Question

A specific example

I need to generate a random number between 0 and 2, inclusive. (or choose randomly between -1, 0, and 1).

The naive approach would be to do something like rand() mod 3 where rand() returns an integer. This approach will not generate statistically random numbers unless the upper bound of rand() is not relatively prime (and the lower bound is 0).

For instance, assuming rand() returned 2 bits (from 0 to 3, inclusive), the modulus would map:

0 -> 0
1 -> 1
2 -> 2
3 -> 0

This skew toward 0 would obviously be much less if more bits would be returned, but regardless, the skew would remain.

The generic question

Is there a way of generating an evenly distributed random number between 0 and n-1, inclusive, where n is relatively prime to 2?

Answer 1

A common approach is to discard random values above the last full cycle, and just ask for a new random number.