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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>4.11 Mutable Pairs and Lists</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.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 Reference</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewselflink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.html"
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it supports <span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._set-mcar%21%29%29" class="RktValLink" data-pltdoc="x">set-mcar!</a></span> and <span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._set-mcdr%21%29%29" class="RktValLink" data-pltdoc="x">set-mcdr!</a></span> mutation
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operations to change the parts of the mutable pair (like traditional Lisp and
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Scheme pairs).</p><p>A <a name="(tech._mutable._list)"></a><span style="font-style: italic">mutable list</span> is analogous to a list created with pairs, but
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instead created with <a href="mpairs.html#%28tech._mutable._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable pairs</span></a>.</p><p>A <a href="mpairs.html#%28tech._mutable._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable pair</span></a> is not a <a href="pairs.html#%28tech._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">pair</span></a>; they are completely
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separate datatypes. Similarly, a <a href="mpairs.html#%28tech._mutable._list%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable list</span></a> is not a
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<a href="pairs.html#%28tech._list%29" class="techoutside" data-pltdoc="x"><span class="techinside">list</span></a>, except that the empty list is also the empty mutable
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list. Instead of programming with mutable pairs and mutable lists,
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data structures such as pairs, lists, and hash tables are practically
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always better choices.</p><p>A <a href="mpairs.html#%28tech._mutable._list%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable list</span></a> can be used as a single-valued sequence (see
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<a href="sequences.html" data-pltdoc="x">Sequences</a>). The elements of the <a href="mpairs.html#%28tech._mutable._list%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable list</span></a> serve as elements
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of the sequence. See also <span class="RktSym"><a href="sequences.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._in-mlist%29%29" class="RktValLink" data-pltdoc="x">in-mlist</a></span>.</p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Mutable_Pair_Constructors_and_Selectors"">4.11.1<tt> </tt><a name="(part._.Mutable_.Pair_.Constructors_and_.Selectors)"></a>Mutable Pair Constructors and Selectors</h5><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._mpair~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mpair~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">mpair?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">v</span> is
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a <a href="mpairs.html#%28tech._mutable._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable pair</span></a>, <span class="RktVal">#f</span> otherwise.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._mcons))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mcons%29%29" class="RktValDef RktValLink" data-pltdoc="x">mcons</a></span></span><span class="hspace"> </span><span class="RktVar">a</span><span class="hspace"> </span><span class="RktVar">d</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mpair~3f%29%29" class="RktValLink" data-pltdoc="x">mpair?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">a</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">d</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns a newly allocated
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<a href="mpairs.html#%28tech._mutable._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable pair</span></a> whose first
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element is <span class="RktVar">a</span> and second element is <span class="RktVar">d</span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._mcar))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mcar%29%29" class="RktValDef RktValLink" data-pltdoc="x">mcar</a></span></span><span class="hspace"> </span><span class="RktVar">p</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">p</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mpair~3f%29%29" class="RktValLink" data-pltdoc="x">mpair?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns the first element of the
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<a href="mpairs.html#%28tech._mutable._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable pair</span></a> <span class="RktVar">p</span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._mcdr))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mcdr%29%29" class="RktValDef RktValLink" data-pltdoc="x">mcdr</a></span></span><span class="hspace"> </span><span class="RktVar">p</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">p</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mpair~3f%29%29" class="RktValLink" data-pltdoc="x">mpair?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns the second element of the
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<a href="mpairs.html#%28tech._mutable._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable pair</span></a> <span class="RktVar">p</span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._set-mcar!))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._set-mcar%21%29%29" class="RktValDef RktValLink" data-pltdoc="x">set-mcar!</a></span></span><span class="hspace"> </span><span class="RktVar">p</span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void~3f%29%29" class="RktValLink" data-pltdoc="x">void?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">p</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mpair~3f%29%29" class="RktValLink" data-pltdoc="x">mpair?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Changes the <a href="mpairs.html#%28tech._mutable._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable pair</span></a> <span class="RktVar">p</span> so that its first element is
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<span class="RktVar">v</span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._set-mcdr!))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._set-mcdr%21%29%29" class="RktValDef RktValLink" data-pltdoc="x">set-mcdr!</a></span></span><span class="hspace"> </span><span class="RktVar">p</span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void~3f%29%29" class="RktValLink" data-pltdoc="x">void?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">p</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mpair~3f%29%29" class="RktValLink" data-pltdoc="x">mpair?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Changes the <a href="mpairs.html#%28tech._mutable._pair%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutable pair</span></a> <span class="RktVar">p</span> so that its second element is
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<span class="RktVar">v</span>.</div></p><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="pairs.html" title="backward to "4.10 Pairs and Lists"" data-pltdoc="x">← prev</a> <a href="data.html" title="up to "4 Datatypes"" data-pltdoc="x">up</a> <a href="vectors.html" title="forward to "4.12 Vectors"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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