b53.py

import sys
from bisect import bisect_right
from itertools import chain, compress

print("Input an even number (0 to exit):")
ub = 50000
is_prime = [0, 0, 1, 1] + [0] * (ub - 3)
is_prime[5::6] = is_prime[7::6] = [1] * int(ub / 6)
primes = [2, 3]
append = primes.append

for n in chain(range(5, ub, 6), range(7, ub, 6)):
    if is_prime[n]:
        append(n)
        is_prime[n * 3::n * 2] = [0] * ((ub - n) // (n * 2))
primes.sort()

for n in map(int, sys.stdin):
    if not n:
        break
    if n % 2:
        print("Number of combinations:")
        print(is_prime[n - 2])
    else:
        print("Number of combinations:")
        print(len([1 for p in primes[:bisect_right(primes, n / 2)] if is_prime[n - p]]))