Curiously, why does my abs-function return -0?

Question

The following abs-function sometimes returns -0 (minus zero)

inline float abs(float a){
    return( a>=0.0f? a :-a);
}

To be more specific, the statement sprintf(str, "%.2f", abs(-0.00f) ); produces "-0.00", and that is annoying since the string is displayed to the user.

Question:

1) Why does it produce -0?

2) How to fix it?

PS: I am using xcode's (objective) c compiler.

Answer 1

1. Because -0.0 == 0.0 and thus -0.0 >= 0.0 is true.

2. Use fabs (or fabsf for float instead of double) rather than trying to reinvent it.

Answer 2

1. -0 is an artifact of binary representation, 0 with the sign bit set. Wikipedia has a comprehensive article on signed zero if you would like further details.

2. Use fabs() as people have said above. If you really really really want to inline, chain your compares:

   inline float abs(float a) { return (a > 0.f) ? a : ( (a < 0.f) ? -a : 0); }