Here’s a quick and simple way to generate combinations (Knuth):

t = #balls to pick
n = #balls in total
pos = range(0, t)+[n]+[0]
j = 0

while(j < t):
	j = 0
	while(pos[j]+1 == pos[j+1]):
		pos[j] = j
		j += 1
	pos[j] += 1
	f(pos)

I use this for a decoding algorithm to create Hamming balls. It works like a charm.