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7.5 KiB
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162 lines
7.5 KiB
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>7.8.3. Simple Iteration Constructs</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Simple Iteration Constructs">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html2637 HREF="node90.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2635 HREF="node86.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2629 HREF="node88.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2639 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2640 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2638 HREF="node90.html"> Mapping</A>
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<B>Up:</B> <A NAME=tex2html2636 HREF="node86.html"> Iteration</A>
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<B> Previous:</B> <A NAME=tex2html2630 HREF="node88.html"> General Iteration</A>
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<HR> <P>
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<H2><A NAME=SECTION001183000000000000000>7.8.3. Simple Iteration Constructs</A></H2>
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<P>
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The constructs <tt>dolist</tt> and <tt>dotimes</tt> execute a body of code
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once for each value taken by a single variable. They are expressible
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in terms of <tt>do</tt>, but capture very common patterns of use.
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<P>
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Both <tt>dolist</tt> and <tt>dotimes</tt> perform
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a body of statements repeatedly. On each iteration a specified
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variable is bound to an element of interest that the body may
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examine. <tt>dolist</tt> examines successive elements of a list,
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and <tt>dotimes</tt> examines integers from 0 to <i>n</i>-1
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for some specified positive integer <i>n</i>.
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<P>
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The value of any of these constructs may be specified by an optional result
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form, which if omitted defaults to the value <tt>nil</tt>.
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<P>
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The <tt>return</tt> statement may be used to return
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immediately from a <tt>dolist</tt> or <tt>dotimes</tt> form,
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discarding any following iterations
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that might have been performed; in effect, a <tt>block</tt> named <tt>nil</tt>
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surrounds the construct.
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The body of the loop is implicitly a <tt>tagbody</tt> construct;
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it may contain tags to serve as the targets of <tt>go</tt> statements.
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Declarations may appear before the body of the loop.
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<P>
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<BR><b>[Macro]</b><BR>
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<pre>
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dolist (<i>var</i> <i>listform</i> [<i>resultform</i>])
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{<i>declaration</i>}* {<i>tag</i> | <i>statement</i>}*
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</pre>
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<P><tt>dolist</tt> provides straightforward iteration over the elements of a list.
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First <tt>dolist</tt>
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evaluates the form <i>listform</i>,
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which should produce a list. It then executes the body
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once for each element in the list, in order, with
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the variable <i>var</i> bound to the element.
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Then <i>resultform</i> (a single form, <i>not</i> an implicit <tt>progn</tt>)
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is evaluated, and the result is the value of the <tt>dolist</tt>
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form. (When the <i>resultform</i> is evaluated, the control variable <i>var</i>
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is still bound and has the value <tt>nil</tt>.)
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If <i>resultform</i> is omitted, the result is <tt>nil</tt>.
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<P>
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<P><pre>
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(dolist (x '(a b c d)) (prin1 x) (princ " ")) => <tt>nil</tt>
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after printing ``<tt>a b c d </tt>'' (note the trailing space)
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</pre><P>
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<P>
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An explicit <tt>return</tt> statement may be used to terminate the loop
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and return a specified value.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(MAPPING-DESTRUCTIVE-INTERACTION) <A NAME=7341> </A>
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to restrict user side effects; see section <A HREF="node92.html#STRUCTURETRAVERSALSECTION">7.9</A>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Macro]</b><BR>
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<pre>
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dotimes (<i>var</i> <i>countform</i> [<i>resultform</i>])
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{<i>declaration</i>}* {<i>tag</i> | <i>statement</i>}*
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</pre>
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<P><tt>dotimes</tt> provides straightforward iteration over a sequence of integers.
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The expression
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<tt>(dotimes (<i>var</i> <i>countform</i> <i>resultform</i>) . <i>progbody</i>)</tt>
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evaluates the form <i>countform</i>, which should produce an integer. It then
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performs <i>progbody</i> once for each integer from zero (inclusive) to
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<i>count</i> (exclusive), in order, with the variable <i>var</i> bound to the
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integer; if the value of <i>countform</i> is zero or negative,
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then the <i>progbody</i> is
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performed zero times. Finally, <i>resultform</i> (a single form, <i>not</i> an
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implicit <tt>progn</tt>) is evaluated, and the result is the value of the
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<tt>dotimes</tt> form. (When the <i>resultform</i> is evaluated, the control
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variable <i>var</i> is still bound and has as its value the number of times
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the body was executed.)
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If <i>resultform</i> is omitted, the result is <tt>nil</tt>.
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<P>
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An explicit <tt>return</tt> statement may be used to terminate the loop
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and return a specified value.
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<P>
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Here is an example of the use of <tt>dotimes</tt> in processing strings:
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<P><pre>
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;;; True if the specified subsequence of the string is a
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;;; palindrome (reads the same forwards and backwards).
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(defun palindromep (string <tt>&optional</tt>
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(start 0)
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(end (length string)))
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(dotimes (k (floor (- end start) 2) <tt>t</tt>)
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(unless (char-equal (char string (+ start k))
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(char string (- end k 1)))
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(return <tt>nil</tt>))))
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(palindromep "Able was I ere I saw Elba") => <tt>t</tt>
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(palindromep "A man, a plan, a canal-Panama!") => <tt>nil</tt>
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(remove-if-not #'alpha-char-p ;Remove punctuation
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"A man, a plan, a canal-Panama!")
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=> "AmanaplanacanalPanama"
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(palindromep
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(remove-if-not #'alpha-char-p
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"A man, a plan, a canal-Panama!")) => <tt>t</tt>
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(palindromep
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(remove-if-not
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#'alpha-char-p
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"Unremarkable was I ere I saw Elba Kramer, nu?")) => <tt>t</tt>
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(palindromep
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(remove-if-not
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#'alpha-char-p
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"A man, a plan, a cat, a ham, a yak,
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a yam, a hat, a canal-Panama!")) => <tt>t</tt>
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(palindromep
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(remove-if-not
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#'alpha-char-p
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"Ja-da, ja-da, ja-da ja-da jing jing jing")) => <tt>nil</tt>
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</pre><P>
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<P>
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Altering the value of <i>var</i> in the body of the loop (by using <tt>setq</tt>,
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for example) will have unpredictable, possibly implementation-dependent
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results. A Common Lisp compiler may choose to issue a warning if such a variable
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appears in a <tt>setq</tt>.
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<P>
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<hr>
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<b>Compatbility note:</b> The <tt>dotimes</tt> construct is the closest thing
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in Common Lisp to the Interlisp <tt>rptq</tt> construct.
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<hr>
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<P>
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See also <tt>do-symbols</tt>, <tt>do-external-symbols</tt>,
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and <tt>do-all-symbols</tt>.
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<P>
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<BR> <HR><A NAME=tex2html2637 HREF="node90.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2635 HREF="node86.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2629 HREF="node88.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2639 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2640 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2638 HREF="node90.html"> Mapping</A>
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<B>Up:</B> <A NAME=tex2html2636 HREF="node86.html"> Iteration</A>
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<B> Previous:</B> <A NAME=tex2html2630 HREF="node88.html"> General Iteration</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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