Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!zephyr.ens.tek.com!vice!bobb
From: bobb@vice.ICO.TEK.COM (Bob Beauchaine)
Newsgroups: comp.lang.pascal
Subject: Re: Is this a bug in TurboPascal
Message-ID: <7401@vice.ICO.TEK.COM>
Date: 3 May 91 21:16:24 GMT
References: <1991May1.021059.2129@ux1.cso.uiuc.edu> <1991May1.060345.26511@newsserver.sfu.ca> <1991May1.134303.7289@maytag.waterloo.edu>
Reply-To: bobb@vice.ICO.TEK.COM (Bob Beauchaine)
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 40

In article <1991May1.134303.7289@maytag.waterloo.edu> dmurdoch@watstat.waterloo.edu (Duncan Murdoch) writes:
>Expressions like
>
>  real1 * int1 * int2
>and
>  int1 * int2 * real1
>
>are especially dangerous, because you don't know in advance how TP will pair
>the operands, and it makes a difference when overflows can occur.  (In TP
>6.0, with the values above, the results of those two expressions are
>65536.0 and 0.0 respectively, because on Wednesdays it operates from left
>to right.  I haven't tried the program on other days of the week, so I'm
>not sure what happens then.)
>

  The only (?) sure way to get around this is to typecast.  Choosing a 
  temporary result variable can guarantee you a correct value, no
  matter what day of the week it is.  For instance, you can't overflow
  a long integer with the product of two integers, no matter what the
  magnitude of the two integers.  So something like this will work,
  independent of the compiler's ordering of terms and factors, because
  of the precedence of parenthesis:
    
       real1 * longint(int1 * int2)
       or
       longint(int1 * int2) * real1

  Note, however, that you can only have two *distinct* operands inside
  the typecast, or the problem can occur all over again inside the 
  parenthesis.


  I found this out, as usual, by having a program crash from overflow.

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\ 

Bob Beauchaine bobb@vice.ICO.TEK.COM 

C: The language that combines the power of assembly language with the 
   flexibility of assembly language.