Today problem is a probability problem.


This problem was asked by Facebook.

Given a stream of elements too large to store in memory, pick a random element from the stream with uniform probability.


There are many variations of such problems, and before solving it, I want to show some basic examples that I met.

Most trivial one is picking one random element from an array.

Every programming language has a function to generate a pseudo-random number (int or float) within the given range. If you think of an array A as of N numbers, it’s clear how to pick random one: generate number i from 0 to N, and a[i] is the answer.

func oneRandom(a []int) int {
    i := rand.Intn(len(a))
    return aa[i]

Sometimes you need to pick N random elements from an array.

In this case, you can use the same approach and pick N indexes from 0 to len(A).

func nRandom(a []int, n int) []int {
    result := make([]int, n) 
    for i := 0; i < n; i++ {
        result[i] = oneRandom(a)
    return result

However, if you need to pick N random elements, they also have to be different.

In this case, you can pick N different indexes and make sure that they are different:

func nDifferentRandom1(a []int, n int) []int {
    m := map[int]bool{}
    for len(m) != n {
        j := rand.Intn(len(a))
        m[j] = true

    result := make([]int, n)
    for i := range m {
        result[i] = a[i]
    return result

But it’s not very efficient, because you can spend much time trying to pick the last index when you need to pick 8 elements from an array of length 10.

In this case, you can use the well-known zero-allocation algorithm to do so. The idea is to move elements you picked to the beginning of an array, and choose from others next:

func nDifferentRandom2(a []int, n int) []int{
    for i := 0; i < n; i++ {
        chooseFrom := a[i:]                            // define a slice to pick from
        choosenIndex := rand.Intn(len(chooseFrom))     // pick a random index from i to N
        a[i], a[choosenIndex] = a[choosenIndex], a[i]  // swap choosenElement with i
    // after all, we have choosen elements at the 
    // begining of an array, and probability is always same.
    return a[:n] 

Let’s return to problem #15 and try to solve it using previous examples.

What do we have:

  1. a stream of elements too large to store in memory
  2. should pick 1 random element

We understand that to pick a random element from the stream with uniform probability, we need to process hole stream somehow (without storing to memory).

Well, we always can iterate over a stream (but just once).

If we knew the length of a stream, we could use the approach from the first example, that would be perfect.

In first example each element of an array have a linked number - it’s index. Moreover, we used a function that returns number from 0 to n with uniform probability within that range to pick one.

So, in this case, we can assign a random value to each element with the same probability, and choose between them based on it linked number.

Let’s do it: for each element, we generate a float number between [0..1]. Also, we remember maximum value that we generated, an element from the stream associated with it - random element.


func solution(in <-chan int) <-chan int {
	res := make(chan int, 1)
	go func() {
		var result int
		var lastprob float64
		for a := range in {
			prob := rand.Float64()
			if prob > lastprob {
				result = a
				lastprob = prob
		res <- result
	return res