Python – Check if a number is prime

In this post, we will see how we can check if a number is prime or not

Logic

A number is prime, if it has NO divisors other than 1 or itself [in other words, it should not be divisible by any number in between 1 and itself]

e.g

5 => is a prime number [5 is not divisible by any number in between 1 and 5, i.e not divisible by 2/3/4]

9 => is NOT a prime number [9 is divisible by 3]

Implementation

num = 9

# num = int(input("Enter a number: "))

if num > 1:
    for i in range(2, num // 2):
        if (num % i) == 0:
            print(num, "is not a prime number")
            print(num, "is divisible by", i)
            break
        else:
            print(num, "is a prime number")

num // 2 => divide with 2 (discard remainder)

Used num // 2 => we are checking divisibility half way of the number [no need to check full range]

If interested the implementation in java, refer here