Float Comparison Returns False

[Loopback]

On 2011-07-07 20:35, Steven Schveighoffer wrote:
> On Thu, 07 Jul 2011 14:19:05 -0400, Loopback <elliott.darfink at 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?
>
> Because they aren't. Just because they are equal to 5 decimal places
> (which by the way is an inaccurate printout of their value), does not
> mean they are fully equal.
>
> Be very careful when comparing floating point numbers. Generally you
> want to use an epsilon to say they are "close enough".
>
> -Steve

Thank you for your answers!

I do want to ask though what an alternative would be in this case, to
compare the two different values. You mentioned something about
"epsilons" but I have no experience within this field. I would really
appreciate an example or something similar so I could understand your
statement.

 From what I can see these are two identical values, I would be more than
glad if someone could explain just what is the difference between these
two "non-equal" values and how make them "equal". Perhaps I should use
ints and long instead since they don't seem to suffer from this
"problem"?