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165 lines
7.7 KiB
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>26.10. Conditional Execution</TITLE>
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<BODY>
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<meta name="description" value=" Conditional Execution">
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<meta name="keywords" value="clm">
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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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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html4664 HREF="node249.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4662 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4656 HREF="node247.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4666 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4667 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4665 HREF="node249.html"> Unconditional Execution</A>
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<B>Up:</B> <A NAME=tex2html4663 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4657 HREF="node247.html"> Variable Initializations</A>
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<HR> <P>
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<H1><A NAME=SECTION0030100000000000000000>26.10. Conditional Execution</A></H1>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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<A NAME=LOOPCONDSECTION>The</A>
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loop keywords <tt>if</tt>, <tt>when</tt>, and <tt>unless</tt> designate constructs that
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are useful when you want some loop clauses to operate under a specified
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condition.
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<P>
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If the specified condition is true, the succeeding loop clause
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is executed. If the specified condition is not true, the succeeding clause is
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skipped, and program control moves to the clause that follows the loop
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keyword <tt>else</tt>. If the specified condition is not true and no
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<tt>else</tt> clause is specified, the entire conditional construct
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is skipped. Several clauses can be connected into
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one compound clause with the loop keyword <tt>and</tt>.
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The end of the conditional clause can be marked with the keyword <tt>end</tt>.
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<pre>
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if <em>expr</em> <i>clause</i> {and <i>clause</i>}*
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[else <i>clause</i> {and <i>clause</i>}*] [end]
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when <em>expr</em> <i>clause</i> {and <i>clause</i>}*
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[else <i>clause</i> {and <i>clause</i>}*] [end]
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unless <em>expr</em> <i>clause</i> {and <i>clause</i>}*
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[else <i>clause</i> {and <i>clause</i>}*] [end]
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</pre>
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<P>The constructs <tt>when</tt> and <tt>if</tt> allow conditional execution of
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loop clauses. These constructs are synonyms and
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can be used interchangeably. [Compare this to the <i>macro</i> <tt>when</tt>,
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which does not allow an ``else'' part.-GLS]
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<P>
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If the value of the test expression <i>expr</i> is non-<tt>nil</tt>, the expression
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<i>clause1</i> is evaluated. If the test expression evaluates to <tt>nil</tt>
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and an <tt>else</tt> construct is specified, the statements that follow the
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<tt>else</tt> are evaluated; otherwise, control passes to the next clause.
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<P>
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The <tt>unless</tt> construct is equivalent to <tt>when</tt> (<tt>not</tt>
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<i>expr</i>) and <tt>if</tt> (<tt>not</tt> <i>expr</i>).
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If the value of the test expression <i>expr</i> is <tt>nil</tt>, the expression
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<i>clause1</i> is evaluated. If the test expression evaluates to
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non-<tt>nil</tt>
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and an <tt>else</tt> construct is specified, the statements that follow the
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<tt>else</tt> are evaluated; otherwise, control passes to the next clause.
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[Compare this to the <i>macro</i> <tt>unless</tt>,
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which does not allow an ``else'' part-or do I mean a ``then'' part?! Ugh.
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To prevent confusion, I strongly recommend as a matter of style
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that <tt>else</tt> not be used with <tt>unless</tt> loop clauses.-GLS]
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<P>
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The <i>clause</i> arguments must be either accumulation, unconditional,
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or conditional clauses (see section <A HREF="node240.html#LOOPKINDSSECTION">26.3.2</A>).
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Clauses that follow the test expression can be grouped by using the
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loop keyword <tt>and</tt> to produce a compound
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clause.
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<P>
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The loop keyword <tt>it</tt> can be used to refer to the result of
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the test expression in a clause. If multiple clauses are connected with <tt>and</tt>,
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the <tt>it</tt> construct must be used in the first
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clause in the block. Since <tt>it</tt> is a loop keyword, <tt>it</tt> may not be used
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as a local variable within a loop.
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<P>
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If <tt>when</tt> or <tt>if</tt> clauses are nested, each <tt>else</tt> is
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paired with the closest preceding <tt>when</tt> or <tt>if</tt> construct that has
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no associated <tt>else</tt>.
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<P>
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The optional loop keyword <tt>end</tt> marks the end of the clause. If this
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keyword is not specified, the next loop keyword marks the end. You can use
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<tt>end</tt> to distinguish the scoping of compound clauses.
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<P><pre>
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;;; Group conditional clauses into a block.
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(loop for i in numbers-list
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when (oddp i)
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do (print i)
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and collect i into odd-numbers
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and do (terpri)
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else ;<tt>I</tt> is even
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collect i into even-numbers
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finally
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(return (values odd-numbers even-numbers)))
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</pre><P>
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<P><pre>
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;;; Collect numbers larger than 3.
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(loop for i in '(1 2 3 4 5 6)
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when (and (> i 3) i)
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collect it) ;<tt>it</tt> refers to <tt>(and (> i 3) i)</tt>
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=> (4 5 6)
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</pre><P>
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<P><pre>
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;;; Find a number in a list.
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(loop for i in '(1 2 3 4 5 6)
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when (and (> i 3) i)
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return it)
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=> 4
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</pre><P>
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<P><pre>
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;;; The preceding example is similar to the following one.
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(loop for i in '(1 2 3 4 5 6)
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thereis (and (> i 3) i))
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=> 4
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</pre><P>
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<P><pre>
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;;; An example of using UNLESS with ELSE (yuk).`-GLS
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(loop for turtle in teenage-mutant-ninja-turtles do
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(loop for x in '(joker brainiac shredder krazy-kat)
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unless (evil x)
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do (eat (make-pizza :anchovies t))
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else unless (and (eq x 'shredder) (attacking-p x))
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do (cut turtle slack);When the evil Shredder attacks,
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else (fight turtle x)));those turtle boys don't cut no slack
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</pre><P>
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<P><pre>
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;;; Nest conditional clauses.
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(loop for i in list
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when (numberp i)
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when (bignump i)
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collect i into big-numbers
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else ;Not <tt>(bignump i)</tt>
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collect i into other-numbers
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else ;Not <tt>(numberp i)</tt>
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when (symbolp i)
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collect i into symbol-list
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else ;Not <tt>(symbolp i)</tt>
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(error "found a funny value in list ~S, value ~S~%"
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"list i))
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;;; Without the END marker, the last AND would apply to the
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;;; inner IF rather than the outer one.
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(loop for x from 0 to 3
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do (print x)
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if (zerop (mod x 2))
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do (princ " a")
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and if (zerop (floor x 2))
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do (princ " b")
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end
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and do (princ " c"))
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</pre><P>
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html4664 HREF="node249.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4662 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4656 HREF="node247.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4666 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4667 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4665 HREF="node249.html"> Unconditional Execution</A>
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<B>Up:</B> <A NAME=tex2html4663 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4657 HREF="node247.html"> Variable Initializations</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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