113 lines
4.8 KiB
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113 lines
4.8 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>26.11. Unconditional Execution</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Unconditional 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=tex2html4676 HREF="node250.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4674 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4668 HREF="node248.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4678 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4679 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4677 HREF="node250.html"> Miscellaneous Features</A>
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<B>Up:</B> <A NAME=tex2html4675 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4669 HREF="node248.html"> Conditional Execution</A>
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<HR> <P>
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<H1><A NAME=SECTION0030110000000000000000>26.11. Unconditional 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=LOOPUNCONDSECTION>The</A>
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loop construct <tt>do</tt> (or <tt>doing</tt>) takes one or more expressions
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and simply evaluates them in order.
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<P>
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The loop construct <tt>return</tt> takes one expression and returns its value. It
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is equivalent to the clause <tt>do (return <i>value</i>)</tt>.
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<tt>do</tt> <tt>{<i>expr</i>}*</tt> <tt><BR></tt><tt>doing</tt> <tt>{<i>expr</i>}*</tt><P>The <tt>do</tt> construct simply evaluates the specified expressions
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wherever they occur in the expanded form of <tt>loop</tt>.
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<P>
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The <i>expr</i> argument can be any non-atomic Common Lisp form.
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Each <i>expr</i> is evaluated in every iteration.
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<P>
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The constructs <tt>do</tt>, <tt>initially</tt>, and <tt>finally</tt> are the
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only loop keywords that take an arbitrary number of forms and group
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them as if using an implicit <tt>progn</tt>.
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Because every loop clause must begin with a loop keyword, you would use
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the keyword <tt>do</tt> when no control action other than execution is
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required.
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<P>
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Examples:
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<P><pre>
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;;; Print some numbers.
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(loop for i from 1 to 5
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do (print i)) `;Prints 5 lines
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1
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2
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3
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4
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5
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=> NIL
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;;; Print numbers and their squares.
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;;; The DO construct applies to multiple forms.
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(loop for i from 1 to 4
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do (print i)
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(print (* i i))) `;Prints 8 lines
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1
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1
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2
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4
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3
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9
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4
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16
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=> NIL
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</pre><P>
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<tt>return</tt> <em>expr</em><P>The <tt>return</tt> construct terminates a
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loop and returns the value of
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the specified expression as the value of the loop. This construct
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is similar to the Common Lisp special form <tt>return-from</tt> and the
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Common Lisp macro <tt>return</tt>.
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<P>
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The Loop Facility supports the <tt>return</tt> construct for backward
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compatibility with older <tt>loop</tt> implementations.
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The <tt>return</tt> construct returns immediately and does not execute
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any <tt>finally</tt> clause that is given.
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<P>
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Examples:
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<P><pre>
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;;; Signal an exceptional condition.
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(loop for item in '(1 2 3 a 4 5)
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when (not (numberp item))
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return (cerror "enter new value"
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"non-numeric value: ~s"
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item)) `;Signals an error
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>>Error: non-numeric value: A
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;;; The previous example is equivalent to the following one.
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(loop for item in '(1 2 3 a 4 5)
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when (not (numberp item))
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do (return
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(cerror "enter new value"
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"non-numeric value: ~s"
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item))) `;Signals an error
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>>Error: non-numeric value: A
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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=tex2html4676 HREF="node250.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4674 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4668 HREF="node248.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4678 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4679 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4677 HREF="node250.html"> Miscellaneous Features</A>
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<B>Up:</B> <A NAME=tex2html4675 HREF="node235.html"> Loop</A>
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|
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<B> Previous:</B> <A NAME=tex2html4669 HREF="node248.html"> Conditional Execution</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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