Which is the fastest way to get the absolute value of a number

Question

Which is the fastest way to implement an operation that returns the absolute value of a number?

x=root(x²)

or

if !isPositiv         


        

Answer 1

Calculating the square root is probably one of the worst things you could do because it is really slow. Usually there is a library function for doing this; something like Math.Abs(). Multiplying with -1 is also unnecessary; just return -x. So a good solution would be the following.

(x >= 0) ? x : -x

The compiler will probably optimize this to a single instructions. Conditions may be quite expensive on modern processors because of the long execution pipelines -the calculations must be thrown away if a branch was misspredicted and the processor started executing the instructions from the wrong code path. But because of the mentioned compiler optimization you need not care in this case.