Example to check whether an integer is a 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.
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.
Table of Contents
Example 1: Using a flag variable
# Program to check if a number is prime or not
num = 29
# To take input from the user
#num = int(input("Enter a number: "))
# define a flag variable
flag = False
# prime numbers are greater than 1
if num > 1:
# check for factors
for i in range(2, num):
if (num % i) == 0:
# if factor is found, set flag to True
flag = True
# break out of loop
break
# check if flag is True
if flag:
print(num, "is not a prime number")
else:
print(num, "is a prime number")
In this program, we have checked if num 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, so we set flag to True and break out of the loop.
Outside the loop, we check if flag is True or False.

If it is
True
,
num
is not a prime number. 
If it is
False
,
num
is a prime number.
Note: We can improve our program by decreasing 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, range(2,num//2) or range(2,math.floor(math.sqrt(num)+1)). The latter range is based on the fact that a composite number must have a factor less than or equal to the square root of that number. Otherwise, the number is prime.
You can change the value of variable num in the above source code to check whether a number is prime or not for other integers.
In Python, we can also use the for…else statement to do this task without using an additional flag variable.
Example 2: Using a for…else statement
# Program to check if a number is prime or not
num = 407
# To 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
Here, we have used a for..else statement to check if num is prime.
It works on the logic that the else clause of the for loop runs if and only if we don’t break out the for loop. That condition is met only when no factors are found, which means that the given number is prime.
So, in the else clause, we print that the number is prime.