Simple example of using modulo (modulus) to test for a prime number.

modulo.py

# testing for a prime number - use of modulo (modulus)
# also, good example of a for loop

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."
print "Then try it with any number you like (but not too large a number!)."
n = raw_input("Enter a number: ")

n = int(n)

for counter in range(2, n):
	# below, % is the modulus sign
	# see http://learnpythonthehardway.org/book/ex3.html 
	
	remainder = n % counter
	
	# 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, counter, remainder)
	if remainder == 0:
		print "Sorry, %d is not a prime number." % n
		break
	if counter == n - 1:
		print "\nHooray! %d is a prime number.\n" % n

Questions for students:

1. Why do we need line 10?
2. In line 12, why do we start with 2? Why not 0? Why not 1?
3. In line 12, we end with n. In line 26, we have n - 1. Why?