229 lines
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229 lines
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Accessor CAR, CDR, CAAR, CADR, CDAR...</TITLE>
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<META HTTP-EQUIV="Author" CONTENT="Kent M. Pitman">
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="f_rplaca.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_conses.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="f_cp_tre.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<HR>
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<A NAME="car"><A NAME="cdr"><A NAME="caar"><A NAME="cadr"><A NAME="cdar"><A NAME="cddr"><A NAME="caaar"><A NAME="caadr"><A NAME="cadar"><A NAME="caddr"><A NAME="cdaar"><A NAME="cdadr"><A NAME="cddar"><A NAME="cdddr"><A NAME="caaaar"><A NAME="caaadr"><A NAME="caadar"><A NAME="caaddr"><A NAME="cadaar"><A NAME="cadadr"><A NAME="caddar"><A NAME="cadddr"><A NAME="cdaaar"><A NAME="cdaadr"><A NAME="cdadar"><A NAME="cdaddr"><A NAME="cddaar"><A NAME="cddadr"><A NAME="cdddar"><A NAME="cddddr"><I>Accessor</I> <B>CAR, CDR, CAAR, CADR, CDAR, CDDR, CAAAR, CAADR, CADAR, CADDR, CDAAR, CDADR, CDDAR, CDDDR, CAAAAR, CAAADR, CAADAR, CAADDR, CADAAR, CADADR, CADDAR, CADDDR, CDAAAR, CDAADR, CDADAR, CDADDR, CDDAAR, CDDADR, CDDDAR, CDDDDR</B></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>car</B> <I>x</I> => <I>object</I><P>
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<B>cdr</B> <I>x</I> => <I>object</I><P>
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<B>caar</B> <I>x</I> => <I>object</I><P>
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<B>cadr</B> <I>x</I> => <I>object</I><P>
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<B>cdar</B> <I>x</I> => <I>object</I><P>
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<B>cddr</B> <I>x</I> => <I>object</I><P>
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<B>caaar</B> <I>x</I> => <I>object</I><P>
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<B>caadr</B> <I>x</I> => <I>object</I><P>
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<B>cadar</B> <I>x</I> => <I>object</I><P>
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<B>caddr</B> <I>x</I> => <I>object</I><P>
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<B>cdaar</B> <I>x</I> => <I>object</I><P>
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<B>cdadr</B> <I>x</I> => <I>object</I><P>
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<B>cddar</B> <I>x</I> => <I>object</I><P>
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<B>cdddr</B> <I>x</I> => <I>object</I><P>
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<B>caaaar</B> <I>x</I> => <I>object</I><P>
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<B>caaadr</B> <I>x</I> => <I>object</I><P>
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<B>caadar</B> <I>x</I> => <I>object</I><P>
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<B>caaddr</B> <I>x</I> => <I>object</I><P>
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<B>cadaar</B> <I>x</I> => <I>object</I><P>
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<B>cadadr</B> <I>x</I> => <I>object</I><P>
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<B>caddar</B> <I>x</I> => <I>object</I><P>
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<B>cadddr</B> <I>x</I> => <I>object</I><P>
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<B>cdaaar</B> <I>x</I> => <I>object</I><P>
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<B>cdaadr</B> <I>x</I> => <I>object</I><P>
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<B>cdadar</B> <I>x</I> => <I>object</I><P>
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<B>cdaddr</B> <I>x</I> => <I>object</I><P>
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<B>cddaar</B> <I>x</I> => <I>object</I><P>
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<B>cddadr</B> <I>x</I> => <I>object</I><P>
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<B>cdddar</B> <I>x</I> => <I>object</I><P>
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<B>cddddr</B> <I>x</I> => <I>object</I><P>
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<TT>(setf (</TT><B>car</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cadr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cddr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caaar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caadr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cadar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caddr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdaar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdadr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cddar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdddr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caaaar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caaadr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caadar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caaddr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cadaar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cadadr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>caddar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cadddr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdaaar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdaadr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdadar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdaddr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cddaar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cddadr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cdddar</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<TT>(setf (</TT><B>cddddr</B> <I>x</I><TT>) </TT><I>new-object</I><TT>)</TT><P>
