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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: Loop terminates when binary coded decimal is used.
Keywords: Nonterminating Repeat loop, real, representation, binary coded decimal
Message-ID: <4679@ur-cc.UUCP>
Date: 3 Jan 90 18:58:34 GMT
References: <4658@ur-cc.UUCP> <4660@ur-cc.UUCP> <803@tuewsd.lso.win.tue.nl>
Reply-To: lowj_ltd@uhura.cc.rochester.edu (John Alan Low, aka "Travis" Low)
Followup-To: comp.lang.pascal
Organization: University of Rochester
Lines: 32


                                                     
In article <803@tuewsd.lso.win.tue.nl> wsinkees@lso.win.tue.nl (Kees Huizing) writes:
>>In article <4658@ur-cc.UUCP> I wrote: 
>>>Program Foo;
>>>var  x : real;
>>>Begin  x := 0.10;
>>>       Repeat
>>>       x := x + 0.01;
>>>       Until x = 0.30; End.
>It's not you that is stupid, it's (Turbo?) Pascal.  Is it not the purpose of
>a high level pogramming language to hide the funny details of implementation
>and representation for the user?  As it shows, the equality operator
>"=" does not on reals what it pretends to do.  I accept that it cannot
>detect perfect equality of reals, but what it could do is detect equality
>upto some representation dependent accuracy.  
>Why is it not done this way?

     After I read this, I compiled the program with the Turbo Pascal 3.0
     turbobcd.com compiler. Guess what? The loop terminates nicely. Also,
     expressions such as 1/3 + 2/3 come out equal to 1.0. So the binary-
     coded representation of decimals works nicely, and the "=" works the
     way it's supposed to.

     Is there some reason this representation is not common practice? 
     Can anybody shed some light on this? I think that later versions of
     Turbo Pascal do not implement this representation, so there must be
     some good reason, right? 

---Travis 

Ich suche die Leidenschaft, die keine Leiden schafft.