def isPrime(n): """Checks if n is prime""" #Check for obvious True or False before entering loop, for efficiency. if n <= 1 or type(n) != int: ##A prime must be a positive whole number. ##1 is not a prime return False if n in [2, 3, 5, 7, 11, 13, 17, 19]: ##Quick check of the first few primes. return True if not n & 1: ##Check to see if n is even. ##2 is the only even prime. return False sifter = int(n**0.5) ##Only the first sqrt(n), any more is redundant. counter = 3 ##Start the counter at 11, the next known prime. #Enter loop to check all other possibilities. while counter <= sifter: if n % counter == 0: ##Test if ctr is a multiple of n. return False counter += 2 ##Only need to check the odd numbers. return True for i in range(500): if isPrime(i): print (i, 'isprime')
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