Simple prime number generator in Python

Question

Could someone please tell me what I\'m doing wrong with this code? It is just printing \'count\' anyway. I just want a very simple prime generator (nothing fancy).

Answer 1

Here is what I have:

def is_prime(num):
    if num < 2:         return False
    elif num < 4:       return True
    elif not num % 2:   return False
    elif num < 9:       return True
    elif not num % 3:   return False
    else:
        for n in range(5, int(math.sqrt(num) + 1), 6):
            if not num % n:
                return False
            elif not num % (n + 2):
                return False

    return True

It's pretty fast for large numbers, as it only checks against already prime numbers for divisors of a number.

Now if you want to generate a list of primes, you can do:

# primes up to 'max'
def primes_max(max):
    yield 2
    for n in range(3, max, 2):
        if is_prime(n):
            yield n

# the first 'count' primes
def primes_count(count):
    counter = 0
    num = 3

    yield 2

    while counter < count:
        if is_prime(num):
            yield num
            counter += 1
        num += 2

using generators here might be desired for efficiency.

And just for reference, instead of saying:

one = 1
while one == 1:
    # do stuff

you can simply say:

while 1:
    #do stuff