Learn about modulo

# testing for a prime number - use of modulo (modulus)
# also, a good example (for beginners) of use of a while loop
# journalism students often ask how one would use modulo - this shows them one case

n = 0
print "\nWe will test a number to find out whether it is prime."
print "Try it with 25. Then try it with 7 or 13."
n = raw_input("Enter a whole number: ")

def testfor100(a):
	a = int(a)
	# below is a while loop that quits as soon as 
	# you enter a number less than 100
	while a >= 100:
		print "Please enter a number that is less than 100."
		a = raw_input("> ")
		a = int(a)
	return a

# this runs the function above
# the n in parens below feeds the value of n into the function where it has (a)
n = testfor100(n)

for num in range(2, n):
	# below, % is the modulus sign
	# see http://learnpythonthehardway.org/book/ex3.html 
	remainder = n % num
	# below, % is used with d to represent a variable 
	# called formatters, or format strings: %s %d %r
	# see http://learnpythonthehardway.org/book/ex5.html
	print "%d / %d leaves a remainder of %d" % (n, num, remainder)
	if remainder == 0:
		print "Sorry, %d is not a prime number." % n
		break
	if num == n - 1:
		print "\nHooray! %d is a prime number.\n" % n