Question

I know I can pick a random element out of an array with the sample method but this leaves the possibility of an element being picked more than once. I could shuffle the array first and then go from first to last element in order but I understand this is memory intensive and I am looking for a less intensive method if possible!

Answer 1

Shuffling an array is not memory intensive. Ruby has a default in place shuffle implementation, it’s called Array.shuffle!. Looking at the source code for this you can see (it’s C):

rb_ary_shuffle_bang(ary)
    VALUE ary;
{
    long i = RARRAY(ary)->len;

    rb_ary_modify(ary);
    while (i) {
        long j = rb_genrand_real()*i;
        VALUE tmp = RARRAY(ary)->ptr[--i];
        RARRAY(ary)->ptr[i] = RARRAY(ary)->ptr[j];
        RARRAY(ary)->ptr[j] = tmp;
    }
    return ary;
}

This implementation follows the classic Fisher-Yates algorithm.

So:

1. Shuffle the array in place using shuffle!. Time complexity is O(n), no extra memory needed.
2. Iterate over the array. Time complexity is O(n), no extra memory needed (only an integer to hold the current index).

Overall you have what you need with no extra memory and time complexity O(n).