Strange output when comparing same float values?

Question

I read the above links on floating points, but even getting strange output.

#include<stdio.h>
int main()
{
    float x = 0.5;

    if (x == 0.5)
        printf("IF");

    else if (x == 0.5f)
        printf("ELSE IF");

    else
        printf("ELSE");
}

Now, according to the promotion rules, Shouldn't "ELSE IF" must be printed ?

But, here it is printing "IF"

EDIT : Is it because 0.5 = 0.1 in binary and everything is 0 after that and loss of precision hence no effects, so comparison IF returns true.

Had it been 0.1, 0.2, 0.3, 0.4, 0.6, 0.7.... , then Else If block returns true.

Pardon me asking same question, because I have read from the above links that floats comparison must never be done.

But, What is the reason of this unexpected behaviour ?

The usual arithmetic conversion rules states that if one of the operands is a double then the other operand is converted to a double. So the condition x == 0.5 is actually (double) x == 0.5 which of course is true. However, due to the problems with floating point values on computers (see e.g. Is floating point math broken? for details), there are very few values for which this is true. — Some programmer dude
I still think it's a little unclear exactly what you're asking, but if I understand it correctly (and go with the answer provided by Gerhardh) then yes you're right. — Some programmer dude
@Someprogrammerdude Yes, That's exactly what I infer. Thanks !! — user7425291
"They are never accurate." This is common misconception. Floats are as accurate as integers for values they were designed to be representing. It's just that programmers often have wrong expectations and various type conversion rules of C are not helping. — user694733

Gerhardh Gerhardh · Accepted Answer · 2017-01-17T07:35:52

As you have read the links regarding problems with floating point types and comparisons, you are probably expecting that 0.5 is rounded during conversion and hence the comparison should fail. But 0.5 is a power of 2 and can be represented perfectly without any rounding in a float or double type variable. Therefore the comparison results in TRUE.

After your edited your question: Yes, if you took 0.1 or one of the other values you mention, you should run into the else part.

Strange output when comparing same float values?

3 Answers