Question

Got this question from algorithms design manual by Steven Skiena.

It is required to select k (value given) numbers to form a subset S’ from a given set S having n numbers, such that selection probability for each number is equal (k/n).
n is unknown (i was thinking of taking S as a link-list for this).
also, we can have only pass through the set S.

Answer 1

Something like this

for elem in S
  if random() < (k - S'.size)/S.size // This is float division
    S'.add(elem)

The first element is chosen with probability k/n, the second one with (n-k)/n * k/(n-1) + k/n * (k-1)/(n-1) which reduces to k/n, etc.