81 lines
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81 lines
3.8 KiB
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Section 6.1.4.2</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="06_ada.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="06_ad.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="06_adc.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
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6.1.4.2 Examples of ALWAYS, NEVER, and THEREIS clauses</H2> <P>
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<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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(loop for i from 0 to 10
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always (< i 11))
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=> T
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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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;; If I exceeds 10 return I; otherwise, return NIL.
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;; The THEREIS construct terminates this loop.
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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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;;; The FINALLY clause is not evaluated in these examples.
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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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(loop never t
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finally (print "you won't see this"))
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=> NIL
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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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;; The FOR construct terminates this loop, so the FINALLY clause
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;; is evaluated.
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(loop for i from 1 to 10
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thereis (> i 11)
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finally (prin1 'got-here))
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>> GOT-HERE
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=> NIL
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;; If this code could be used to find a counterexample to Fermat's
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;; last theorem, it would still not return the value of the
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;; counterexample because all of the THEREIS clauses in this example
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;; only return T. But if Fermat is right, that won't matter
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;; because this won't 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) (expt y n))
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(expt z n))))))
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</PRE>
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</TT> <P>
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