86 lines
4.8 KiB
HTML
86 lines
4.8 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 LIST-LENGTH</TITLE>
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<A NAME="list-length"><I>Function</I> <B>LIST-LENGTH</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>list-length</B> <I>list</I> => <I>length</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> or a <A REL=DEFINITION HREF="26_glo_c.htm#circular_list"><I>circular list</I></A>. <P>
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<I>length</I>---a non-negative <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A>, or <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<P><B>Description:</B><P>
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<P>
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Returns the <A REL=DEFINITION HREF="26_glo_l.htm#length"><I>length</I></A> of <I>list</I> if <I>list</I> is a <A REL=DEFINITION HREF="26_glo_p.htm#proper_list"><I>proper list</I></A>. Returns <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> if <I>list</I> is a <A REL=DEFINITION HREF="26_glo_c.htm#circular_list"><I>circular list</I></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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(list-length '(a b c d)) => 4
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(list-length '(a (b c) d)) => 3
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(list-length '()) => 0
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(list-length nil) => 0
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(defun circular-list (&rest elements)
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(let ((cycle (copy-list elements)))
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(nconc cycle cycle)))
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(list-length (circular-list 'a 'b)) => NIL
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(list-length (circular-list 'a)) => NIL
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(list-length (circular-list)) => 0
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</PRE>
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</TT> <P>
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<P><B>Side Effects:</B> None.
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<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_c.htm#circular_list"><I>circular list</I></A>. <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_length.htm#length"><B>length</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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<A REL=DEFINITION HREF="#list-length"><B>list-length</B></A> could be implemented as follows: <P>
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<PRE>
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(defun list-length (x)
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(do ((n 0 (+ n 2)) ;Counter.
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(fast x (cddr fast)) ;Fast pointer: leaps by 2.
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(slow x (cdr slow))) ;Slow pointer: leaps by 1.
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(nil)
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;; If fast pointer hits the end, return the count.
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(when (endp fast) (return n))
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(when (endp (cdr fast)) (return (+ n 1)))
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;; If fast pointer eventually equals slow pointer,
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;; then we must be stuck in a circular list.
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;; (A deeper property is the converse: if we are
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;; stuck in a circular list, then eventually the
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;; fast pointer will equal the slow pointer.
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;; That fact justifies this implementation.)
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(when (and (eq fast slow) (> n 0)) (return nil))))
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</PRE>
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</TT> <P>
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