A float set to 0.1 compares false against 0.1

Question

 int main()   
    {
            float f = 0.1;
            if (f == 0.1)
                printf("True");
            else
                printf("False");
        }

I am just a beginner in c. I don't understand the behavior of the above program. The output is false. Why??

Do not ignore compiler's warnings. There's always a reson for them... — Adriano Repetti, Dec 31 '13 at 10:17
@Adriano Nice quote in your profile. But writing a ununderstandable code is a good technique to insure you'll never be fired ;) — Maroun, Dec 31 '13 at 10:18
Probably bad form to post the code as the title to a question. — Clifford, Dec 31 '13 at 10:21
@Adriano What compiler warning would you expect here? This program could have been written by someone who knew what they was doing. How is the compiler supposed to know the difference? — Pascal Cuoq, Dec 31 '13 at 10:54
When this question comes up, I traditionally mention this blog post that I wrote: http://blog.frama-c.com/index.php?post/2011/11/08/Floating-point-quiz — Pascal Cuoq, Dec 31 '13 at 10:55
@PascalCuoq if VC he'lll get C4244 by default (...possible loss of data). If GCC well I **Always** keep -Wconversion... — Adriano Repetti, Dec 31 '13 at 11:40

Bathsheba · Accepted Answer · 2013-12-31T10:40:55.780

7

0.1 is a double literal.

As 0.1 cannot be represented exactly in floating point, a float set to 0.1 will compare false against a double set to 0.1.

Your comparison would behave as intended if you use a float literal: 0.1f (note the suffixed f: not to be confused with your variable name):

float foo /*renamed for clarity*/= 0.1;
if (foo == 0.1f){
     /*this will compare true*/

edited Dec 31 '13 at 10:40

answered Dec 31 '13 at 10:15

Bathsheba

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might want to add that, if `float` is the desired type, then `float foo = 0.1f;` and `if (foo == 0.1f) puts("True");` would probably be what most people write – Elias Van Ootegem Dec 31 '13 at 10:33
@EliasVanOotegem: good point: I've edited. Perhaps a full answer would mention promotion of the floating point type to a double type, but I think that's outside the question scope. – Bathsheba Dec 31 '13 at 10:42

A float set to 0.1 compares false against 0.1

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