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234 lines
9.3 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>26.7. End-Test Control</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" End-Test Control">
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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=tex2html4628 HREF="node246.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4626 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4620 HREF="node244.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4630 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4631 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4629 HREF="node246.html"> Value Accumulation</A>
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<B>Up:</B> <A NAME=tex2html4627 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4621 HREF="node244.html"> Iteration Control</A>
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<HR> <P>
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<H1><A NAME=SECTION003070000000000000000>26.7. End-Test Control</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=LOOPTESTSECTION>The</A>
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loop keywords <tt>always</tt>, <tt>never</tt>, <tt>thereis</tt>,
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<tt>until</tt>, and <tt>while</tt> designate constructs that use a single test
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condition to determine when loop iteration should terminate.
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<P>
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The constructs <tt>always</tt>, <tt>never</tt>, and <tt>thereis</tt> provide
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specific values to be returned when a loop terminates.
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Using <tt>always</tt>, <tt>never</tt>, or <tt>thereis</tt> with
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value-returning accumulation clauses can produce unpredictable results.
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In all other respects these
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constructs behave like the <tt>while</tt> and <tt>until</tt> constructs.
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<P>
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The macro <tt>loop-finish</tt> can be used at any time to cause normal
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termination. In normal termination, <tt>finally</tt> clauses are
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executed and default return values are returned.
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<P>
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End-test control constructs can be used anywhere within the loop
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body. The termination conditions are tested in the order in which
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they appear.
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<tt>while</tt> <em>expr</em> <tt><BR></tt><tt>until</tt> <em>expr</em><P>The <tt>while</tt> construct allows iteration to continue until the specified
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expression <i>expr</i> evaluates to <tt>nil</tt>. The expression
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is re-evaluated at the location of the <tt>while</tt> clause.
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<P>
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The <tt>until</tt> construct is equivalent to
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<tt>while</tt> (<tt>not</tt> <i>expr</i>). If the value of the
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specified expression is non-<tt>nil</tt>, iteration terminates.
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<P>
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You can use <tt>while</tt> and <tt>until</tt>
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at any point in a loop. If a <tt>while</tt> or <tt>until</tt> clause causes
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termination, any clauses that precede it in the source
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are still evaluated.
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<P>
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Examples:
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<P><pre>
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;;; A classic "while-loop".
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(loop while (hungry-p) do (eat))
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;;; UNTIL NOT is equivalent to WHILE.
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(loop until (not (hungry-p)) do (eat))
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;;; Collect the length and the items of STACK.
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(let ((stack '(a b c d e f)))
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(loop while stack
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for item = (length stack) then (pop stack)
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collect item))
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=> (6 A B C D E F)
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;;; Use WHILE to terminate a loop that otherwise wouldn't
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;;; terminate. Note that WHILE occurs after the WHEN.
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(loop for i fixnum from 3
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when (oddp i) collect i
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while (< i 5))
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=> (3 5)
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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>always</tt> <em>expr</em> <tt><BR></tt><tt>never</tt> <em>expr</em> <tt><BR></tt><tt>thereis</tt> <em>expr</em><P>The <tt>always</tt> construct takes one form and terminates the loop
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if the form ever evaluates to <tt>nil</tt>; in this case, it returns
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<tt>nil</tt>. Otherwise, it provides a default return value of <tt>t</tt>.
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<P>
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The <tt>never</tt> construct takes one form and terminates the loop
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if the form ever evaluates to non-<tt>nil</tt>; in this case, it returns
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<tt>nil</tt>. Otherwise, it provides a default return value of <tt>t</tt>.
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<P>
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The <tt>thereis</tt> construct takes one form and terminates the loop
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if the form ever evaluates to non-<tt>nil</tt>; in this case, it returns
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that value.
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<P>
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If the <tt>while</tt> or <tt>until</tt> construct causes termination,
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control is passed to the loop epilogue, where any <tt>finally</tt>
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clauses will be executed. Since <tt>always</tt>, <tt>never</tt>, and
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<tt>thereis</tt> use the Common Lisp macro <tt>return</tt> to terminate
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iteration, any <tt>finally</tt> clause that is specified is not
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evaluated.
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<P>
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Examples:
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<P><pre>
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;;; Make sure I is always less than 11 (two ways).
