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84 lines
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>15.3. Alteration of List Structure</TITLE>
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<meta name="description" value=" Alteration of List Structure">
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<meta name="keywords" value="clm">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3401 HREF="node151.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3399 HREF="node147.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3393 HREF="node149.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3403 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3404 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3402 HREF="node151.html"> Substitution of Expressions</A>
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<B>Up:</B> <A NAME=tex2html3400 HREF="node147.html"> Lists</A>
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<B> Previous:</B> <A NAME=tex2html3394 HREF="node149.html"> Lists</A>
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<HR> <P>
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<H1><A NAME=SECTION001930000000000000000>15.3. Alteration of List Structure</A></H1>
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<P>
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The functions <tt>rplaca</tt> and <tt>rplacd</tt>
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may be used to make alterations in already existing
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list structure, that is, to change the <i>car</i> or <i>cdr</i> of an
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existing cons.
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One may also use <tt>setf</tt> in conjunction with <tt>car</tt> and <tt>cdr</tt>.
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<P>
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The structure is not copied but is destructively altered;
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hence caution should be exercised when using these functions, as
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strange side effects can occur if portions of list structure become
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shared.
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The <tt>nconc</tt>, <tt>nreverse</tt>, <tt>nreconc</tt>,
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and <tt>nbutlast</tt> functions, already
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described,
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have the same property, as do certain of the generic sequence
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functions such as <tt>delete</tt>.
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However, they are normally not
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used for this side effect; rather, the list-structure modification
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is purely for efficiency, and compatible non-modifying functions
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are provided.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>rplaca <i>x</i> <i>y</i></tt><P><tt>(rplaca <i>x</i> <i>y</i>)</tt> changes the <i>car</i> of <i>x</i> to <i>y</i> and returns
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(the modified) <i>x</i>. <i>x</i> must be a cons, but <i>y</i> may be any
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Lisp object.
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For example:
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<P><pre>
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(setq g '(a b c))
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(rplaca (cdr g) 'd) => (d c)
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Now g => (a d c)
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</pre><P>
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<P>
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<BR><b>[Function]</b><BR>
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<tt>rplacd <i>x</i> <i>y</i></tt><P><tt>(rplacd <i>x</i> <i>y</i>)</tt> changes the <i>cdr</i> of <i>x</i> to <i>y</i> and returns
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(the modified) <i>x</i>. <i>x</i> must be a cons, but <i>y</i> may be
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any Lisp object.
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For example:
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<P><pre>
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(setq x '(a b c))
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(rplacd x 'd) => (a . d)
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Now x => (a . d)
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</pre><P>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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The functions <tt>rplaca</tt> and <tt>rplacd</tt> go back to the earliest
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origins of Lisp, along with <tt>car</tt>, <tt>cdr</tt>, and <tt>cons</tt>.
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Nowadays, however, they seem to be falling by the wayside.
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More and more Common Lisp programmers use <tt>setf</tt> for nearly
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all structure modifications: <tt>(rplaca x y)</tt> is rendered
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as <tt>(setf (car x) y)</tt> or perhaps as <tt>(setf (first x) y)</tt>.
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Even more likely is that a <tt>defstruct</tt> structure or a CLOS class
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is used in place of a list, if the data structure is at all complicated;
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in this case <tt>setf</tt> is used with a slot accessor.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html3401 HREF="node151.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3399 HREF="node147.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3393 HREF="node149.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3403 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3404 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3402 HREF="node151.html"> Substitution of Expressions</A>
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<B>Up:</B> <A NAME=tex2html3400 HREF="node147.html"> Lists</A>
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<B> Previous:</B> <A NAME=tex2html3394 HREF="node149.html"> Lists</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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