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Date: Thu, 29 Jul 2010 17:08:24 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Float conversion
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Date: 2010-07-29T17:08:24+02:00
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On 29.07.10 14:46, Dmitry A. Kazakov wrote:
> On Thu, 29 Jul 2010 05:23:42 -0700 (PDT), Henrique wrote:
> 
>> I have some problems in converting float variables in Ada. Look at the
>> code below.
>>
>> Why var4 is not equal var1?
> 
> Because they are not. Floating-point operations are inexact.
> 

Interestingly, a quick rewrite to C translated with gcc gives a different
impression (of equality),

#include <stdio.h>

int main()
{
  typedef float My_Float;


#define CONVERSION_CONSTANT 6076.11
  const My_Float CONVERSION_CONSTANT2 = 1.0 / CONVERSION_CONSTANT;


    My_Float Var1 = 999.9;
    My_Float Var2 = Var1*CONVERSION_CONSTANT;
    My_Float Var3 = Var2/CONVERSION_CONSTANT;
    My_Float Var4 = Var2*CONVERSION_CONSTANT2;

    fputs("var3: ", stdout); printf("%.15f", Var3);
    if (Var3 = Var1)
      fputs("=", stdout);
    else if (Var3 > Var1)
      fputs(">", stdout);
    else
      fputs("<", stdout);

    fputs("\n", stdout);
    fputs("var4: ", stdout); printf("%.15f", Var4);
    if (Var4 = Var1)
      fputs("=", stdout);
    else if (Var4 > Var1)
      fputs(">", stdout);
    else
      fputs("<", stdout);


    fputs("\n", stdout);
    return 0;
}


$ ./a.out
var3: 999.900024414062500=
var4: 999.900085449218750=