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<span id="adjoin"></span><div class="header">
<p>
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<hr>
<span id="adjoin-_005bFunction_005d"></span><h4 class="subsection">14.2.44 adjoin [Function]</h4>
<p><code>adjoin</code> <i>item list <span class="roman">&key</span> key test test-not</i> ⇒ <i>new-list</i>
</p>
<span id="Arguments-and-Values_003a_003a-313"></span><h4 class="subsubheading">Arguments and Values::</h4>
<p><i>item</i>—an <i>object</i>.
</p>
<p><i>list</i>—a <i>proper list</i>.
</p>
<p><i>test</i>—a <i>designator</i> for a <i>function</i> of two <i>arguments</i>
that returns a <i>generalized boolean</i>.
</p>
<p><i>test-not</i>—a <i>designator</i> for
a <i>function</i> of two <i>arguments</i>
that returns a <i>generalized boolean</i>.
</p>
<p><i>key</i>—a <i>designator</i> for a <i>function</i> of one argument,
or <b>nil</b>.
</p>
<p><i>new-list</i>—a <i>list</i>.
</p>
<span id="Description_003a_003a-414"></span><h4 class="subsubheading">Description::</h4>
<p>Tests whether <i>item</i> is the same as an existing element of <i>list</i>.
If the <i>item</i> is not an existing element,
<b>adjoin</b> adds it to <i>list</i> (as if by <b>cons</b>)
and returns the resulting <i>list</i>;
otherwise, nothing is added and the original <i>list</i> is returned.
</p>
<p>The <i>test</i>, <i>test-not</i>, and <i>key</i>
affect how it is determined whether <i>item</i> is the same as an <i>element</i> of <i>list</i>.
For details, see <a href="Satisfying-a-Two_002dArgument-Test.html">Satisfying a Two-Argument Test</a>.\ifvmode\else\endgraf
\ifdim \prevdepth>-1000pt
\NIS\parskip \normalparskip\relax\fi
</p>
<span id="Examples_003a_003a-297"></span><h4 class="subsubheading">Examples::</h4>
<div class="example">
<pre class="example"> (setq slist '()) ⇒ NIL
(adjoin 'a slist) ⇒ (A)
slist ⇒ NIL
(setq slist (adjoin '(test-item 1) slist)) ⇒ ((TEST-ITEM 1))
(adjoin '(test-item 1) slist) ⇒ ((TEST-ITEM 1) (TEST-ITEM 1))
(adjoin '(test-item 1) slist :test 'equal) ⇒ ((TEST-ITEM 1))
(adjoin '(new-test-item 1) slist :key #'cadr) ⇒ ((TEST-ITEM 1))
(adjoin '(new-test-item 1) slist) ⇒ ((NEW-TEST-ITEM 1) (TEST-ITEM 1))
</pre></div>
<span id="Exceptional-Situations_003a_003a-133"></span><h4 class="subsubheading">Exceptional Situations::</h4>
<p>Should be prepared to signal an error of <i>type</i> <b>type-error</b>
if <i>list</i> is not a <i>proper list</i>.
</p>
<span id="See-Also_003a_003a-338"></span><h4 class="subsubheading">See Also::</h4>
<p><a href="pushnew.html">pushnew</a>
,
</p>
<p><a href="Traversal-Rules-and-Side-Effects.html">Traversal Rules and Side Effects</a>
</p>
<span id="Notes_003a_003a-224"></span><h4 class="subsubheading">Notes::</h4>
<p>The <tt>:test-not</tt> parameter is deprecated.
</p>
<div class="example">
<pre class="example"> (adjoin item list :key fn)
≡ (if (member (fn item) list :key fn) list (cons item list))
</pre></div>
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