44 lines
3.7 KiB
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44 lines
3.7 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: Section 2.4.8.16</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="02_dho.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="02_dh.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="02_dhq.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
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2.4.8.16 Sharpsign Sharpsign</H2> <P>
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<TT>#</TT><I>n</I><TT>#</TT> <P>
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<TT>#</TT><I>n</I><TT>#</TT>, where <I>n</I> is a required unsigned decimal <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A>, provides a reference to some <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> labeled by <TT>#</TT><I>n</I><TT>=</TT>; that is, <TT>#</TT><I>n</I><TT>#</TT> represents a pointer to the same (<A REL=DEFINITION HREF="f_eq.htm#eq"><B>eq</B></A>) <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> labeled by <TT>#</TT><I>n</I><TT>=</TT>. For example, a structure created in the variable <TT>y</TT> by this code: <P>
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<PRE>
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(setq x (list 'p 'q))
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(setq y (list (list 'a 'b) x 'foo x))
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(rplacd (last y) (cdr y))
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</PRE>
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</TT> could be represented in this way: <P>
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<PRE>
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((a b) . #1=(#2=(p q) foo #2# . #1#))
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</PRE>
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</TT> Without this notation, but with <A REL=DEFINITION HREF="v_pr_lev.htm#STprint-lengthST"><B>*print-length*</B></A> set to <TT>10</TT> and <A REL=DEFINITION HREF="v_pr_cir.htm#STprint-circleST"><B>*print-circle*</B></A> set to <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>, the structure would print in this way: <P>
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<PRE>
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((a b) (p q) foo (p q) (p q) foo (p q) (p q) foo (p q) ...)
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</PRE>
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</TT> A reference <TT>#</TT><I>n</I><TT>#</TT> may only occur after a label <TT>#</TT><I>n</I><TT>=</TT>; forward references are not permitted. The reference may not appear as the labeled object itself (that is, <TT>#</TT><I>n</I><TT>=#</TT><I>n</I><TT>#</TT>) may not be written because the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> labeled by <TT>#</TT><I>n</I><TT>=</TT> is not well defined in this case. <P>
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