# Quickly Generating Primes Below n With the Sieve of Eratosthenes

The uses for prime numbers in computer science are nearly endless. They are useful for everything from hashing, cryptology, factorization and all sorts of applications in-between.

There exists a great number of algorithms that allow us to quickly generate primes, but today we are going to take a look at a popular method known as a prime sieve. There are a number of different implementations of prime sieves, but one of the simplest to implement is known as the Sieve of Eratosthenes. This algorithm is great for quickly generating smaller prime numbers (but it may not be the best choice for generating very large primes).

# How it Works

In general, the Sieve of Eratosthenes works by generating a list of number from 2 to n. The algorithm will then work through the list, marking all the composite numbers. Here is a more detailed breakdown of the implementation:

1. Create a list of integers from 2 to n. We start at 2 because it’s the smallest prime
2. Set p=2
3. Iterate over the multiples of p by counting to n from 2p in increments of p. These are the numbers that get marked as composites in the list.
4. Find the first number greater than p in the list that is not marked. If one does not exist, we are done. If one does exist, however, set p to this new value and repeat from step 3.

This method has a complexity of $O\left(N \cdot log\left(log\left(N\right)\right)\right)$.

# Implementation in Python

Let’s take a look at how we can implement a Sieve of Eratosthenes in Python:

def get_primes():
D = {}
p = 2

while 1:
if p not in D:
yield p
D[p*p] = [q]
else:
for q in D[p]:
D.setdefault[q+p, []).append(q)
del D[p]
p += 1


For this implementation I have modified things a bit to yield an infinite prime generator.

Let’s take a moment to consider an example of how this could be used in a practical scenario. Let’s take, for example, problem 10 from Project Euler, which asks that we find the sum of all the primes below 2 million. Using our gen_primes() method we can easily solve this with the following:

primes = gen_primes()
print(sum(itertools.takewhile(lambda x: x < 2000000, primes)))


## Another Practical Example

Before I wrap up this post, let’s consider just one more practical use for our gen_primes() method. Assume that we needed to find out what the nth prime is. For the purpose of example, let’s just say we want to find the 500th prime number. It turns out this can be done easily with the following:

primes = gen_primes()
print(next(itertools.islice(primes, 500, None), None))


Running this will reveal that the 500th prime number is 3,581.

# Wrap Up

As I hope you can see, the Sieve of Eratosthenes is a simple way to generate prime numbers that can prove useful in a number of situations. I hope you’ve found this helpful!