Prime Number Checker Using Python

ECX 30 Days of Code and Design

Day 9

Is Prime?

Task

Write a function that takes an integer as input and determines whether it is a prime number or not. A prime number is a positive integer divisible only by one and itself. Nonetheless, 1 is not a prime number.

My Approach

This is the code below. I would explain the code in bits throughout this article.

def prime_number(int_input):
    """Checks if a user's input is either a prime or composite number."""
    prime = True

    # Negative numbers are not prime numbers
    if int_input < 0:
        print('Negative numbers are not prime numbers.')
        prime = False

    # 0 and 1 are not prime numbers
    if int_input == 0 or int_input == 1:
        print(str(int_input) + ' is a composite (not a prime) number.')
        prime = False

    # Divide the number inputted by the user from two to the int_input - 1
    for i in range(int_input):

        # Prevent ZeroDivisionError and division by 1
        if i == 0 or i == 1:
            continue

        # Check if the number is a composite (not a prime) number
        if int_input % i == 0:
            print(str(int_input) + ' is a composite (not a prime) number.')
            prime = False
            break

    # Print if the number is prime
    if prime is True:
        print(str(int_input) + ' is a prime number.')


# User's input
try:
    print(' Prime Number Checker '.center(30, '*'))
    print('Check if a number is a prime or composite number.')
    user_input = int(input('Enter number: '))

    # Function call
    prime_number(user_input)
except ValueError:
    print('Invalid input! Input only integers')

First, we create the function, prime_number(), which is to take an integer parameter. We then create a variable, prime, which we equate to True. If the number the user inputs is a prime number, prime stays True, else it is made False.

def prime_number(int_input):
    """Checks if a user's input is either a prime or composite number."""
    prime = True

Next, we check if the user's input is a negative number (negative numbers are not prime numbers), zero, or one.

    if int_input < 0:
        print('Negative numbers are not prime numbers.')
        prime = False

    if int_input == 0 or int_input == 1:
        print(str(int_input) + ' is a composite (not a prime) number.')
        prime = False

Next, we divide the user's input by numbers from 2 to (user input - 1). We won't divide the user's input by 0 or 1. If we get a quotient without a remainder, it is a composite (not a prime) number, thus prime is made False, and the loop breaks.

    for i in range(int_input):

        if i == 0 or i == 1:
            continue

        if int_input % i == 0:
            print(str(int_input) + ' is a composite (not a prime) number.')
            prime = False
            break

If the number is not composite, it is a prime number and it is printed out.

    if prime is True:
        print(str(int_input) + ' is a prime number.')

Finally, we prompt the user to input the number they want to check if it is a prime number or not and we call the prime_number() function with the user's input as its argument. A try except block is used to catch value error and prevent crash of the program.

try:
    print(' Prime Number Checker '.center(30, '*'))
    print('Check if a number is a prime or composite number.')
    user_input = int(input('Enter number: '))

    # Function call
    prime_number(user_input)
except ValueError:
    print('Invalid input! Input only integers')

Output

Negative Number

**** Prime Number Checker ****
Check if a number is a prime or composite number.
Enter number: -20
Negative numbers are not prime numbers.

Composite Number

**** Prime Number Checker ****
Check if a number is a prime or composite number.
Enter number: 125
125 is a composite (not a prime) number.

Prime Number

**** Prime Number Checker ****
Check if a number is a prime or composite number.
Enter number: 11
11 is a prime number.

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