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From: phys169@csc.canterbury.ac.nz
Newsgroups: comp.lang.pascal
Subject: Re: Is this a bug in TurboPascal? (answer: no)
Message-ID: <1991May1.191049.557@csc.canterbury.ac.nz>
Date: 1 May 91 07:10:49 GMT
References: <1991May1.021059.2129@ux1.cso.uiuc.edu>
Organization: University of Canterbury, Christchurch, New Zealand
Lines: 37

In article <1991May1.021059.2129@ux1.cso.uiuc.edu>, husak@ux1.cso.uiuc.edu (BugsBunny) writes:
> I'm running the following in TP and get the wrong answer.. Please correct
> me if I'm wrong...
> 
> program Is_this_wrong; 
> var
>     xx,yy,c1,c2,c3,c4: integer;
> begin
>     xx := 639; yy := 479; 
>     c1 := Trunc(50*xx/639); c2 := Trunc(50*yy/479);
>     c3 := Trunc(300*xx/639); c4 := Trunc(250*yy/479);
>     writeln(xx,yy,c1,c2,c3,c4);
> end.
> 
This is a common problem: you are multiplying two integers to get a temporary
integer result. For example, when calculating 300*xx/639 it works out 300*xx
to 16 bits accuracy (same accuracy as the 300 and xx), which is not the 191700
you would expect. Either you should change the declaration of xx & yy to be
longint (or reals, if you like), or change the code to..
  c1 := Trunc(50.0*xx/639.0); c2 := Trunc(50.0*yy/479.0);
  c3 := Trunc(300.0*yy/639);  c4 := Trunc(250.0*yy/479.0);

The reason this works is that 300.0*yy is a real result, which goes on to the
division by 639 (or 639.0, either will do, but 639.0 is preferrable).

Note that 300*479/639 gives the expected result with TP 5, since 300*479 is
evaluated at compile time to give a longint result, so it knows to use 32 bits
of accuracy. Another interesting point is that 120*b (where b is a byte
variable set to 100) gives the correct answer, when you might have thought it
would be clipped to 8 bits. Presumably 100 is an integer constant, not a byte
constant.

BTW, it is a good idea to make sure range checking is on; it catches some
cases like this (but not this particular one, I'm afraid). 

Hope this helps,
Mark Aitchison, Physics, University of Canterbury, New Zealand.