Python prime calculator - Not updating boolean value

Question

from sys import argv

def prime():
  for i in range(2,num):
      if num % i == 0:
          print(f"{num} can be divisible by {i}.")
          checker = False

num = int(input("Enter the number you want to check is prime: "))
checker = True

if num < 2:
    print(f"{num} is a prime number.")
elif num == 0:
    print(f"{num} is not a prime number.")
elif num > 0:
    prime()
    print(f"Is this number a prime - {checker}.")
else:
    print(f"Please write number larger than 0.")

Hello guys, I've just started to code and this is probably a really simple question. I want my code to either find the factors of a number or print if a number is a prime - but my boolean value in the "prime" function never updates to False. Not sure why!

Thank you.

Do you get any errors when you run this code, or does it print out `True`? — castle-bravo, Mar 18 '18 at 03:47
No errors, it just prints out True even if it's not a prime. — tobytobias, Mar 18 '18 at 03:50

score 0 · Accepted Answer · answered Mar 18 '18 at 03:50

The reason why your prime() function is not 'updating' to False is because there is no return statement in the function.

With it, the function looks like this:

def prime():
  checker = True
  for i in range(2,num):
      if num % i == 0:
          print(f"{num} can be divisible by {i}.")
          checker = False
  return checker

And the updated code looks like this:

from sys import argv

def prime():
  checker = True
  for i in range(2,num):
      if num % i == 0:
          print(f"{num} can be divisible by {i}.")
          checker = False
  return checker


num = int(input("Enter the number you want to check is prime: "))

if num < 2:
    print(f"{num} is a prime number.")
elif num == 0:
    print(f"{num} is not a prime number.")
elif num > 0:
    is_prime = prime()
    print(f"Is this number a prime - {is_prime}.")
else:
    print(f"Please write number larger than 0.")

However, this isn't the only way to solve your problem: it's not recommended and frowned upon by many programmers as bad practice - use the global statement (which looks like that's what you were trying to do originally).

With it, the function looks like this:

def prime():
  global checker
  for i in range(2,num):
      if num % i == 0:
          print(f"{num} can be divisible by {i}.")
          checker = False

And the updated code looks like this:

from sys import argv

def prime():
  global checker
  for i in range(2,num):
      if num % i == 0:
          print(f"{num} can be divisible by {i}.")
          checker = False


num = int(input("Enter the number you want to check is prime: "))
checker = True

if num < 2:
    print(f"{num} is a prime number.")
elif num == 0:
    print(f"{num} is not a prime number.")
elif num > 0:
    prime()
    print(f"Is this number a prime - {checker}.")
else:
    print(f"Please write number larger than 0.")

No worries - always happy to help! – lyxαl Mar 18 '18 at 03:59 — lyxαl, Mar 18 '18 at 03:59

score 0 · Answer 2 · answered Mar 18 '18 at 03:52

0

def prime():
       global checker
       """ remaining logic as is"""

I think this would work. Sorry for format as I'm writing on phone.

answered Mar 18 '18 at 03:52

Niranjan Agnihotri

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Python prime calculator - Not updating boolean value

2 Answers2