80 lines
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80 lines
5.5 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: Function BUTLAST, NBUTLAST</TITLE>
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<A NAME="butlast"><A NAME="nbutlast"><I>Function</I> <B>BUTLAST, NBUTLAST</B></A></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>butlast</B> <I>list <TT>&optional</TT> n</I> => <I>result-list</I><P>
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<B>nbutlast</B> <I>list <TT>&optional</TT> n</I> => <I>result-list</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>list</I>---a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A>, which might be a <A REL=DEFINITION HREF="26_glo_d.htm#dotted_list"><I>dotted list</I></A> but must not be a <A REL=DEFINITION HREF="26_glo_c.htm#circular_list"><I>circular list</I></A>. <P>
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<I>n</I>---a non-negative <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A>. <P>
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<I>result-list</I>---a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#butlast"><B>butlast</B></A> returns a copy of <I>list</I> from which the last <I>n</I> conses have been omitted. If <I>n</I> is not supplied, its value is 1. If there are fewer than <I>n</I> conses in <I>list</I>, <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> is returned and, in the case of <A REL=DEFINITION HREF="#nbutlast"><B>nbutlast</B></A>, <I>list</I> is not modified. <P>
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<A REL=DEFINITION HREF="#nbutlast"><B>nbutlast</B></A> is like <A REL=DEFINITION HREF="#butlast"><B>butlast</B></A>, but <A REL=DEFINITION HREF="#nbutlast"><B>nbutlast</B></A> may modify <I>list</I>. It changes the <A REL=DEFINITION HREF="26_glo_c.htm#cdr"><I>cdr</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A> <I>n</I>+1 from the end of the <I>list</I> to <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<P><B>Examples:</B><P>
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<PRE>
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(setq lst '(1 2 3 4 5 6 7 8 9)) => (1 2 3 4 5 6 7 8 9)
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(butlast lst) => (1 2 3 4 5 6 7 8)
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(butlast lst 5) => (1 2 3 4)
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(butlast lst (+ 5 5)) => NIL
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lst => (1 2 3 4 5 6 7 8 9)
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(nbutlast lst 3) => (1 2 3 4 5 6)
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lst => (1 2 3 4 5 6)
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(nbutlast lst 99) => NIL
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lst => (1 2 3 4 5 6)
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(butlast '(a b c d)) => (A B C)
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(butlast '((a b) (c d))) => ((A B))
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(butlast '(a)) => NIL
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(butlast nil) => NIL
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(setq foo (list 'a 'b 'c 'd)) => (A B C D)
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(nbutlast foo) => (A B C)
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foo => (A B C)
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(nbutlast (list 'a)) => NIL
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(nbutlast '()) => NIL
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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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Should signal an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_tp_err.htm#type-error"><B>type-error</B></A> if <I>list</I> is not a <A REL=DEFINITION HREF="26_glo_p.htm#proper_list"><I>proper list</I></A> or a <A REL=DEFINITION HREF="26_glo_d.htm#dotted_list"><I>dotted list</I></A>. Should signal an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_tp_err.htm#type-error"><B>type-error</B></A> if <I>n</I> is not a non-negative <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A>. <P>
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<P><B>See Also:</B> None.
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<P>
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<P><B>Notes:</B><P>
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<P>
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<PRE>
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(butlast list n) == (ldiff list (last list n))
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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/iss022.htm">BUTLAST-NEGATIVE:SHOULD-SIGNAL</A><LI> <A REL=CHILD HREF="../Issues/iss138.htm">DOTTED-LIST-ARGUMENTS:CLARIFY</A><P></UL><HR>
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