Check for Prime Numbers in Python

In this Python lesson, we will practice how to write a program to check if a number is a prime number or not. Examples of prime numbers are, 2, 3, 4, 5, 7, 13, 17, 19 and so on. We will carry out this process using a conditional statement and loop system.

Note: A Prime number is any positive number that is divisible by 1 and itself alone. 1 is neither a prime nor a composite number. A composite number is a positive integer that can be formed by multiplying two smaller positive integers. Equivalently, it is a positive integer that has at least one divisor other than 1 and itself.

Example: Check Prime Number Using a for…else statement

# Request input from user
num = int(input('Enter a number: '))

# prime numbers must be greater than 1
if num > 1:
   # check for factors
   for i in range(2,num):
       if (num % i) == 0:
           print('{0} is not a prime number'.format(num))
           break
   else:
       print('{0} is a prime number'.format(num))
       
# if input number is less than or equal to 1
else:
   print('{0} is not a prime number'.format(num))

Output

Enter a number: 17
17 is a prime number

In the above program, we checked for prime numbers using the for..else statement. We ensured that numbers less than or equal to 1 are not considered prime numbers because prime numbers must be greater than 1 and must not also be a negative numbers.

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