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?
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.
Edit: more complete solution, and fixed off-by-one error:
var prev, randNumber; for(var i = 0; i < noBoxes.length; i+=randNumber) { do { randNumber = 2 + Math.floor(Math.random() * 4); } while(randNumber === prev); prev = randNumber; } 
produce random number in the range of 1..n-1; if random number == previous number, then random number = n; (assuming we count from 1...)Like so: http://jsbin.com/eyitav/
var loopingValues = [2, 3, 4, 5], len, value, rand; while ( (len=loopingValues.length) ) { rand = ~~( ( Math.random() % 1 ) * len); value = loopingValues.splice(rand, 1)[0]; //alert( value ) } Now you have your loop with value as the value 2, 3, 4 and 5 in random order.
Or easier to read: http://jsbin.com/ejizuw/
var loopingValues = [2, 3, 4, 5], value, rand; while ( loopingValues.length ) { rand = Math.random(); if ( rand === 1 ) { rand = 0; } rand = rand * loopingValues.length; rand = Math.floor(rand); value = loopingValues[rand]; loopingValues.splice(rand, 1); alert(value); } You can get your series in an array before you start looping-
var noBoxes={length:100}; var R= [2], r= 2, n= 2; while(R.length<noBoxes.length){ n= R[R.length-1]; while(r== n) r= 2+Math.floor(Math.random()*4) R.push(r); } And then loop through the array- for(var i = 0; i < noBoxes.length; i+=R[i]) { .... } 