Example to check whether an integer is a prime number (Check Prime Number) or not using for loop and if…else statement. If the number is not prime, it’s explained in output why it is not a prime number.

To understand this example, you should have the knowledge of following Python programming topics:

- Python if…else Statement
- Python for Loop
- Python break and continue

A positive integer greater than 1 which has no other factors except 1 and the number itself is called a prime number. 2, 3, 5, 7 etc. are prime numbers as they do not have any other factors. But 6 is not prime (it is composite) since, 2 x 3 = 6.

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#### Python Program to Check Prime Number

# Python program to check if the input number is prime or notnum = 407 # take input from the user # num = int(input("Enter a number: ")) # prime numbers are greater than 1 if num > 1: # check for factors for i in range(2,num): if (num % i) == 0: print(num,"is not a prime number") print(i,"times",num//i,"is",num) break else: print(num,"is a prime number") # if input number is less than # or equal to 1, it is not prime else: print(num,"is not a prime number")

#### Output

407 is not a prime number 11 times 37 is 407

In this program, variable num is checked if it’s prime or not. Numbers less than or equal to 1 are not prime numbers. Hence, we only proceed if the num is greater than 1.

We check if num is exactly divisible by any number from 2 to num – 1. If we find a factor in that range, the number is not prime. Else the number is prime.

We can decrease the range of numbers where we look for factors.

In the above program, our search range is from 2 to num – 1.

We could have used the range, [2, num / 2] or [2, num ** 0.5]. The later range is based on the fact that a composite number must have a factor less than the square root of that number; otherwise, the number is prime.

You can change the value of variable num in the above source code and test for other integers (if you want).

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