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

could you try that,

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

Answer 2

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);

Answer 3

I like Neal's answer although this is begging for some recursion. Here it is in java, you'll still get the general idea. Note that you'll hit an infinite loop if you pull out more numbers than MAX, I could have fixed that but left it as is for clarity.

edit: saw neal added a while loop so that works great.

public class RandCheck {
    private List<Integer> numbers;
    private Random rand;
    private int MAX = 100;

    public RandCheck(){
        numbers = new ArrayList<Integer>();
        rand = new Random();
    }

    public int getRandomNum(){
        return getRandomNumRecursive(getRand());
    }

    private int getRandomNumRecursive(int num){
        if(numbers.contains(num)){
            return getRandomNumRecursive(getRand());
        } else {
            return num;
        }
    }

    private int getRand(){
        return rand.nextInt(MAX);
    }

    public static void main(String[] args){
        RandCheck randCheck = new RandCheck();

        for(int i = 0; i < 100; i++){
            System.out.println(randCheck.getRandomNum());
        }
    }
}