Python Eratosthenes Sieve Algorithm Optimization

Question

I\'m attempting to implement the Sieve of Eratosthenes. The output seems to be correct (minus \"2\" that needs to be added) but if the input to the function is larger than 1

Answer 1

This code takes 2 seconds to generate primes less than 10M (it is not mine, i found it somewer on google)

def erat_sieve(bound):
    if bound < 2:
        return []
    max_ndx = (bound - 1) // 2
    sieve = [True] * (max_ndx + 1)
    #loop up to square root
    for ndx in range(int(bound ** 0.5) // 2):
        # check for prime
        if sieve[ndx]:
            # unmark all odd multiples of the prime
            num = ndx * 2 + 3
            sieve[ndx+num:max_ndx:num] = [False] * ((max_ndx-ndx-num-1)//num + 1)
    # translate into numbers
    return [2] + [ndx * 2 + 3 for ndx in range(max_ndx) if sieve[ndx]]