51 lines
2.3 KiB
HTML
51 lines
2.3 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Originally converted to HTML using LaTeX2HTML 95 (Thu Jan 19 1995) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE> Iteration Using Dolist</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Iteration Using Dolist">
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<meta name="keywords" value="lp">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<BR> <HR>
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<A HREF="node37.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node29.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node35.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <BR>
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<A HREF="lp.html"><B>Contents</B></A>
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<B> Next:</B>
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<A HREF="node37.html"> When To Use </A>
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<B>Up:</B>
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<A HREF="node29.html"> Recursion and Iteration</A>
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<B> Previous:</B>
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<A HREF="node35.html"> Local Variables Using </A>
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<BR> <HR> <P>
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<H1> Iteration Using Dolist</H1>
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<P>
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Dolist is very much like dotimes, except that the iteration is controlled by the length of a list, rather than by the value of a count. Here is the general form for dolist:
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<BLOCKQUOTE>
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<PRE> (dolist (<next-element> <target-list> <result> ) <body> )
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</PRE>
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</BLOCKQUOTE>
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In a dolist statment result and body work just as in dotimes. Next-element is the name of a variable which is intially set to the first element of target-list (which must therefore be a list). The next time through, next-element is set to the second element of target-list and so on until the end of target-list is reached, at which point result-form is evaluated and returned.
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<P>
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Here is num-sublists done as an example:
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<BLOCKQUOTE>
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<PRE> (defun num-sublists-i (lis)
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(let ((result 0))
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(dolist (next lis result)
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(if (listp next)
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(setf result (1+ result))))))
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</PRE>
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</BLOCKQUOTE>
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The function num-sublists-i works because result is incremented only whenever the current element of the list indicated by next is itself a list. Notice that in the case where the next element of the list is not itself a list there is no need to do anything. Thus, in the if statement of the definition it is possible to omit the else clause entirely.
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<P>
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<BR> <HR>
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<P>
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<ADDRESS>
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<I>© Colin Allen & Maneesh Dhagat <BR>
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March 2007 </I>
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</ADDRESS>
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</BODY>
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