Question

When multiplying a floating point number that is very close to 1 with an int > 0, can it ever be interpreted as 1.

That is, if Math.random() returns its highest possible result (which is 1 step below 1.0), will

(int)(Math.random() * 8)

be 8 or 7?

For a practical example, can this often-used construct give an index out of bounds error:

someArray[(int)(Math.random() * someArray.length)];

I’m specifically interested in answers for Java and ActionScript 3, but I suppose they all use the same rules for floating point arithmetic, and answers for any platform would be useful.

Update: Though I already accepted an answer, I’d still appreciate confirmation that this can’t go wrong in ActionScript 3 either, since a colleague reporting that he saw it go wrong once is what partly prompted me to ask this question.

Answer 1

Because 8 is a power of 2, multiplying a float by it will never add or remove precision from the value unless you overflow. Multiplying by other numbers, and in particular floating point numbers (other than a negative power of 2, e.g. 0.25, 0.0625, etc.), will reduce precision.