How to incrementally sample without replacement?

Question

Python has my_sample = random.sample(range(100), 10) to randomly sample without replacement from [0, 100).

Suppose I have sampled n

Answer 1

This is my version of the Knuth shuffle, that was first posted by Tim Peters, prettified by Eric and then nicely space-optimized by necromancer.

This is based on Eric’s version, since I indeed found his code very pretty :).

import random
def shuffle_gen(n):
    # this is used like a range(n) list, but we don’t store
    # those entries where state[i] = i.
    state = dict()
    for remaining in xrange(n, 0, -1):
        i = random.randrange(remaining)
        yield state.get(i,i)
        state[i] = state.get(remaining - 1,remaining - 1)
        # Cleanup – we don’t need this information anymore
        state.pop(remaining - 1, None)

usage:

out = []
gen = shuffle_gen(100)
for n in range(100):
    out.append(gen.next())
print out, len(set(out))