# Sieve of Eratosthenes

In this notebook, I implement a few versions of the sieve of Eratosthenes for finding prime numbers.

Starting from $n=2$ and given the set $P$ of prime numbers up until $n-1$, we check if $n$ is divisible by any $p \in P$. If not, $P \leftarrow P + {n}$.

from math import sqrt

from math import sqrt, pi
import numpy as np

cached_primes = [2]
evaluated = 2


The first version of the sieve works just like explained at the top.

def eratosthenes(n):
primes = []
for i in range(2, n):
if all([i % p != 0 for p in primes]):
primes.append(i)

return primes


The second version starts with 2 already in the set of primes and only checks the primality of odd numbers. It should be twice as fast.This second version starts with 2 already in the list of primes and only checks odd numbers. It should be twice as fast.

def eratosthenes_no_evens(n):
primes = [2]
for i in range(3, n, 2):
if all([i % p != 0 for p in primes]):
primes.append(i)

return primes


The last version uses all the gimmicks from the previous ones, but also caches the primes found in previous runs and reuses them in subsequent calls of the function.

def eratosthenes_cached(n):
global evaluated, cached_primes

if evaluated < n:
start = evaluated if evaluated % 2 != 0 else evaluated + 1
for i in range(start, n, 2):
if all([i % p != 0 for p in cached_primes]):
cached_primes.append(i)

evaluated = n

for i in range(len(cached_primes)):
if cached_primes[i] >= n:
return cached_primes[:i]
return cached_primes


At this point, I suspected that calls to functions such as all and any were not optimal. They could implement some kind of early stopping, but I wanted to try a different aspproach in case they didn’t. These versions use a for/else loop to add primes to the set, instead of a call to the all() function.

def eratosthenes_no_any(n):
primes = []
for i in range(2, n):
for p in primes:
if i % p == 0:
break
else:
primes.append(i)

return primes

def eratosthenes_no_evens_no_any(n):
primes = [2]
for i in range(3, n, 2):
for p in primes:
if i % p == 0:
break
else:
primes.append(i)

return primes

def eratosthenes_cached_no_any(n):
global evaluated, cached_primes

if evaluated < n:
start = evaluated if evaluated % 2 != 0 else evaluated + 1
for i in range(start, n, 2):
for p in cached_primes:
if i % p == 0:
break
else:
cached_primes.append(i)

evaluated = n

for i in range(len(cached_primes)):
if cached_primes[i] >= n:
return cached_primes[:i]
return cached_primes


Check if the output of all functions is equal.

a = eratosthenes(1000)
b = eratosthenes_no_evens(1000)
c = eratosthenes_cached(1000)
d = eratosthenes_no_any(1000)
e = eratosthenes_no_evens_no_any(1000)
f = eratosthenes_cached_no_any(1000)

a == b == c == d == e == f

True


Here we can see that result 2 is roughly twice as fast as result 1. Result 3 is the fastest, since the primes up until 1000 have been cached in the previous cell. After replacing the call to all() (which is not as smart as I thought it was) with the for-break loop, results 4 and 5 are virtually equal in terms of speed. Result 6 was not changed, since its results were already cached.

%timeit eratosthenes(1000)
%timeit eratosthenes_no_evens(1000)
%timeit eratosthenes_cached(1000)
%timeit eratosthenes_no_any(1000)
%timeit eratosthenes_no_evens_no_any(1000)
%timeit eratosthenes_cached_no_any(1000)

6.92 ms ± 41.1 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
3.6 ms ± 43.1 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
12 µs ± 738 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
907 µs ± 24.8 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
920 µs ± 63.7 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
12.2 µs ± 225 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)


Here, I tried to evaluate if caching was indeed efficient. For some reason, it seems it isn’t.

%timeit -r 3 -n 10 eratosthenes(10); eratosthenes(100); eratosthenes(1000); eratosthenes(10000)
%timeit -r 3 -n 10 eratosthenes_no_evens(10); eratosthenes_no_evens(100); eratosthenes_no_evens(1000); eratosthenes_no_evens(10000)
%timeit -r 3 -n 10 eratosthenes_cached(10); eratosthenes_cached(100); eratosthenes_cached(1000); eratosthenes_cached(10000)
%timeit -r 3 -n 10 eratosthenes_no_any(10); eratosthenes_no_any(100); eratosthenes_no_any(1000); eratosthenes_no_any(10000)
%timeit -r 3 -n 10 eratosthenes_no_evens_no_any(10); eratosthenes_no_evens_no_any(100); eratosthenes_no_evens_no_any(1000); eratosthenes_no_evens_no_any(10000)
%timeit -r 3 -n 10 eratosthenes_cached_no_any(10); eratosthenes_cached_no_any(100); eratosthenes_cached_no_any(1000); eratosthenes_cached_no_any(10000)

540 ms ± 10.8 ms per loop (mean ± std. dev. of 3 runs, 10 loops each)
274 ms ± 5.47 ms per loop (mean ± std. dev. of 3 runs, 10 loops each)
500 ms ± 24.2 ms per loop (mean ± std. dev. of 3 runs, 10 loops each)
52.4 ms ± 121 µs per loop (mean ± std. dev. of 3 runs, 10 loops each)
51.9 ms ± 1.1 ms per loop (mean ± std. dev. of 3 runs, 10 loops each)
49.9 ms ± 631 µs per loop (mean ± std. dev. of 3 runs, 10 loops each)

Written on May 21, 2020