Checking if a Number is Prime or Not with Python

In this blog, we will use the Python programming language to write a simple application that checks if the number is prime or not.

Prime numbers are special numbers, greater than 1, that have exactly two factors, themselves and 1.

19 is a prime number. It can only be divided by 1 and 19.

9 is not a prime number. It can be divided by 3 as well as 1 and 9.

num = int(input('Enter the number: ')) 

Above code is the way to get input from the user, making it int data type and assigning it to a variable called num.

# If given number is greater than 1
if num > 1:
 
    # Iterate from 2 to n / 2
    for i in range(2, int(num/2)+1):
 
        # If num is divisible by any number between
        # 2 and n / 2, it is not prime
        if (num % i) == 0:
            print(num, "is not a prime number")
            break
    else:
        print(num, "is a prime number")
 
else:
    print(num, "is not a prime number")

Since the integer 1 is neither prime nor composite we first check the num to be greater than 1. If the num is greater than 1 then we write a for loop.

The loop starts from 2 and goes till num/2. Because the largest proper divisor of a number N can never be greater than N/2. Recall that range method does not include the second argument.

The next condition checks if the number is divisible by i, if yes, it prints out the message that the number is not prime, else, it says the number is prime. And the last else condition is a way of considering the numbers that are not greater than 1 (not prime).

Let us run the code and see an example:

Enter the number:11

When we enter 11 as an input, the output says:

11 is a prime number

Because 11 is only divisible by itself and 1, the output is correct.

Let us see another one:

Enter the number: 86

When we enter 86 as an input,

86 is not a prime number

is the result. Because 86 can be divided by 1, 2, 43, 86, it is a composite number not a prime number.

You can find the GitHub repo for the jupyter notebook and python code at this link