Comparison issue

[H. S. Teoh]

On Mon, Mar 19, 2012 at 08:50:02AM -0400, bearophile wrote:
> James Miller:
> 
> >         writeln(v1 == 1); //false
> >         writeln(v1 == 1.0); //false
> >         writeln(v1 == 1.0f); //false
> >         writeln(v1+1 == 2.0f); //true

Using == to compare floating point values is wrong. Due to the nature of
floating point computation, there's always a possibility of roundoff
error. Therefore, the correct way to compare floats is:

	immutable real epsilon = 1.0e-12; // adjustable accuracy here
	if (abs(y-x) < epsilon) {
		// approximately equal
	} else {
		// not equal
	}


> Maybe I'd like to deprecate and then statically forbid the use of ==
> among floating point values, and replace it with a library-defined
> function.
[...]

I agree. Using == for any floating point values is pretty much never
right. Either we should change the definition of == for floats to use
abs(y-x)<epsilon for some given epsilon value, or we should prohibit it
altogether, and force people to always write abs(y-x)<epsilon.


T

-- 
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