Is floating-point == ever OK?

Question

Just today I came across third-party software we're using and in their sample code there was something along these lines:

// Defined in somewhere.h static const double BAR = 3.14;  // Code elsewhere.cpp void foo(double d) {     if (d == BAR)         ... } 

I'm aware of the problem with floating-points and their representation, but it made me wonder if there are cases where float == float would be fine? I'm not asking for when it could work, but when it makes sense and works.

Also, what about a call like foo(BAR)? Will this always compare equal as they both use the same static const BAR?

Answer 1

Yes, you are guaranteed that whole numbers, including 0.0, compare with ==

Of course you have to be a little careful with how you got the whole number in the first place, assignment is safe but the result of any calculation is suspect

ps there are a set of real numbers that do have a perfect reproduction as a float (think of 1/2, 1/4 1/8 etc) but you probably don't know in advance that you have one of these.

Just to clarify. It is guaranteed by IEEE 754 that float representions of integers (whole numbers) within range, are exact.

float a=1.0; float b=1.0; a==b  // true 

But you have to be careful how you get the whole numbers

float a=1.0/3.0; a*3.0 == 1.0  // not true !!