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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>4.12 Simple Dispatch: case</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="../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" 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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="tocviewselflink" 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" 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class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">▼</a></td><td>4 </td><td><a href="scheme-forms.html" class="tocviewlink" data-pltdoc="x">Expressions and Definitions</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">4.1 </td><td><a href="syntax-notation.html" class="tocviewlink" data-pltdoc="x">Notation</a></td></tr><tr><td align="right">4.2 </td><td><a href="binding.html" class="tocviewlink" data-pltdoc="x">Identifiers and Binding</a></td></tr><tr><td align="right">4.3 </td><td><a href="application.html" class="tocviewlink" data-pltdoc="x">Function Calls (Procedure Applications)</a></td></tr><tr><td align="right">4.4 </td><td><a href="lambda.html" class="tocviewlink" data-pltdoc="x">Functions (Procedures):<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">lambda</span></span></a></td></tr><tr><td align="right">4.5 </td><td><a href="define.html" class="tocviewlink" data-pltdoc="x">Definitions:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">define</span></span></a></td></tr><tr><td align="right">4.6 </td><td><a href="let.html" class="tocviewlink" data-pltdoc="x">Local Binding</a></td></tr><tr><td align="right">4.7 </td><td><a href="conditionals.html" class="tocviewlink" data-pltdoc="x">Conditionals</a></td></tr><tr><td align="right">4.8 </td><td><a href="begin.html" class="tocviewlink" data-pltdoc="x">Sequencing</a></td></tr><tr><td align="right">4.9 </td><td><a href="set_.html" class="tocviewlink" data-pltdoc="x">Assignment:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">set!</span></span></a></td></tr><tr><td align="right">4.10 </td><td><a href="quote.html" class="tocviewlink" data-pltdoc="x">Quoting:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">quote</span></span> and <span class="RktVal">'</span></a></td></tr><tr><td align="right">4.11 </td><td><a href="qq.html" class="tocviewlink" data-pltdoc="x">Quasiquoting:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">quasiquote</span></span> and <span class="RktVal">‘</span></a></td></tr><tr><td align="right">4.12 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Simple Dispatch:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">case</span></span></a></td></tr><tr><td align="right">4.13 </td><td><a href="parameterize.html" class="tocviewlink" data-pltdoc="x">Dynamic Binding:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">parameterize</span></span></a></td></tr></table></div></div></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="qq.html" title="backward to "4.11 Quasiquoting: quasiquote and lsquo"" data-pltdoc="x">← prev</a> <a href="scheme-forms.html" title="up to "4 Expressions and Definitions"" data-pltdoc="x">up</a> <a href="parameterize.html" title="forward to "4.13 Dynamic Binding: parameterize"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""case"">4.12<tt> </tt><a name="(part._case)"></a>Simple Dispatch: <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=case.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">case</a></span></h4><p>The <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=case.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">case</a></span> form dispatches to a clause by matching the result
of an expression to the values for the clause:</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=case.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">case</a></span><span class="hspace"> </span><span class="RktVar">expr</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVar">datum</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></blockquote><p>Each <span class="RktVar">datum</span> will be compared to the result of <span class="RktVar">expr</span>
using <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">equal?</a></span>, and then the corresponding <span class="RktSym">body</span>s are
evaluated. The <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=case.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">case</a></span> form can dispatch to the correct clause
in <span style="font-style: italic">O</span>(<span style="font-style: italic">log N</span>)<span style="font-style: italic"></span> time for <span style="font-style: italic">N</span> <span class="RktSym">datum</span>s.</p><p>Multiple <span class="RktVar">datum</span>s can be supplied for each clause, and the
corresponding <span class="RktVar">body</span>s are evaluated if any of the
<span class="RktVar">datum</span>s match.</p><p><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><p>Example:</p></td></tr><tr><td><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._random%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">random</a></span><span class="hspace"> </span><span class="RktVal">6</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a\n"</span><span class="hspace"> </span><span class="RktSym">v</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=case.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">case</a></span><span class="hspace"> </span><span class="RktSym">v</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVal">0</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">zero</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="RktVal">1</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">one</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="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">two</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="RktVal">3</span><span class="hspace"> </span><span class="RktVal">4</span><span class="hspace"> </span><span class="RktVal">5</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">many</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">4</span></p></td></tr><tr><td><p><span class="RktRes">'many</span></p></td></tr></table></blockquote></td></tr></table></p><p>The last clause of a <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=case.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">case</a></span> form can use <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._else%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">else</a></span>, just
like <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._cond%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">cond</a></span>:</p><p><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><p>Example:</p></td></tr><tr><td><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=case.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">case</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._random%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">random</a></span><span class="hspace"> </span><span class="RktVal">6</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="RktVal">0</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">zero</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="RktVal">1</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">one</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="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">two</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._else%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">else</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">many</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'two</span></p></td></tr></table></blockquote></td></tr></table></p><p>For more general pattern matching (but without the dispatch-time
guarantee), use <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span>, which is introduced in
<a href="match.html" data-pltdoc="x">Pattern Matching</a>.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="qq.html" title="backward to "4.11 Quasiquoting: quasiquote and lsquo"" data-pltdoc="x">← prev</a> <a href="scheme-forms.html" title="up to "4 Expressions and Definitions"" data-pltdoc="x">up</a> <a href="parameterize.html" title="forward to "4.13 Dynamic Binding: parameterize"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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