devdocs/elisp/rearrangement.html


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 <h4 class="subsection">Functions that Rearrange Lists</h4>    <p>Here are some functions that rearrange lists destructively by modifying the <small>CDR</small>s of their component cons cells. These functions are destructive because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value. </p> <p>See <code>delq</code>, in <a href="sets-and-lists">Sets And Lists</a>, for another function that modifies cons cells. </p> <dl> <dt id="nconc">Function: <strong>nconc</strong> <em>&amp;rest lists</em>
</dt> <dd>
  <p>This function returns a list containing all the elements of <var>lists</var>. Unlike <code>append</code> (see <a href="building-lists">Building Lists</a>), the <var>lists</var> are <em>not</em> copied. Instead, the last <small>CDR</small> of each of the <var>lists</var> is changed to refer to the following list. The last of the <var>lists</var> is not altered. For example: </p> <div class="example"> <pre class="example">(setq x (list 1 2 3))
     ⇒ (1 2 3)
</pre>
<pre class="example">(nconc x '(4 5))
     ⇒ (1 2 3 4 5)
</pre>
<pre class="example">x
     ⇒ (1 2 3 4 5)
</pre>
</div> <p>Since the last argument of <code>nconc</code> is not itself modified, it is reasonable to use a constant list, such as <code>'(4 5)</code>, as in the above example. For the same reason, the last argument need not be a list: </p> <div class="example"> <pre class="example">(setq x (list 1 2 3))
     ⇒ (1 2 3)
</pre>
<pre class="example">(nconc x 'z)
     ⇒ (1 2 3 . z)
</pre>
<pre class="example">x
     ⇒ (1 2 3 . z)
</pre>
</div> <p>However, the other arguments (all but the last) should be mutable lists. </p> <p>A common pitfall is to use a constant list as a non-last argument to <code>nconc</code>. If you do this, the resulting behavior is undefined (see <a href="self_002devaluating-forms">Self-Evaluating Forms</a>). It is possible that your program will change each time you run it! Here is what might happen (though this is not guaranteed to happen): </p> <div class="example"> <pre class="example">(defun add-foo (x)            ; <span class="roman">We want this function to add</span>
  (nconc '(foo) x))           ;   <span class="roman"><code>foo</code> to the front of its arg.</span>
</pre>

<pre class="example">(symbol-function 'add-foo)
     ⇒ (lambda (x) (nconc '(foo) x))
</pre>

<pre class="example">(setq xx (add-foo '(1 2)))    ; <span class="roman">It seems to work.</span>
     ⇒ (foo 1 2)
</pre>
<pre class="example">(setq xy (add-foo '(3 4)))    ; <span class="roman">What happened?</span>
     ⇒ (foo 1 2 3 4)
</pre>
<pre class="example">(eq xx xy)
     ⇒ t
</pre>

<pre class="example">(symbol-function 'add-foo)
     ⇒ (lambda (x) (nconc '(foo 1 2 3 4) x))
</pre>
</div> </dd>
</dl><div class="_attribution">
  <p class="_attribution-p">
    Copyright &copy; 1990-1996, 1998-2022 Free Software Foundation, Inc. <br>Licensed under the GNU GPL license.<br>
    <a href="https://www.gnu.org/software/emacs/manual/html_node/elisp/Rearrangement.html" class="_attribution-link">https://www.gnu.org/software/emacs/manual/html_node/elisp/Rearrangement.html</a>
  </p>
</div>