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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
Rodrigo de Azevedo
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    Your program is actually testing for even numbers since it calls `break` after the first `if` check. You can see this by testing 9 - which it declares as prime since it's odd. – davejagoda Oct 24 '16 at 03:45

5 Answers5

6

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.

Amadan
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  • Great point. Else can also be used in lots of other situations in python such as while loops and exception handling. You can read more at http://python-notes.curiousefficiency.org/en/latest/python_concepts/break_else.html. – Adam Van Prooyen Oct 24 '16 at 03:55
  • However sir, why does my code work for other integers? –  Oct 24 '16 at 04:03
  • As hinted to by others, your code only ever tests `nnumber % 2`: if it's even, "Not a Prime Number!", else if it's odd, "Prime Number!". Thus, it will be correct for all primes and even numbers; but all odd non-primes will give you the wrong answer. – Amadan Oct 24 '16 at 04:07
  • why doesn't the same happen when else is out of the for loop? –  Oct 24 '16 at 04:37
  • Because now we're using `for...else`, and not `if...else`, a completely different thing (even though the `else` has the same name, the meaning is completely different, and I explained what happens at the end of my answer). To be a prime, it must be indivisible every time; if it is divisible we can stop, but if it is indivisible we must continue the loop, and not `break`. – Amadan Oct 24 '16 at 04:41
1

If a number is prime, that means no number evenly divides it besides 1 and itself. That mean you need to check every number below it before being able to say the number is prime.

In your code, you are exiting the loop on the first iteration -- regardless of how large the number is.

Adam Van Prooyen
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0

The judgment has a problem, can only be 1 or the prime number is divisible by itself. You judge is only 2 out of the whole is not a prime number, you should use a double loop to do the conditions of judgment

YYYHL
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you are checking for the 1st iteration only. Irrespective of whether it is prime or not it exits from the loop since one of the section i.e either 'if' or 'else' will execute causing the loop to break. The logic would be to check for the entire range of numbers from 2 to (number/2) and if it divides the number at some point it will not be a prime number. If the loop exits after iterating over the entire loop then it is a prime number. Hope you will be able to do it now. Thanks!.

iams0nus
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Your existing code only every tests if the number that you entered is divisible by 2 and then breaks out of the loop, whichever way. You have to check all values lower and only exit if it does find that it's NOT a prime:

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

prime = True

for x in prime_range:
    if nnumber % x == 0:
        prime = False
        break

if prime:
    print 'Prime Number!'
else:
    print 'Not a Prime Number!'
RFV
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