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From: terra@diku.dk (Morten Welinder)
Newsgroups: comp.lang.pascal
Subject: Re: Is this not a bug?
Message-ID: <1991Feb22.103032.489@odin.diku.dk>
Date: 22 Feb 91 10:30:32 GMT
References: <1991Feb20.170152.5401@ux1.cso.uiuc.edu>
Sender: news@odin.diku.dk (Netnews System)
Organization: Department of Computer Science, U of Copenhagen
Lines: 43

mead@uxh.cso.uiuc.edu (Alan David Mead) writes:

>I finally tracked down a problem with an application writen in
>TP5.5:  Since my values are all positive, I used words instead of
>integers.  The ABS() function, however, does not operate as (I)
>expected with word parameters.  This code demonstrates.  While I
>can imagine how the value is produced by ABS(), I think the manual's
>description is deceptive.  What does the net think?
>============================================================
>program damnit;

>const
>  Start : integer = 900;

>var
>  w1,w2 : word;
>  i1, i2 : integer;

>  function Value:word;
>    begin
>      Start := Start - 200;
>      Value := Start;
>    end;

>begin
>  w1 := Value;
>  w2 := Value;
>  writeln( 'ABS( ',w2,' - ',w1,' ) =? ',abs( w2 - w1 ));

>  i1 := Value;
>  i2 := Value;
>  writeln( 'ABS( ',i2,' - ',i1,' ) =? ',abs( i2 - i1 ));
>end.
>--
>Alan Mead : mead@uxh.cso.uiuc.edu

(w1-w2) has type 'word' and so assumed positive; you force an overflow
since w2<w1, but the result is positive.

You have to convert either w1 or w2 to type 'integer' or 'longint' in
order to allow negative results: integer(w1)-integer(w2)

Morten Welinder