Re: Float Comparison Returns False

[Simen Kjaeraas]

On Thu, 07 Jul 2011 20:19:05 +0200, Loopback <elliott.darf...@gmail.com>  
wrote:

Hello!
I've been programming some miscellaneous code and got stuck in an odd
case. While comparing floats, two obviously identical values return
false in comparison.

I am not sure if this is related to float precision or something
similar. This is the code that I have used:

import std.stdio;

void main(string[] args)
{
        while(foo()) {}
}

bool foo()
{
        static bool ss;
        static int loops;
        static float m = 0f;
        
        if(m != 1.73205f)
        {
                m += 0.00500592f;
                
                if(++loops == 346)
                        ss = true;
        }

        if(ss)
        {
                writefln("Variable: %s", m);
                writefln("Constant: %s", 1.73205f);
                writefln("Equality: %s", m == 1.73205f);
                
                return false;
        }
        
        return true;
}

The output of this program is the following:

Variable: 1.73205
Constant: 1.73205
Equality: false

My question is; how come these values compare unequal?

Try adding this in there:

  writefln("Difference: %s", m - 1.73205);

It prints:
Difference: 1.61095e-06


It may also be worth using %a to see the actual values in a float:

  writefln("Variable: %a", m);
  writefln("Constant: %a", 1.73205f);

Variable: 0x1.bb67bcp+0
Constant: 0x1.bb67ap+0

As you can see, these numbers are different. Floating point math
is weird. Two numbers that look the same can be different.
--
  Simen