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<P>
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<P><B>Pronunciation:</B><P>
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<P>
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<A REL=DEFINITION HREF="#cadr"><B>cadr</B></A>: ['ka,duhr] <P>
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<A REL=DEFINITION HREF="#caddr"><B>caddr</B></A>: ['kaduh,duhr] or ['ka,dduhr] <P>
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<A REL=DEFINITION HREF="#cdr"><B>cdr</B></A>: ['k,duhr] <P>
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<A REL=DEFINITION HREF="#cddr"><B>cddr</B></A>: ['kduh,duhr] or ['kuh,dduhr] <P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>x</I>---a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A>. <P>
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<I>object</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>new-object</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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If <I>x</I> is a <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A>, <A REL=DEFINITION HREF="#car"><B>car</B></A> returns the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of that <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A>. If <I>x</I> is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>, <A REL=DEFINITION HREF="#car"><B>car</B></A> returns <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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If <I>x</I> is a <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A>, <A REL=DEFINITION HREF="#cdr"><B>cdr</B></A> returns the <A REL=DEFINITION HREF="26_glo_c.htm#cdr"><I>cdr</I></A> of that <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A>. If <I>x</I> is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>, <A REL=DEFINITION HREF="#cdr"><B>cdr</B></A> returns <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<A REL=DEFINITION HREF="26_glo_f.htm#function"><I>Functions</I></A> are provided which perform compositions of up to four <A REL=DEFINITION HREF="#car"><B>car</B></A> and <A REL=DEFINITION HREF="#cdr"><B>cdr</B></A> operations. Their <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>names</I></A> consist of a <TT>C</TT>, followed by two, three, or four occurrences of <TT>A</TT> or <TT>D</TT>, and finally an <TT>R</TT>. The series of <TT>A</TT>'s and <TT>D</TT>'s in each <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A>'s <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> is chosen to identify the series of <A REL=DEFINITION HREF="#car"><B>car</B></A> and <A REL=DEFINITION HREF="#cdr"><B>cdr</B></A> operations that is performed by the function. The order in which the <TT>A</TT>'s and <TT>D</TT>'s appear is the inverse of the order in which the corresponding operations are performed. The next figure defines the relationships precisely. <P>
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<PRE>
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This <A REL=DEFINITION HREF="26_glo_p.htm#place">place</A> ... Is equivalent to this <A REL=DEFINITION HREF="26_glo_p.htm#place">place</A> ...
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(caar x) (car (car x))
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(cadr x) (car (cdr x))
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(cdar x) (cdr (car x))
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(cddr x) (cdr (cdr x))
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(caaar x) (car (car (car x)))
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(caadr x) (car (car (cdr x)))
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(cadar x) (car (cdr (car x)))
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(caddr x) (car (cdr (cdr x)))
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(cdaar x) (cdr (car (car x)))
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(cdadr x) (cdr (car (cdr x)))
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(cddar x) (cdr (cdr (car x)))
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(cdddr x) (cdr (cdr (cdr x)))
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(caaaar x) (car (car (car (car x))))
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(caaadr x) (car (car (car (cdr x))))
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(caadar x) (car (car (cdr (car x))))
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(caaddr x) (car (car (cdr (cdr x))))
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(cadaar x) (car (cdr (car (car x))))
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(cadadr x) (car (cdr (car (cdr x))))
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(caddar x) (car (cdr (cdr (car x))))
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(cadddr x) (car (cdr (cdr (cdr x))))
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(cdaaar x) (cdr (car (car (car x))))
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(cdaadr x) (cdr (car (car (cdr x))))
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(cdadar x) (cdr (car (cdr (car x))))
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(cdaddr x) (cdr (car (cdr (cdr x))))
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(cddaar x) (cdr (cdr (car (car x))))
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(cddadr x) (cdr (cdr (car (cdr x))))
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(cdddar x) (cdr (cdr (cdr (car x))))
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(cddddr x) (cdr (cdr (cdr (cdr x))))
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</PRE>
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<P><B>Figure 14-6. CAR and CDR variants</B> <P>
