Is - (- x) = x correct in floating point arithmetic?

Question

Suppose x is a floating point number, I want to know whether - ( - x) equals to x in general?

Let us ignore corner cases such as x = max_floating_point_number / min_floating_number.

Answer 1

In general, yes; negation of a floating-point value induces no error or loss of precision. (It just involves flipping a bit.)

The one exception to this is NaN. -(-(NaN)) is NaN, but NaN does not compare equal to any value, even another NaN. That's firmly in the realm of "corner cases", though.