq" data-pltdo
|
||
|
|
or the requested iterations are complete, whichever comes first. If
|
||
|
|
the provided length exceeds the requested number of iterations, then
|
||
|
|
the remaining slots in the vector are initialized to the default
|
||
|
|
argument of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=vectors.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-vector%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">make-vector</a></span>.</p><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""for_and_and_for_or"">11.5<tt> </tt><a name="(part._for_and_and_for_or)"></a><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fand%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/and</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252For%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/or</a></span></h4><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fand%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/and</a></span> form combines iteration results with
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._and%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">and</a></span>, stopping as soon as it encounters <span class="RktVal">#f</span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Fand%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/and</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">"Intro"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#f</span></p></td></tr></table></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252For%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/or</a></span> form combines iteration results with <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._or%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">or</a></span>,
|
||
|
|
stopping as soon as it encounters a true value:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252For%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/or</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">"Intro"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr></table></blockquote><p>As usual, the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Fand%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/and</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252For%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/or</a></span> forms provide the
|
||
|
|
same facility with nested iterations.</p><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""for_first_and_for_last"">11.6<tt> </tt><a name="(part._for_first_and_for_last)"></a><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffirst%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/first</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flast%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/last</a></span></h4><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffirst%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/first</a></span> form returns the result of the first time that
|
||
|
|
the <span class="RktVar">body</span>s are evaluated, skipping further iterations.
|
||
|
|
This form is most useful with a <span class="RktPn">#:when</span> clause.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffirst%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/first</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="hspace"> </span><span class="RktVal">"Index"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:when</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">"Intro"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"Details"</span></p></td></tr></table></blockquote><p>If the <span class="RktVar">body</span>s are evaluated zero times, then the result
|
||
|
|
is <span class="RktVal">#f</span>.</p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flast%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/last</a></span> form runs all iterations, returning the value of
|
||
|
|
the last iteration (or <span class="RktVal">#f</span> if no iterations are run):</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Flast%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/last</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="hspace"> </span><span class="RktVal">"Index"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:when</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">"Index"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"Conclusion"</span></p></td></tr></table></blockquote><p>As usual, the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Ffirst%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/first</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flast%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/last</a></span> forms provide
|
||
|
|
the same facility with nested iterations:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Ffirst%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/first</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Reference"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="hspace"> </span><span class="RktVal">"Index"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:when</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">"Intro"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("Guide" "Details")</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flast%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/last</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span
|
||
|
|
results. Its syntax is slightly different than the syntax of
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>, because accumulation variables must be declared at the
|
||
|
|
beginning:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffold%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/fold</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktVar">accum-id</span><span class="hspace"> </span><span class="RktVar">init-expr</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._......%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">clause</span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._......%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktSym">...+</span><span class="RktPn">)</span></td></tr></table></blockquote><p>In the simple case, only one <span class="RktPn">[</span><span class="RktVar">accum-id</span><span class="stt"> </span><span class="RktVar">init-expr</span><span class="RktPn">]</span> is
|
||
|
|
provided, and the result of the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffold%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/fold</a></span> is the final value
|
||
|
|
for <span class="RktVar">accum-id</span>, which starts out with the value of
|
||
|
|
<span class="RktVar">init-expr</span>. In the <span class="RktVar">clause</span>s and
|
||
|
|
<span class="RktVar">body</span>s, <span class="RktVar">accum-id</span> can be referenced to get its
|
||
|
|
current value, and the last <span class="RktVar">body</span> provides the value of
|
||
|
|
<span class="RktVar">accum-id</span> for the next iteration.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffold%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/fold</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">len</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">len</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-length%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">string-length</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">15</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffold%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/fold</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">prev</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">i</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-naturals%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="h
|
||
|
|
<span class="RktVar">body</span> must produce multiple values, one for each
|
||
|
|
<span class="RktVar">accum-id</span>. The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffold%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/fold</a></span> expression itself produces
|
||
|
|
multiple values for the results.</p><p><div class="SIntrapara">Example:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252Ffold%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for/fold</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">prev</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">counter</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:when</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktSym">prev</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a. ~a\n"</span><span class="hspace"> </span><span class="RktSym">counter</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=values.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._values%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">values</a></span><span class="hspace"> </span><span class="RktSym">chapter</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace">&nb
|
||
|
|
values, individual iterations of a sequence can produce multiple
|
||
|
|
elements. For example, a hash table as a sequence generates two
|
||
|
|
values for each iteration: a key and a value.</p><p>In the same way that <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._let-values%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">let-values</a></span> binds multiple results to
|
||
|
|
multiple identifiers, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> can bind multiple sequence elements
|
||
|
|