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;;; The FOR construct terminates these loops.
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</pre><P>
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<P><pre>
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(loop for i from 0 to 10
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always (< i 11))
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=> T
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</pre><P>
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<P><pre>
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(loop for i from 0 to 10
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never (> i 11))
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=> T
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</pre><P>
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<P><pre>
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;;; If I exceeds 10, return I; otherwise, return NIL.
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;;; The THEREIS construct terminates this loop.
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</pre><P>
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<P><pre>
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(loop for i from 0
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thereis (when (> i 10) i) )
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=> 11
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</pre><P>
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<P><pre>
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;;; The FINALLY clause is not evaluated in these examples.
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</pre><P>
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<P><pre>
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(loop for i from 0 to 10
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always (< i 9)
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finally (print "you won't see this"))
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=> NIL
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</pre><P>
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<P><pre>
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(loop never t
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finally (print "you won't see this"))
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=> NIL
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</pre><P>
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<P><pre>
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(loop thereis "Here is my value"
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finally (print "you won't see this"))
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=> "Here is my value"
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</pre><P>
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<P><pre>
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;;; The FOR construct terminates this loop,
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;;; so the FINALLY clause is evaluated.
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</pre><P>
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<P><pre>
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(loop for i from 1 to 10
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thereis (> i 11)
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finally (print i)) `;Prints 1 line
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11
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=> NIL
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</pre><P>
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<P><pre>
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(defstruct mountain height difficulty (why "because it is there"))
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(setq everest (make-mountain :height '(2.86e-13 parsecs)))
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(setq chocorua (make-mountain :height '(1059180001 microns)))
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(defstruct desert area (humidity 0))
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(setq sahara (make-desert :area '(212480000 square furlongs)))
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`;First there is a mountain, then there is no mountain, then there is <b>...</b>
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(loop for x in (list everest sahara chocorua) `; -GLS
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thereis (and (mountain-p x) (mountain-height x)))
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=> (2.86E-13 PARSECS)
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;;; If you could use this code to find a counterexample to
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;;; Fermat's last theorem, it would still not return the value
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;;; of the counterexample because all of the THEREIS clauses
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;;; in this example return only T. Of course, this code has
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;;; never been observed to terminate.
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(loop for z upfrom 2
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thereis
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(loop for n upfrom 3 below (log z 2)
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thereis
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(loop for x below z
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thereis
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(loop for y below z
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thereis (= (+ (expt x n)
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(expt y n))
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(expt z n))))))
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</pre><P>
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>loop-finish</tt><P>The macro <tt>loop-finish</tt> terminates iteration normally
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and returns any accumulated result. If specified, a <tt>finally</tt> clause
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is evaluated.
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<P>
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In most cases it is not necessary to use <tt>loop-finish</tt>
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because other loop control clauses terminate the loop.
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Use <tt>loop-finish</tt> to provide a normal exit
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from a nested condition inside a loop.
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<P>
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You can use <tt>loop-finish</tt>
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inside nested Lisp code to provide a normal exit from a loop.
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Since <tt>loop-finish</tt> transfers control to the loop epilogue,
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using <tt>loop-finish</tt> within a <tt>finally</tt> expression can cause
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infinite looping.
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<P>
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Implementations are allowed to provide this construct as a local macro
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by using <tt>macrolet</tt>.
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<P>
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Examples:
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<P><pre>
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;;; Print a date in February, but exclude leap day.
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;;; LOOP-FINISH exits from the nested condition.
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(loop for date in date-list
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do (case date
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(29 (when (eq month 'february)
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(loop-finish))
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(format t "~:@(~A~) ~A" month date))))
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;;; Terminate the loop, but return the accumulated count.
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(loop for i in '(1 2 3 stop-here 4 5 6)
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when (symbolp i) do (loop-finish)
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count i)
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=> 3
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;;; This loop works just as well as the previous example.
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(loop for i in '(1 2 3 stop-here 4 5 6)
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until (symbolp i)
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count i)
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=> 3
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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=tex2html4628 HREF="node246.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4626 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4620 HREF="node244.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4630 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4631 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4629 HREF="node246.html"> Value Accumulation</A>
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<B>Up:</B> <A NAME=tex2html4627 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4621 HREF="node244.html"> Iteration Control</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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