127 lines
4.8 KiB
HTML
127 lines
4.8 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> do (SPECIAL FORM)</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" do (SPECIAL FORM)">
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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="node91.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node72.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node89.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="node91.html"> documentation (FUNCTION)</A>
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<B>Up:</B>
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<A HREF="node72.html"> Appendix: Selected Lisp </A>
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<B> Previous:</B>
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<A HREF="node89.html"> defun (MACRO)</A>
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<BR> <HR> <P>
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<H1> do (SPECIAL FORM)</H1>
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<P>
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<b> Format:</b>
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<tt> (do ((<var1> <init1> <update1> )</tt>
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<tt> (<var2> <init2> <update2> )</tt>
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<tt> .</tt>
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<tt> .</tt>
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<tt> (<varN> <initN> <updateN> ))</tt>
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<tt> (<test> <body1> )</tt>
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<tt> <body2> )</tt>
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<P>
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<b> Required arguments:</b>
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2
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<P>
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<tt> ((<var> <init> <update> )...)</tt>: a list of zero or more variable
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clauses; <tt> <var> </tt> is a symbol appropriate as a variable
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name; <tt> <init> </tt>, which is optional, is any Lisp
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expression; <tt> <update> </tt>, which is optional, is any Lisp
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expression.
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<tt> (<test> <body1> )</tt>: <tt> <test> </tt> is any Lisp expression;
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<tt> <body1> </tt> is a sequence of zero or more Lisp
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expressions.
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<P>
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<b> Optional arguments:</b>
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arbitrary
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<P>
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<tt> <body2> </tt>: a sequence of zero or more Lisp expressions
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<P>
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The special form do allows the programmer to specify
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iteration. The first part of do is a list of variables; if <tt> <init> </tt>
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is provided, the <tt> <var> </tt>'s are initialised to the result of evaluating
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the corresponding <tt> <init> </tt>. If no <tt> <init> </tt> is provided, <tt> <var> </tt> is
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initialised to NIL. The optional <tt> <update> </tt> expression may
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specify how the variable is to be modified after every iteration.
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After every iteration, <tt> <var> </tt> is set to result of evaluating
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<tt> <update> </tt>.
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Initialisation and updating for all variables is performed in
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parallel; thus a <tt> <var> </tt> in one clause may not be used in the <tt>
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<init> </tt> or <tt> <update> </tt> of another clause.
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<P>
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The second part of ``do,'' the <tt> <test> </tt>, checks for termination. The
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<tt> <test> </tt> is evaluated before each pass; if it returns a non-NIL value,
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the sequence of expressions in <tt> <body1> </tt> are evaluated one by one; do
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returns the value of the last expression in <tt> <body1> </tt>. If <tt> <body1> </tt>
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contains no expressions, do returns NIL.
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<P>
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The third part of the
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do is the body of the iteration, <tt> <body2> </tt>. At each pass, the sequence
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of expressions in <tt> <body2> </tt> are evaluated one by one.
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If <tt> <body2> </tt> contains an expression of the form <tt> (return <expr> )</tt>, where
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<tt> <expr> </tt> is any Lisp expression, do terminates immediately and returns
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the result of evaluating <tt><expr> </tt>.
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<P>
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<b> Examples:</b>
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<P>
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<BLOCKQUOTE>
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<PRE> (setq sum (+ sum (first lst))))
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25
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> (do ((lst
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'(a (b (c d)) e (f))
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(rest lst))
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(len 0 (+ 1 len))) ; determines length of lst
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((null lst) len)) ; "do," here, has no body
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4
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> (defun my-exp (m n) ; raise m to power of n
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(do ((result 1)
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(exp n))
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((= exp 0) result)
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(setq result (* result m))
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(setq exp (- exp 1))))
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MY-EXP
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> (my-exp 5 3)
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125
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> (defun my-exp2 (m n) ; simpler version of my-exp
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(do ((result 1 (* result m))
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(exp n (- exp 1)))
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((= exp 0) result)))
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</PRE>
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</BLOCKQUOTE>
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<P>
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<BR> <HR>
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<A HREF="node91.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node72.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node89.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="node91.html"> documentation (FUNCTION)</A>
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<B>Up:</B>
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<A HREF="node72.html"> Appendix: Selected Lisp </A>
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|
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<B> Previous:</B>
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|
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<A HREF="node89.html"> defun (MACRO)</A>
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<BR> <HR> <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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November 1999</I>
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</ADDRESS>
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</BODY>
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