Unexpected output - prime numbers

Question

I wrote an answer to a program that gets a number from the user and display whether the number is prime.This is a beginner exercise.

Here is the question:

A prime number is a number that is only evenly divisible by itself and 1. For example, the number 5 is prime because it can only be evenly divided by 1 and 5. The number 6, however, is not prime because it can be divided evenly by 1, 2, 3 and 6. Write a Boolean function named is_prime which takes an integer as an argument and returns true if the argument is a prime number, or false otherwise. Use the function in a program that prompts the user to enter a number then displays a message indicating whether the number is prime.

Program I wrote :

def main():
    # Get a number
    number = get_number()

    # Check the number is a prime
    status = is_prime(number)

    # Display a message if the number is prime or not
    display_prime(status)


def get_number():
    number = int(input('Enter a number: '))
    return number


def is_prime(number):
    if (number % 1 == 0) and (number % number == 0):
        status = True
    else:
        status = False
    return status


def display_prime(status):
    if is_prime(status):
        print('The number is a prime')
    else:
        print('The number is not a prime.')


main()

Here is the output:

Enter a number: 6
The number is a prime

I can't find where I went wrong. Can you help me ?
Thank you in advance.

Any number is this world is divisible by 1 and by itself. A prime number is a number which can never be divided by any other number other than itself or 1. `6` is divisible by `6,1,2,3`, so it is not. — Kris, Dec 24 '21 at 05:19
is_prime will return True for every number. Do you see what is missing there? — buran, Dec 24 '21 at 05:20
And a google search will give you enough examples to understand, how you should do it. — Kris, Dec 24 '21 at 05:21
Does this answer your question? [checking prime number in python](https://stackoverflow.com/questions/56558234/checking-prime-number-in-python) — Kris, Dec 24 '21 at 05:23
You might also want to check what you're calling `display_prime` with. — Mark Ransom, Dec 24 '21 at 05:37

score 2 · Answer 1 · answered Dec 24 '21 at 05:23

Where you went wrong is you ignored the word "only" in the definition of a prime number:

A prime number is a number that is only evenly divisible by itself and 1

Every integer, prime or otherwise, is evenly divisible by itself and 1. The difference is that a prime number is only divisible by itself and 1. Because your function is simply checking that the input is divisible by itself and by 1, every input results in returning true.

What you want to do instead is write a function that determines that the number is not evenly divisible by any other number other than itself and 1.

There are some good examples of how to do that here: https://www.programiz.com/python-programming/examples/prime-number

Also, note that by definition a prime number has exactly two factors, itself and 1. Therefore 1 is not a prime number, because it has exactly 1 factor, that is, 1. So whatever your solution is, make sure that an input of 1 returns false. — James Dunn, Dec 24 '21 at 05:31

Sash Sinha · Answer 2 · 2021-12-24T05:49:00.050

There is no need to call is_prime again in display_prime:

def get_int_input(prompt: str = 'Enter an integer number: ') -> int:
    number = -1
    while True:
        try:
            number = int(input(prompt))
            break
        except ValueError:
            print('Error: Enter an integer, try again...')
    return number

def is_prime(number: int) -> bool:
    """Checks if a number is a prime by division"""
    if number < 2:
        return False
    if number == 2:
        return True
    for x in range(2, int(number**0.5) + 1):
        if number % x == 0:
            return False
    return True

def display_prime(prime_status: bool) -> None:
    """Displays a message if the number is prime or not"""
    if prime_status:
        print('The number is a prime.')
    else:
        print('The number is not a prime.')

def main() -> None:
    number = get_int_input()
    status = is_prime(number)
    display_prime(status)

if __name__ == '__main__':
    main()

Example Usage 1:

Enter an integer number: 6
The number is not a prime.

Example Usage 2:

Enter an integer number: a
Error: Enter an integer, try again...
Enter an integer number: 2860486313
The number is a prime.

I'm sure your code works well. At the moment I can't understand it because I'm a newbie. Thank you for your comment. I will check later. — Jims-Rc, Dec 24 '21 at 05:38
Ok, I understand I just added type information so the types of the parameters in your functions are clear. — Sash Sinha, Dec 24 '21 at 05:40

Unexpected output - prime numbers

2 Answers2