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100 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: Function REVAPPEND, NRECONC</TITLE>
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<A NAME="revappend"><A NAME="nreconc"><I>Function</I> <B>REVAPPEND, NRECONC</B></A></A> <P>
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<P>
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<P><B>Syntax:</B><P>
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<P>
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<B>revappend</B> <I>list tail</I> => <I>result-list</I><P>
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<B>nreconc</B> <I>list tail</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_p.htm#proper_list"><I>proper list</I></A>. <P>
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<I>tail</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>result-list</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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<A REL=DEFINITION HREF="#revappend"><B>revappend</B></A> constructs a <A REL=DEFINITION HREF="26_glo_c.htm#copy"><I>copy</I></A>[2] of <I>list</I>, but with the <A REL=DEFINITION HREF="26_glo_e.htm#element"><I>elements</I></A> in reverse order. It then appends (as if by <A REL=DEFINITION HREF="f_nconc.htm#nconc"><B>nconc</B></A>) the <I>tail</I> to that reversed list and returns the result. <P>
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<A REL=DEFINITION HREF="#nreconc"><B>nreconc</B></A> reverses the order of <A REL=DEFINITION HREF="26_glo_e.htm#element"><I>elements</I></A> in <I>list</I> (as if by <A REL=DEFINITION HREF="f_revers.htm#nreverse"><B>nreverse</B></A>). It then appends (as if by <A REL=DEFINITION HREF="f_nconc.htm#nconc"><B>nconc</B></A>) the <I>tail</I> to that reversed list and returns the result. <P>
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The resulting <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> shares <A REL=DEFINITION HREF="26_glo_l.htm#list_structure"><I>list structure</I></A> with <I>tail</I>. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(let ((list-1 (list 1 2 3))
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(list-2 (list 'a 'b 'c)))
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(print (revappend list-1 list-2))
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(print (equal list-1 '(1 2 3)))
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(print (equal list-2 '(a b c))))
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>> (3 2 1 A B C)
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>> T
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>> T
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=> T
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(revappend '(1 2 3) '()) => (3 2 1)
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(revappend '(1 2 3) '(a . b)) => (3 2 1 A . B)
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(revappend '() '(a b c)) => (A B C)
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(revappend '(1 2 3) 'a) => (3 2 1 . A)
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(revappend '() 'a) => A ;degenerate case
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(let ((list-1 '(1 2 3))
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(list-2 '(a b c)))
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(print (nreconc list-1 list-2))
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(print (equal list-1 '(1 2 3)))
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(print (equal list-2 '(a b c))))
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>> (3 2 1 A B C)
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>> NIL
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>> T
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=> T
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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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<A REL=DEFINITION HREF="#revappend"><B>revappend</B></A> does not modify either of its <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>arguments</I></A>. <A REL=DEFINITION HREF="#nreconc"><B>nreconc</B></A> is permitted to modify <I>list</I> but not <I>tail</I>. <P>
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Although it might be implemented differently, <A REL=DEFINITION HREF="#nreconc"><B>nreconc</B></A> is constrained to have side-effect behavior equivalent to: <P>
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<PRE>
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(nconc (nreverse list) tail)
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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> 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="f_revers.htm#reverse"><B>reverse</B></A>, <A REL=DEFINITION HREF="f_revers.htm#nreverse"><B>nreverse</B></A>, <A REL=DEFINITION HREF="f_nconc.htm#nconc"><B>nconc</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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The following functional equivalences are true, although good <A REL=DEFINITION HREF="26_glo_i.htm#implementation"><I>implementations</I></A> will typically use a faster algorithm for achieving the same effect: <P>
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<PRE>
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(revappend list tail) == (nconc (reverse list) tail)
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(nreconc list tail) == (nconc (nreverse list) tail)
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</PRE>
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</TT> <P>
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<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/iss293.htm">REMF-DESTRUCTION-UNSPECIFIED:X3J13-MAR-89</A><LI> <A REL=CHILD HREF="../Issues/iss138.htm">DOTTED-LIST-ARGUMENTS:CLARIFY</A><P></UL><HR>
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