How to check dependencies of floats

Question

I want to determine (in c++) if one float number is the multiplicative inverse of another float number. The problem is that i have to use a third variable to do it. For inst

Answer 1

What is striking is that whatever the rounding rule is, you expect the outcome of the two versions to be the same (either twice wrong or twice right)!

Most probably, in the first case a promotion to higher accuracy in the FPU registers takes place when evaluating x==1/y, whereas z= 1/y really stores the single-precision result.

Other contributors have explaine why 5==1/0.2 can fail, I needn't repeat that.