docsrc/xlisp/xlisp-doc/reference/null.htm


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<html><head><title>XLISP null</title>

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</head>

<body>

<a href="../start.htm">Nyquist / XLISP 2.0</a>&nbsp; -&nbsp;
<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>null</h1>

<hr>

<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
  <td><nobr>Type:</nobr></td>
  <td><nobr>&nbsp;&nbsp;-&nbsp;&nbsp;</nobr></td>
  <td width="100%"><nobr>predicate function (subr)</nobr></td>
</tr>
<tr valign="top">
  <td><nobr>Source:</nobr></td>
  <td><nobr>&nbsp;&nbsp;-&nbsp;&nbsp;</nobr></td>
  <td width="100%"><nobr>xlbfun.c</nobr></td>
</tr>
</tbody></table></p>

<h2>Syntax</h2>

<dl>
<dt>(null <i>expr</i>)</dt>
<dd><i>expr</i> - the expression to check<br>
returns - <a href="t.htm">&nbsp;T&nbsp;</a> if the list is
empty, <a href="nil.htm">NIL</a> otherwise</dd>
</dl>

<h2>Description</h2>

<p>The 'null' predicate function checks 'expr' for an empty list.
<a href="t.htm">&nbsp;T&nbsp;</a> is returned if the list is
empty, <a href="nil.htm">NIL</a> is returned otherwise. The
'expr' does not have to be a valid list, but if it is not a list then
<a href="nil.htm">NIL</a> is returned as the result.</p>

<h2>Examples</h2>

<pre class="example">
(null '())     <font color="#008844">; returns T - empty list</font>
(null ())      <font color="#008844">; returns T - still empty</font>
(setq a NIL)   <font color="#008844">; set up a variable</font>
(null a)       <font color="#008844">; returns T - value = empty list</font>

(null "a")     <font color="#008844">; returns NIL - not a list</font>
(null 'a)      <font color="#008844">; returns NIL - not a list</font>
</pre>

<p><b>Note:</b> The 'null' predicate function is the same function as the
<a href="not.htm">not</a> predicate function.</p>

<p>See the
<a href="../manual/xlisp-man-018.htm#null">null</a>
predicate function in the <nobr>XLISP 2.0</nobr> manual.</p>

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