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From: togood@roger.lerc.nasa.gov (Chris Miller)
Newsgroups: comp.lang.pascal
Subject: Re: Is this a bug in TurboPascal
Message-ID: <1991May1.121622.14722@eagle.lerc.nasa.gov>
Date: 1 May 91 12:16:22 GMT
References: <1991May1.021059.2129@ux1.cso.uiuc.edu>
Sender: news@eagle.lerc.nasa.gov
Organization: Nasa Lewis Research Center ( Cleveland )
Lines: 40

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.
>
>OUTPUT:
>639,479,50,50,-7,-23
>
>How does it happen that the last two numbers are wrong and the rest right???
>			Really confused in Illinois,
>				Steve

No, it isn't wrong, you are creating a number that is too large for an integer 
variable (i.e. larger than MaxInt):

program RollOver;
var
  i: integer; l: longint;
begin
  writeln('MaxInt =',MaxInt);
  for i := 50 to 52 do begin
    l := longint(i)*639;
    writeln(i, ' ',l,' ',i*639,' ',Trunc(i*639/639));
  end;
end.

OUTPUT:
MaxInt =32767
50 31950 31950 50
51 32589 32589 51
52 33228 -32308 -50