math random number without repeating a previous number

Question

Can't seem to find an answer to this, say I have this:

setInterval(function() {
    m = Math.floor(Math.random()*7);
    $('.foo:nth-of-type('+m+')').fadeIn(300);
}, 300);

How do I make it so that random number doesn't repeat itself. For example if the random number is 2, I don't want 2 to come out again.

Answer 1

There are a number of ways you could achieve this.

Solution A: If the range of numbers isn't large (let's say less than 10), you could just keep track of the numbers you've already generated. Then if you generate a duplicate, discard it and generate another number.

Solution B: Pre-generate the random numbers, store them into an array and then go through the array. You could accomplish this by taking the numbers 1,2,...,n and then shuffle them.

shuffle = function(o) {
    for(var j, x, i = o.length; i; j = parseInt(Math.random() * i), x = o[--i], o[i] = o[j], o[j] = x);
    return o;
};

var randorder = shuffle([0,1,2,3,4,5,6]);
var index = 0;

setInterval(function() {
    $('.foo:nth-of-type('+(randorder[index++])+')').fadeIn(300);
}, 300);

Solution C: Keep track of the numbers available in an array. Randomly pick a number. Remove number from said array.

var randnums = [0,1,2,3,4,5,6];

setInterval(function() {
    var m = Math.floor(Math.random()*randnums.length);
    $('.foo:nth-of-type('+(randnums[m])+')').fadeIn(300);
    randnums = randnums.splice(m,1);
}, 300);