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.
# Python program to check if the input number is prime or not # 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")
Enter a number: 407 407 is not a prime number 11 times 37 is 407
Enter a number: 853 853 is a prime number
In this program, user is asked to enter a number and this program check whether that number is 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 square root of that number. Otherwise the number is prime.