generate random numbers of which the sum is constant

Question

I am thinking if there is anyway to generate a set of random numbers of which the sum is always a constant. For example, 20 can be divided into 5 numbers ( 1, 2,3,4,10) I do

Answer 1

This method does the job, and also allows controlling the "difference degree" between the values (e.g. if you want the array values to be close one to another)

/**
     * Create array of positive integers which exactly sums to a given (integer) number.
     * @param {Number} number of items
     * @param {Number} sum  required sum
     * @param {Number} [d=100] difference degree between the values (0..100)
     */
    randomSumArray: function(len, sum, d) {
        var _sum = 0;
        var arr = [];
        var n, i;

        if (!d && d !== 0) {
            d = 100;
        }

        for (i = 0; i < len; i++) {
            var from = (100 - d) * 1000,
                to = (100 + d) * 1000,
                n = Math.floor(Math.random() * (to - from + 1) + from); //random integer between from..to

            _sum += n;
            arr.push(n);
        }

        var x = sum / _sum;

        _sum = 0; //count sum (again)
        for (var i = 0; i < len; i++) {
            arr[i] = Math.round(arr[i] * x);
            _sum += arr[i];
        }

        var diff = sum - _sum;

        // Correct the array if its sum does not match required sum (usually by a small bit)
        if (diff) {
            x = diff / Math.abs(diff); //x will be 1 or -1
            var j = 0;
            while (diff && j < 1000) { //limit to a finite number of 'corrections'
                i = Math.floor(Math.random() * (len + 1)); //random index in the array
                if (arr[i] + x >= 0) {
                    arr[i] += x;
                    diff -= x;
                }
                j++;
            }
        }

        return arr;
    }