Question

I ran into the challenge where I need a function that returns a random number within a given range from 0 - X. Not only that, but I require the number returned to be unique; not duplicating numbers that have already been returned on previous calls to the function.

Optionally, when this is done (e.g. the range has been ‘exhausted’), just return a random number within the range.

How would one go about doing this?

Answer 1

I wrote this function. It keeps its own array with a history of generated numbers, preventing initial duplicates, continuing to output a random number if all numbers in the range have been outputted once:

// Generates a unique number from a range
// keeps track of generated numbers in a history array
// if all numbers in the range have been returned once, keep outputting random numbers within the range
var UniqueRandom = { NumHistory: new Array(), generate: function(maxNum) {
        var current = Math.round(Math.random()*(maxNum-1));
        if (maxNum > 1 && this.NumHistory.length > 0) {
            if (this.NumHistory.length != maxNum) {
                while($.inArray(current, this.NumHistory) != -1) { current = Math.round(Math.random()*(maxNum-1)); }
                this.NumHistory.push(current);
                return current;
            } else {
                //unique numbers done, continue outputting random numbers, or we could reset the history array (NumHistory = [];)
                return current;
            }
        } else {
            //first time only
            this.NumHistory.push(current);
            return current;
        }
    }
};

Here’s a working Fiddle

I hope this is of use to someone!

Edit: as pointed out by Pointy below, it might get slow with a large range (here is a
fiddle, going over a range from 0-1000, which seems to run fine). However; I didn’t require a very large range, so perhaps this function is indeed not suited if you look to generate and keep track of an enormous range.