Random number generator without dupes in Javascript?

Question

I need help with writing some code that will create a random number from an array of 12 numbers and print it 9 times without dupes. This has been tough for me to accomplish. Any ideas?

Answer 1

If I understand you correctly, you want to shuffle your array.

Loop a couple of times (length of array should do), and in every iteration, get two random array indexes and swap the two elements there. (Update: if you are really serious about this, this may not be the best algorithm).

You can then print the first nine array elements, which will be in random order and not repeat.

Answer 2

This is relatively simple to do, the theory behind it is creating another array which keeps track of which elements of the array you have used.

var tempArray = new Array(12),i,r;
for (i=0;i<9;i++)
    {
    r = Math.floor(Math.random()*12);    // Get a random index
    if (tempArray[r] === undefined)      // If the index hasn't been used yet
        {
        document.write(numberArray[r]);  // Display it
        tempArray[r] = true;             // Flag it as have been used
        }
    else                                 // Otherwise
        {
        i--;                             // Try again
        }
    }

Other methods include shuffling the array, removing used elements from the array, or moving used elements to the end of the array.

Answer 3

var nums = [1,2,3,4,5,6,7,8,9,10,11,12];
var gen_nums = [];

function in_array(array, el) {
   for(var i = 0, j = array.length; i < j; i++) 
       if(array[i] == el) return true;
   return false;
}

function get_rand(array) {
    var rand = array[Math.floor(Math.random()*array.length)];
    if(!in_array(gen_nums, rand)) {
       gen_nums.push(rand); 
       return rand;
    }
    return get_rand(array);
}

for(var i = 0; i < 9; i++) {
    document.write(get_rand(nums));
}

Answer 4

Try this once:

//Here o is the array;
var testArr = [6, 7, 12, 15, 17, 20, 21];
    shuffle = function(o){ //v1.0
                        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;
                };
shuffle(testArr);

Answer 5

The most effective and efficient way to do this is to shuffle your numbers then print the first nine of them. Use a good shuffle algorithm.What Thilo suggested will give you poor results. See here.

Edit Here's a brief Knuth Shuffle algorithm example:

void shuffle(vector<int> nums)
{
  for (int i = nums.size()-1; i >= 0; i--)
  {
    // this line is really shorthand, but gets the point across, I hope.
    swap(nums[i],nums[rand()%i]);
  }
}