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From: lowj_ltd@uhura.cc.rochester.edu (John Alan Low)
Newsgroups: comp.lang.pascal
Subject: Re: Problem in Turbo Pascal 4.0
Summary: I understand now, thanks to all who emailed me!
Keywords: Nonterminating Repeat loop
Message-ID: <4660@ur-cc.UUCP>
Date: 30 Dec 89 19:06:58 GMT
References: <4658@ur-cc.UUCP>
Reply-To: lowj_ltd@uhura.cc.rochester.edu (John Alan Low, aka "Travis" Low)
Organization: University of Rochester
Lines: 35

In article <4658@ur-cc.UUCP> I wrote: 
>I am having a problem with Turbo Pascal 4.0. The repeat loop in the following
>loop never terminates. 
>Program Foo;
>var  x : real;
>Begin
>x := 0.10;
>     Repeat
>     x := x + 0.01;
>     Until x = 0.30;
>End.

     Thanks to everybody who emailed me on this. I forgot that 0.01 is a
     repeating decimal in binary. I never will again.

     For those who are as mystified as I was:
     Since 0.01 cannot be represented as a non-repeating decimal in binary,
     there is a floating point roundoff error. So x is never equal to 0.30,
     just very close. Strangely enough, although I put no formatting in the
     writeln statement, I missed the error. I don't know if it showed up at
     all, I was very tired when I wrote it.

     The two simplest solutions were:
       1) replace the terminating condition with "Until x >= 0.30".
       2) replace the terminating condition with "Until abs(x - 0.30) < delta",
          where delta is some predetermined error factor.
     The best solution in terms of speed was:
          Make x an integer (start at 10) and increment by 1 each time. Where
          x is used in the loop, subsitute x/100. Terminate when x = 30.
 
     Thanks again for all the help. (Boy, do I feel stupid)

---Travis

Ich suche die Leidenschaft, die keine Leiden schafft.