to multiple iteration identifiers:</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>While <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">let</a></span> must be changed to <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._let-values%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">let-values</a></span>
|
||
|
|
to bind multiple identifiers, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> simply allows a
|
||
|
|
parenthesized list of identifiers instead of a single
|
||
|
|
identifier in any clause.</p></blockquote></blockquote></blockquote><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#hash</span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktVal">"apple"</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVal">"banana"</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVal">3</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a count: ~a\n"</span><span class="hspace"> </span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td><p><span class="RktOut">apple count: 1</span></p></td></tr><tr><td><p><span class="RktOut">banana count: 3</span></p></td></tr></table></td></tr></table></blockquote><p>This extension to multiple-value bindings works for all <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span>
|
||
|
|
variants. For example, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/list</a></span> nests iterations, builds a
|
||
|
|
list, and also works with multiple-valued sequences:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%252Flist%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#hash</span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktVal">"apple"</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVal">"banana"</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVal">3</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">i</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">k</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("apple" "banana" "banana" "banana")</span></p></td></tr></table></blockquote><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Breaking_an_Iteration"">11.9<tt> </tt><a name="(part._.Breaking_an_.Iteration)"></a>Breaking an Iteration</h4><p>An even more complete syntax of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> is</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">clause</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">body-or-break</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="hspace"> </span><span class="RktVar">body</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="spe
|
||
|
|
be included among the binding clauses and body of the iteration. Among
|
||
|
|
the binding clauses, <span class="RktPn">#:break</span> is like <span class="RktPn">#:unless</span>
|
||
|
|
but when its <span class="RktVar">boolean-expr</span> is true, all sequences within the
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> are stopped. Among the <span class="RktVar">body</span>s,
|
||
|
|
<span class="RktPn">#:break</span> has the same effect on sequences when its
|
||
|
|
<span class="RktVar">boolean-expr</span> is true, and it also prevents later
|
||
|
|
<span class="RktVar">body</span>s from evaluation in the current iteration.</p><p>For example, while using <span class="RktPn">#:unless</span> between clauses effectively
|
||
|
|
skips later sequences as well as the body,</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Story"</span><span class="hspace"> </span><span class="RktVal">"Reference"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:unless</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">"Story"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a ~a\n"</span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td><p><span class="RktOut">Guide Intro</span></p></td></tr><tr><td><p><span class="RktOut">Guide Details</span></p></td></tr><tr><td><p><span class="RktOut">Guide Conclusion</span></p></td></tr><tr><td><p><span class="RktOut">Reference Intro</span></p></td></tr><tr><td><p><span class="RktOut">Reference Details</span></p></td></tr><tr><td><p><span class="RktOut">Reference Conclusion</span></p></td></tr></table></td></tr></table></blockquote><p>using <span class="RktPn">#:break</span> causes the entire <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> iteration
|
||
|
|
to terminate:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Story"</span><span class="hspace"> </span><span class="RktVal">"Reference"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:break</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">"Story"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a ~a\n"</span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td><p><span class="RktOut">Guide Intro</span></p></td></tr><tr><td><p><span class="RktOut">Guide Details</span></p></td></tr><tr><td><p><span class="RktOut">Guide Conclusion</span></p></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Story"</span><span class="hspace"> </span><span class="RktVal">"Reference"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace">  
|
||
|
|
but it does not immediately terminate the iteration. Instead, it
|
||
|
|
allows at most one more element to be drawn for each sequence and at
|
||
|
|
most one more evaluation of the <span class="RktVar">body</span>s.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%252A%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for*</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Guide"</span><span class="hspace"> </span><span class="RktVal">"Story"</span><span class="hspace"> </span><span class="RktVal">"Reference"</span><span class="RktVal">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">"Intro"</span><span class="hspace"> </span><span class="RktVal">"Details"</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktVal">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:final</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._and%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktVal">"Story"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="hspace"> </span><span class="RktVal">"Conclusion"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a ~a\n"</span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktSym">chapter</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td><p><span class="RktOut">Guide Intro</span></p></td></tr><tr><td><p><span class="
|
||
|
|
you write by hand as a recursive-function invocation. A hand-written
|
||
|
|
loop, however, is normally specific to a particular kind of data, such
|
||
|
|
as lists. In that case, the hand-written loop uses selectors like
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">car</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._cdr%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">cdr</a></span> directly, instead of handling all forms
|
||
|
|
of sequences and dispatching to an appropriate iterator.</p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> forms can provide the performance of hand-written
|
||
|
|
loops when enough information is apparent about the sequences to
|
||
|
|
iterate. Specifically, the clause should have one of the following
|
||
|
|
<span class="RktVar">fast-clause</span> forms:</p><p><table cellspacing="0" cellpadding="0"><tr><td align="right" valign="baseline"><span class="hspace"> </span><span class="RktVar">fast-clause</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">[</span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">fast-seq</span><span class="RktPn">]</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVar">id</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">fast-seq</span><span class="RktPn">]</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">id</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">fast-indexed-seq</span><span class="RktPn">]</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktSym">...</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">fast-parallel-seq</span><span class="RktPn">]</span></td></tr></table></p><p><table cellspacing="0" cellpadding="0"><tr><td align="right" valign="baseline"><span class="hspace"> </span><span class="RktVar">fast-seq</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=sequences.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._in-range%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">in-range</a></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" va
|
||
|
|
patterns that provide good performance is extensible, just like the
|
||
|
|
set of sequence values. The documentation for a sequence constructor
|
||
|
|
should indicate the performance benefits of using it directly in
|
||
|
|
a <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._for%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">for</a></span> <span class="RktVar">clause</span>.</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="magnify.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=for.html&version=8.6" class="Sq" data-pltdoc="x">Iterations and Comprehensions: <span class="RktSym"><span class="RktStxLink">for</span></span>, <span class="RktSym"><span class="RktStxLink">for/list</span></span>, ...</a> in <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=index.html&version=8.6" class="Sq" data-pltdoc="x">The Racket Reference</a> provides more on iterations and comprehensions.</p></blockquote></blockquote></blockquote><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="conts.html" title="backward to "10.3 Continuations"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Guide"" data-pltdoc="x">up</a> <a href="match.html" title="forward to "12 Pattern Matching"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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