Question

I need to produce a random number ranging between 2 and 5 which is different each time. For example, I don’t want two 3’s in a row, or two 5’s in a row. I then have to feed this into a for() loop, like this:

for(var i = 0; i < noBoxes.length; i+=randNumber) {
    //....
}

How do I go about doing this?

Answer 1

Generate a random number up to n-1 and add it on, modulo the original range (shifting because the min is not 0):

i = some random int from 2 to 5
delta = randInt(3) // range of possible values from 2 to 5 is 4, minus 1 to
                   // prevent getting all the way round to i again
nextval = (i-2+delta)%4+2  // shift i down by the minimum, add the
                           // delta and modulo the range

This works because it adds up to 1 below the range, so it can never get back to the original number. For example, i=3, random int 0 to 2, so the max is (i-2+2)%3+2=3%3+2=0+2=2.

function differentRandInt(min,max,current) {
    var shiftedcurrent = current-min;
    var range = max-min+1;
    var delta = Math.floor((Math.random()*(range-1))+1);
    return (shiftedcurrent + delta)%range + min;
}

So if i=3, then i-min is 1, the range after adding the delta is 2,3,4, modulo 4 yielding 2,3,0, so adding the min gives us 4,5,2.