Randomly generate a set of m integers from an array of size n

Question

Full Question is:

Write a method to randomly generate a set of m integers from an array of size n. Each element must have equal probability of being chosen`

This question is picked from "Crack the coding interview" and the solution is this:

We can swap the element with an element at the beginning of the array and then “remember” that the array now only includes elements j and greater. That is, when we pick subset[0] to be array[k], we replace array[k] with the first element in the array. When we pick subset[1], we consider array[0] to be “dead” and we pick a random element y between 1 and array size(). We then set subset[1] equal to array[y], and set array[y] equal to array[1]. Elements 0 and 1 are now “dead”. Subset[2] is now chosen from array[2] through array[array size()], and so on.

My question is that if we're shrinking the array from which we're picking random numbers then probability of each number being picked 1/remaining_num_elements. How does it stay equal for all the elements?

Answer 1

Think about it like you're picking m random numbers from a bag of n numbers, with the first j elements representing the numbers in your hand and the remaining elements representing the numbers still in the bag. (You iterate j from 0 to m - 1 to pull out numbers, as your book suggests. j, then, represents the number of integers that you have already pulled out of the bag.)

If you're picking m integers from a bag in real life, then every time you pick a new number, you take from the bag only and not from your hand. Thus, the remaining_num_elements shrinks at each step.

When you think this way, it should be simple to see that this method guarantees that each element has equal probability of being chosen.