http://d.puremagic.com/issues/show_bug.cgi?id=1756

           Summary: comparing a constant to artihmetic expression with
                    floating point types fails
           Product: D
           Version: 2.008
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: sven.stoenner@googlemail.com


the direct comparison of a constant to an arithmetic expression does not work correctly when using variables:

// float
float a = 0.6f, 0.8f;
float c = a / b;
assert(c == 0.6f / 0.8f); // ok
assert(c == a / b); // fails
assert(0.75f == 0.6f / 0.8f); // ok
assert(0.75f == a / b); // fails

// double
double a = 0.6, b = 0.8;
double c = a / b;
assert(c == 0.6 / 0.8); // fails
assert(c == a / b); // fails
assert(0.75 == 0.6 / 0.8); // ok
assert(0.75 == a / b); // fails


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d-bugmail@puremagic.com wrote:
> http://d.puremagic.com/issues/show_bug.cgi?id=1756
> 
>            Summary: comparing a constant to artihmetic expression with
>                     floating point types fails
>            Product: D
>            Version: 2.008
>           Platform: PC
>         OS/Version: Windows
>             Status: NEW
>           Severity: normal
>           Priority: P2
>          Component: DMD
>         AssignedTo: bugzilla@digitalmars.com
>         ReportedBy: sven.stoenner@googlemail.com
> 
> 
> the direct comparison of a constant to an arithmetic expression does not work
> correctly when using variables:
> 
> // float
> float a = 0.6f, 0.8f;

Is this exactly what you tried?
It should of course be:
float a = 0.6f, b = 0.8f;

http://d.puremagic.com/issues/show_bug.cgi?id=1756





------- Comment #2 from sven.stoenner@googlemail.com  2007-12-31 07:55 -------
> Is this exactly what you tried?
> It should of course be:
> float a = 0.6f, b = 0.8f;
> 

no, i've tried
float a = 0.6f, b = 0.8f;

this failure is occured while editing this report, sorry.


-- 

<d-bugmail@puremagic.com> wrote in message news:bug-1756-3@http.d.puremagic.com/issues/...
> http://d.puremagic.com/issues/show_bug.cgi?id=1756

> the direct comparison of a constant to an arithmetic expression does not
> work
> correctly when using variables:
>
> // float
> float a = 0.6f, 0.8f;
> float c = a / b;
> assert(c == 0.6f / 0.8f); // ok
> assert(c == a / b); // fails
> assert(0.75f == 0.6f / 0.8f); // ok
> assert(0.75f == a / b); // fails
>
> // double
> double a = 0.6, b = 0.8;
> double c = a / b;
> assert(c == 0.6 / 0.8); // fails
> assert(c == a / b); // fails
> assert(0.75 == 0.6 / 0.8); // ok
> assert(0.75 == a / b); // fails

You forgot one.

// real
real a = 0.6, b = 0.8;
real c = a / b;
assert(c == 0.6 / 0.8); // ok
assert(c == a / b); // ok
assert(0.75 == 0.6 / 0.8); // ok
assert(0.75 == a / b); // ok

They all pass when you use real because the compiler does constant folding by casting all floating point constants to real.  When you then compare it to a float or double, it fails because the constant is more precise than the float or double.

http://d.puremagic.com/issues/show_bug.cgi?id=1756





------- Comment #4 from wbaxter@gmail.com  2007-12-31 13:11 -------
You should almost never rely on == to compare floating point numbers.
The failure you're seeing is a perfect example of why not.
Here's the first thing I could google up about it:
http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm
but if you'll look you should find dozens of references saying the same thing.

That said, I can see why you would expect those particular cases to work, but in general you should never be surprised by equality failing in floating point comparisons.  There are just too many things that can go wrong.


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http://d.puremagic.com/issues/show_bug.cgi?id=1756


bugzilla@digitalmars.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WONTFIX




------- Comment #5 from bugzilla@digitalmars.com  2008-08-28 17:56 -------
Due to roundoff error, expressions that are mathematically the same are not the same with computer floating point.


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