94 lines
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94 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: Macro SHIFTF</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_setf_.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_data_a.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="m_rotate.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="shiftf"><I>Macro</I> <B>SHIFTF</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>shiftf</B> <I><I>place</I><B>+</B> newvalue</I> => <I>old-value-1</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>. <P>
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<I>newvalue</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>; evaluated. <P>
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<I>old-value-1</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> (the old <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of the first <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="#shiftf"><B>shiftf</B></A> modifies the values of each <I>place</I> by storing <I>newvalue</I> into the last <I>place</I>, and shifting the values of the second through the last <I>place</I> into the remaining <I>places</I>. <P>
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If <I>newvalue</I> produces more values than there are store variables, the extra values are ignored. If <I>newvalue</I> produces fewer values than there are store variables, the missing values are set to <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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In the form <TT>(shiftf </TT><I>place1</I><TT> </TT><I>place2</I><TT> ... </TT><I>placen</I><TT> </TT><I>newvalue</I><TT>)</TT>, the values in <I>place1</I> through <I>placen</I> are <A REL=DEFINITION HREF="26_glo_r.htm#read"><I>read</I></A> and saved, and <I>newvalue</I> is evaluated, for a total of <TT>n</TT>+1 values in all. Values 2 through <TT>n</TT>+1 are then stored into <I>place1</I> through <I>placen</I>, respectively. It is as if all the <I>places</I> form a shift register; the <I>newvalue</I> is shifted in from the right, all values shift over to the left one place, and the value shifted out of <I>place1</I> is returned. <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>places</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 x (list 1 2 3) y 'trash) => TRASH
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(shiftf y x (cdr x) '(hi there)) => TRASH
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x => (2 3)
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y => (1 HI THERE)
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(setq x (list 'a 'b 'c)) => (A B C)
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(shiftf (cadr x) 'z) => B
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x => (A Z C)
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(shiftf (cadr x) (cddr x) 'q) => Z
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x => (A (C) . Q)
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(setq n 0) => 0
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(setq x (list 'a 'b 'c 'd)) => (A B C D)
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(shiftf (nth (setq n (+ n 1)) x) 'z) => B
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x => (A Z C D)
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</PRE>
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</TT> <P>
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<P><B>Affected By:</B><P>
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<P>
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<A REL=DEFINITION HREF="m_defi_3.htm#define-setf-expander"><B>define-setf-expander</B></A>, <A REL=DEFINITION HREF="m_defset.htm#defsetf"><B>defsetf</B></A>, <A REL=DEFINITION HREF="v_mexp_h.htm#STmacroexpand-hookST"><B>*macroexpand-hook*</B></A> <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_setf_.htm#setf"><B>setf</B></A>, <A REL=DEFINITION HREF="m_rotate.htm#rotatef"><B>rotatef</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>(shiftf </TT><I>place1</I><TT> </TT><I>place2</I><TT> ... </TT><I>placen</I><TT> </TT><I>newvalue</I><TT>)</TT> is roughly equivalent to <P>
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<PRE>
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(let ((var1 place1)
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(var2 place2)
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...
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(varn placen)
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(var0 newvalue))
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(setf place1 var2)
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(setf place2 var3)
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...
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(setf placen var0)
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var1)
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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 each <TT>place</TT> twice, whereas <A REL=DEFINITION HREF="#shiftf"><B>shiftf</B></A> evaluates them once. For example, <P>
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<PRE>
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(setq n 0) => 0
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(setq x (list 'a 'b 'c 'd)) => (A B C D)
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(prog1 (nth (setq n (+ n 1)) x)
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(setf (nth (setq n (+ n 1)) x) 'z)) => B
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x => (A B Z D)
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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 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/iss309.htm">SETF-MULTIPLE-STORE-VARIABLES:ALLOW</A><P></UL><HR>
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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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