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<A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> can also be used with any of these functions to change an existing component of <I>x</I>, but <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> will not make new components. So, for example, the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of a <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A> can be assigned with <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> of <A REL=DEFINITION HREF="#car"><B>car</B></A>, but the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> cannot be assigned with <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> of <A REL=DEFINITION HREF="#car"><B>car</B></A>. Similarly, the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of a <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A> whose <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> is a <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A> can be assigned with <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> of <A REL=DEFINITION HREF="#caar"><B>caar</B></A>, but neither <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>nor a <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A> whose car is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> can be assigned with <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> of <A REL=DEFINITION HREF="#caar"><B>caar</B></A>. <P>
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The argument <I>x</I> is permitted to be a <A REL=DEFINITION HREF="26_glo_d.htm#dotted_list"><I>dotted list</I></A> or a <A REL=DEFINITION HREF="26_glo_c.htm#circular_list"><I>circular list</I></A>. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(car nil) => NIL
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(cdr '(1 . 2)) => 2
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(cdr '(1 2)) => (2)
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(cadr '(1 2)) => 2
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(car '(a b c)) => A
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(cdr '(a b c)) => (B C)
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</PRE>
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</TT> <P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>Exceptional Situations:</B><P>
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<P>
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The functions <A REL=DEFINITION HREF="#car"><B>car</B></A> and <A REL=DEFINITION HREF="#cdr"><B>cdr</B></A> should signal <A REL=DEFINITION HREF="e_tp_err.htm#type-error"><B>type-error</B></A> if they receive an argument which is not a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A>. The other functions (<A REL=DEFINITION HREF="#caar"><B>caar</B></A>, <A REL=DEFINITION HREF="#cadr"><B>cadr</B></A>, ... <A REL=DEFINITION HREF="#cddddr"><B>cddddr</B></A>) should behave for the purpose of error checking as if defined by appropriate calls to <A REL=DEFINITION HREF="#car"><B>car</B></A> and <A REL=DEFINITION HREF="#cdr"><B>cdr</B></A>. <P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_rplaca.htm#rplaca"><B>rplaca</B></A>, <A REL=DEFINITION HREF="f_firstc.htm#first"><B>first</B></A>, <A REL=DEFINITION HREF="f_rest.htm#rest"><B>rest</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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The <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of a <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A> can also be altered by using <A REL=DEFINITION HREF="f_rplaca.htm#rplaca"><B>rplaca</B></A>, and the <A REL=DEFINITION HREF="26_glo_c.htm#cdr"><I>cdr</I></A> of a <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A> can be altered by using <A REL=DEFINITION HREF="f_rplaca.htm#rplacd"><B>rplacd</B></A>. <P>
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<PRE>
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(car x) == (first x)
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(cadr x) == (second x) == (car (cdr x))
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(caddr x) == (third x) == (car (cdr (cdr x)))
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(cadddr x) == (fourth x) == (car (cdr (cdr (cdr x))))
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</PRE>
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</TT> <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issue</A>, <I>not part of the specification</I>, applies to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss138.htm">DOTTED-LIST-ARGUMENTS:CLARIFY</A><P></UL><HR>
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<A REL=NAVIGATOR HREF="../Front/StartPts.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Starting Points]" SRC="../Graphics/StartPts.gif" ALIGN=Bottom></A><A REL=TOC HREF="../Front/Contents.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Contents]" SRC="../Graphics/Contents.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Master.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Index]" SRC="../Graphics/Index.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Symbol.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Symbols]" SRC="../Graphics/Symbols.gif" ALIGN=Bottom></A><A REL=GLOSSARY HREF="../Body/26_a.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Glossary]" SRC="../Graphics/Glossary.gif" ALIGN=Bottom></A><A HREF="../Front/X3J13Iss.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Issues]" SRC="../Graphics/Issues.gif" ALIGN=Bottom></A><BR>
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<A REL=COPYRIGHT HREF="../Front/Help.htm#Legal"><I>Copyright 1996-2005, LispWorks Ltd. All rights reserved.</I></A><P>
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