Checking if a double is equal to -0.00

Question

I am making this big program in C, which is a part of my homework. My problem is that my program is outputing x = -0.00 instead of x = 0.00. I have tried comparing like if(x==-0.00) x=fabs(x) but I've read that it won't work like that with doubles. So my question is are there any other ways to check if double is equal to negative zero?

Answer 1

You can use the standard macro signbit(arg) from math.h. It will return nonzero value if arg is negative and ​0​ otherwise.

From the man page:

signbit() is a generic macro which can work on all real floating- point types. It returns a nonzero value if the value of x has its sign bit set.

This is not the same as x < 0.0, because IEEE 754 floating point allows zero to be signed. The comparison -0.0 < 0.0 is false, but signbit(-0.0) will return a nonzero value.

NaNs and infinities have a sign bit.

Also, from cppreference.com:

This macro detects the sign bit of zeroes, infinities, and NaNs. Along with copysign, this macro is one of the only two portable ways to examine the sign of a NaN.

Answer 2

Very few calculations actually give you a signed negative zero. What you're probably observing is a negative value close to zero that has been truncated by your formatting choice when outputting the value.

Note that -0.0 is defined to be equal to 0.0, so a simple comparison to 0.0 is enough to verify a signed zero.

If you want to convert an exact signed zero -0.0 to 0.0 then add 0.0 to it.