Question

I have a variable that has a number between 1-3.

I need to randomly generate a new number between 1-3 but it must not be the same as the last one.

It happens in a loop hundreds of times.

What is the most efficient way of doing this?

Answer 1

May the powers of modular arithmetic help you!!

This function does what you want using the modulo operator:

/**
 * generate(1) will produce 2 or 3 with probablity .5
 * generate(2) will produce 1 or 3 with probablity .5
 * ... you get the idea.
 */
function generate(nb) {
    rnd = Math.round(Math.random())
    return 1 + (nb + rnd) % 3
}

if you want to avoid a function call, you can inline the code.