Random selection

Question

Given two integer numbers N and n (N >= n > 0), how do I generate random selection (without repetition!) of [0, N) with length = n? E.g. Given N = 5, n = 3 possible solution

Answer 1

In python, this would be really easy:

selection = random.shuffle(range(N))[:n]

This is O(N) in memory, since the list of valid values is generated first and then shuffled in place, so it fails on your requirement :(

You could try something like this:

N = 5
n = 3
selection = set()
while len(selection) < n:
    selection += pick_random_int(0, N)

Which is essentially what Jon Skeet proposed. This will work well for n << N, but start to fail horribly with n close to N. In that case, though, the O(n) and O(N) memory solutions will converge anyway and your requirement is moot ;)