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<html><head><title>XLISP case</title>
<link rel="stylesheet" type="text/css" href="reference.css">
</head>
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
<hr>
<h1>case</h1>
<hr>
<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
<td><nobr>Type:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>special form (fsubr)</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xlcont.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<p><div class="box">
<dl>
<dt>(<b>case</b> <i>expr</i> [(<i>value action</i>) ... ])</dt>
<dd><i>expr</i> - an expression that can be compared via <a href="eql.htm">eql</a><br>
<i>value</i> - an unevaluated expression or list of unevaluated expressions<br>
<i>action</i> - one or more expressions<br>
returns - the value of the last expression of the matching case</dd>
</dl>
</div></p>
<h2>Description</h2>
<p>The 'case' special form first evaluates 'expr', the return value of this
evaluation is then compared against all the 'value' entries:</p>
<pre class="example">
(case <font color="#0000CC">expr</font>
(<font color="#0000CC">value-</font><font color="#AA0000">1</font> <font color="#0000CC">action-</font><font color="#AA0000">1</font>)
(<font color="#0000CC">value-</font><font color="#AA0000">2</font> <font color="#0000CC">action-</font><font color="#AA0000">2</font>)
<font color="#008844">...</font>
(<font color="#0000CC">value-</font><font color="#AA0000">n</font> <font color="#0000CC">action-</font><font color="#AA0000">n</font>))
</pre>
<p>If 'value' is a single atom, the atom is compared against 'expr':</p>
<pre class="example">
> (case 'a
('a "a")
('b "b"))
"a"
</pre>
<p>If 'value' is a list, each of the elements of the list are compared
against 'expr':</p>
<pre class="example">
> (case 'a
((1 2 3 4) "number")
((a b c d) "alpha"))
"alpha"
</pre>
<p>The 'action' associated with the first 'value' that matches 'expr' is
evaluated and returned as the result of the 'case' special form.</p>
<p>If no 'value' matches, <a href="nil.htm">NIL</a> is returned:</p>
<pre class="example">
> (case 'c
('a "a")
('b "b"))
NIL
</pre>
<p>If the last 'value' is the symbol <a href="t.htm"> T </a> and
no other 'value' has matched 'expr', then 'case' will evaluate the 'action'
associated <nobr>with <a href="t.htm"> T </a></nobr>:</p>
<pre class="example">
> (case 3
(1 "one")
(2 "two")
(t "no match"))
"no match"
</pre>
<p>If there are multiple <a href="t.htm"> T </a> entries, the
first is considered to be the end of the 'case':</p>
<pre class="example">
> (case 9
(1 "one")
(t "first t")
(t "second t"))
"first t"
</pre>
<p><b>Note:</b> The 'case' special form does not work with a list or string
as the 'expr' because 'case' uses <a href="eql.htm">eql</a> which cannot
compare lists or strings:</p>
<pre class="example">
> (case "a" <font color="#AA0000">; doesn't work!</font>
("a" 'a)
("b" 'b))
NIL
</pre>
<p>The <a href="cond.htm">cond</a> special form can be used to test Lisp
expressions that cannot be handled by 'case'.</p>
<h2>Examples</h2>
<pre class="example">
(case) => NIL
(case 'a) => NIL
(defun c-type (expr)
(case (type-of expr)
(flonum "float")
(fixnum "integer")
(string "string")
(cons "non-empty list")
(nil "empty list")
(t "other")))
(c-type 1.2) => "float"
(c-type 3) => "integer"
(c-type "ab") => "string"
(c-type '(a b)) => "non-empty list"
(c-type '()) => "empty list"
(c-type 'a) => "other"
</pre>
<p>See <a href="defun.htm">defun</a>,
<nobr><a href="type-of.htm">type-of</a></nobr>.</p>
<p>See also:</p>
<ul>
<li><nobr>Contents → <a href="../manual/contents.htm#control-constructs">Control Constructs</a></nobr></li>
</ul>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
<hr>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
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