Picking 2 random elements from array

Question

What is the most efficient way to select 2 unique random elements from an array (ie, make sure the same element is not selected twice).

I have so far:

Answer 1

If you shuffle the array and splice the number of elements you want to return, the return value will contain as many items as it can, if you ask for more items than are in the array. You can shuffle the actual array or a copy, with slice().

Array.prototype.getRandom= function(num, cut){
    var A= cut? this:this.slice(0);
    A.sort(function(){
        return .5-Math.random();
    });
    return A.splice(0, num);
}
var a1= [1, 2, 3, 4, 5];
a1.getRandom(2)
>>[4, 2]

If you want to remove the selected items from the original array, so that a second call will not include the elements the first call returned, pass a second argument: getRandom(3,true);

window.Memry=window.Memry || {};
Memry.a1= [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

Memry.a1.getRandom(3,true);
>>[5,10,7]
Memry.a1.getRandom(3,true);
>>[3,9,6]
Memry.a1.getRandom(3,true);
>>[8,4,1]
Memry.a1.getRandom(3,true);
>>[2]