Farhan T. Sami Farhan T. Sami - 1 month ago 7
Python Question

Program that checks if a number is prime number

Hello I have created this program to check if a number is a prime number. It works but for some reason says that 999 is a prime number. Where is my mistake. It would be great if someone explained. Thank You!

Here is my program:

number = raw_input('Enter a Number: ')
nnumber = int(number)
prime_range = range(2, nnumber)

for x in prime_range:

if nnumber % x == 0:
print 'Not a Prime Number!'
break

else:
print 'Prime Number!'
break

Answer

Trace it. x starts with 2, then tests 999 % 2; it is 1, so else is executed, "Prime number!" is printed, and loop is broken out of. Program ends.

Instead, you need to print "Prime number!" only when you tested all possibilities for x. The easiest way to do that is to unindent else: (and delete break there):

for x in prime_range:

    if nnumber % x == 0:
        print 'Not a Prime Number!'
        break

else:
    print 'Prime Number!'

Python executes else of a for when for completes withoout being broken: exactly what you want.

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