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home / developersection / forums / write a function to check if a number is prime. with explanation
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Write a function to check if a number is prime. with explanation

Ravi Vishwakarma 1917 02 Sep 2025

Write a function to check if a number is prime. with explanation

python python-3.4python
Updated 03 Sep 2025
Ravi Vishwakarma
Ravi Vishwakarma

IT-Hardware & Networking

Ravi Vishwakarma is a dedicated Software Developer with a passion for crafting efficient and innovative solutions. With a keen eye for detail and years of experience, he excels in developing robust software systems that meet client needs. His expertise spans across multiple programming languages and technologies, making him a valuable asset in any software development project.

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1 Answers

Anubhav Sharma
Anubhav Sharma 02 Sep 2025
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Prime Number Reminder:

  • A prime number is a number greater than 1 that has no divisors other than 1 and itself.
  • Examples: 2, 3, 5, 7, 11, 13 are prime.
  • 4, 6, 8, 9, 10 are not prime because they can be divided by numbers other than 1 and themselves.

Python Program

def is_prime_number(n):
    # Prime numbers are greater than 1
    if n <= 1:
        return False
    
    # Check divisibility from 2 to sqrt(n) -> (n**0.5)
    for i in range(2, int(n**0.5) + 1):
        if n % i == 0:  # if divisible by i
            return False
    return True


# Test the function
num = int(input("Enter a number: "))
if is_prime_number(num):
    print(num, "is a prime number")
else:
    print(num, "is not a prime number")

Explanation

Base Case:

  • Numbers 0, 1, and negative numbers are not prime.
if n <= 1:
    return False

Check divisibility:

for i in range(2, int(n**0.5) + 1):
  • We only check divisors up to √n instead of n-1.
  • Why? If n has a factor larger than √n, then the corresponding smaller factor would already have been found.
  • This makes it efficient.

Check modulus:

if n % i == 0:
    return False
  • If n divides evenly by i, then it’s not prime.

Return True if no divisors found:

return True

Output :

Enter a number: 29
29 is a prime number
Enter a number: 20
20 is not a prime number
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