68 lines
5.2 KiB
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68 lines
5.2 KiB
HTML
<!-- 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: Macro POP</TITLE>
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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="m_push.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_firstc.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="pop"><I>Macro</I> <B>POP</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>pop</B> <I>place</I> => <I>element</I><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>place</I>---a <A REL=DEFINITION HREF="26_glo_p.htm#place"><I>place</I></A>, the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of which is a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> (possibly, but necessarily, a <A REL=DEFINITION HREF="26_glo_d.htm#dotted_list"><I>dotted list</I></A> or <A REL=DEFINITION HREF="26_glo_c.htm#circular_list"><I>circular list</I></A>). <P>
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<I>element</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> (the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of the contents of <I>place</I>). <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#pop"><B>pop</B></A> <A REL=DEFINITION HREF="26_glo_r.htm#read"><I>reads</I></A> the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of <I>place</I>, remembers the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of the <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> which was retrieved, <A REL=DEFINITION HREF="26_glo_w.htm#write"><I>writes</I></A> the <A REL=DEFINITION HREF="26_glo_c.htm#cdr"><I>cdr</I></A> of the <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> back into the <I>place</I>, and finally <A REL=DEFINITION HREF="26_glo_y.htm#yield"><I>yields</I></A> the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of the originally retrieved <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A>. <P>
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For information about the <A REL=DEFINITION HREF="26_glo_e.htm#evaluation"><I>evaluation</I></A> of <A REL=DEFINITION HREF="26_glo_s.htm#subform"><I>subforms</I></A> of <I>place</I>, see <A REL=CHILD HREF="05_aaa.htm">Section 5.1.1.1 (Evaluation of Subforms to Places)</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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(setq stack '(a b c)) => (A B C)
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(pop stack) => A
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stack => (B C)
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(setq llst '((1 2 3 4))) => ((1 2 3 4))
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(pop (car llst)) => 1
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llst => ((2 3 4))
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</PRE>
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</TT> <P>
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<P><B>Side Effects:</B><P>
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<P>
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The contents of <I>place</I> are modified. <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> None.
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<P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="m_push.htm#push"><B>push</B></A>, <A REL=DEFINITION HREF="m_pshnew.htm#pushnew"><B>pushnew</B></A>, <A REL=CHILD HREF="05_a.htm">Section 5.1 (Generalized Reference)</A> <P>
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<P><B>Notes:</B><P>
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<P>
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The effect of <TT>(pop </TT><I>place</I><TT>)</TT> is roughly equivalent to <P>
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<PRE>
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(prog1 (car place) (setf place (cdr place)))
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</PRE>
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</TT> except that the latter would evaluate any <A REL=DEFINITION HREF="26_glo_s.htm#subform"><I>subforms</I></A> of <I>place</I> three times, while <A REL=DEFINITION HREF="#pop"><B>pop</B></A> evaluates them only once. <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issues</A>, <I>not part of the specification</I>, apply to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss279.htm">PUSH-EVALUATION-ORDER:FIRST-ITEM</A><LI> <A REL=CHILD HREF="../Issues/iss138.htm">DOTTED-LIST-ARGUMENTS:CLARIFY</A><P></UL><HR>
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