Python program to check Prime number

In this tutorial we will write a Python program to check Prime number . Before reading further we recommend to read about below tutorial-

• If-Else Statement in Python
• Python For Loop
• Break statement in Python

What is Prime number

A positive integer number which is greater than 1 and only divisible by 1 and that number is called Prime number .

Example :

• 2 is a prime number as 2 is only divisible by 1 and 3 .
• 3 is a prime number as 3 is only divisible by 1 and 3.
• 4 is not a prime number because it is not only divisible by 1 and 4 , it is also divisible by 2 .

Python program to check Prime number

Steps :

• First we will define a function named is_prime() to check if a number is prime number or not . This function will take a number as function argument .
• Then we will define a flag variable as True . It means if the number is not divisible by any other number except 1 and that number then it is a prime number .
• After that we will check if the given number is less than 2 or not . If the given number is less than 2 then we simply return None and exit from the function .
• Then we will check if the given number is divisible by any other number except 1 and that number starting from 2 and ending at square root of the given number using for loop . To determine square root of the given number we need to import math module and use math.sqrt() function .
• If the given number is divisible by any other number except 1 and that number then we simply change the value of the flag variable as False and exit from the loop using break statement .
• After that we will return the flag variable .
• If the function return None then the given number is less than 2 , if the function return True then the given number is a prime number and if the function return False then the given number is not a prime number

import math

# defining a function to check 
# if a given number is prime or not
def is_prime(number):
    # declaring a flag variable 
    prime=True
    # if the given number is less than 2 
    # then the function will return None
    if number<2:
        return None
    else:
        # trying to divide the number by the numbers 
        # starting from 2
        for i in range(2,int(math.sqrt(number)+1)):
            if number%i==0:
                # if the number is divisable by any number
                # staring from 2 then change the flag variable
                # and exit immediately from the loop
                prime=False
                break
    return prime


# calling the function
print(is_prime(1))  
print(is_prime(7))
print(is_prime(9))  
print(is_prime(34))  
print(is_prime(113))  

None
True
False
False
True

• In the above code first we have defined the is_prime() function which will check if the given number is prime or not .
• As 1 is less than 2 so is_prime(1) has returned None .
• As 7 is only divisible by 1 and 7 so it is a prime number . For this reason is_prime(7) has returned True .
• As 9 is not only divisible by 1 and 9 but also divisible by 3 so it is not a prime number . For this reason is_prime(9) has returned False .
• As 34 is not only divisible by 1 and 34 but also divisible by 2 and 17 so it is not a prime number . For this reason is_prime(34) has returned False .
• As 113 is only divisible by 1 and 113 so it is a prime number . For this reason is_prime(113) has returned True .

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