Checking if a Number is Prime in Python

In the world of mathematics, a prime number is a special type of number. A prime number is greater than 1 and can only be divided by 1 and itself without leaving a remainder. This means it has no other divisors. For example, the numbers 2, 3, 5, and 7 are prime, while 4, 6, and 8 are not because they can be divided by numbers other than 1 and themselves.

Why Check for Prime Numbers?

Prime numbers are important in various fields, including cryptography, computer science, and number theory. Knowing whether a number is prime can help us understand its properties and how it relates to other numbers.

Writing the Python Program

Now, let’s write a simple Python program to check if a number is prime. We’ll break it down into easy-to-understand steps.

Step 1: Get User Input

First, we need to ask the user to enter a number. We will store this number in a variable.

number = int(input("Enter a number: "))

Step 2: Check if the Number is Less Than 2

Since prime numbers are greater than 1, we need to check if the number is less than 2. If it is, we can immediately say it is not a prime number.

if number < 2:
    print(f"{number} is not a prime number.")

Step 3: Loop Through Possible Divisors

If the number is 2 or greater, we can start checking for factors. We will loop from 2 to the square root of the number. We only need to check up to the square root because if a number has a factor larger than its square root, the corresponding factor must be smaller.

Here’s how to implement that:

import math

is_prime = True  # Assume the number is prime initially

for i in range(2, int(math.sqrt(number)) + 1):
    if number % i == 0:  # Check if 'i' divides 'number' without a remainder
        is_prime = False  # Set is_prime to False if a factor is found
        break  # No need to check further if we found a divisor

Step 4: Print the Result

Finally, we need to tell the user whether the number is prime or not based on the checks we performed.

if is_prime:
    print(f"{number} is a prime number.")
else:
    print(f"{number} is not a prime number.")

Complete Code

Putting it all together, here’s the complete code for the program:

import math

# Step 1: Get user input
number = int(input("Enter a number: "))

# Step 2: Check if the number is less than 2
if number < 2:
    print(f"{number} is not a prime number.")
else:
    is_prime = True  # Assume the number is prime initially

    # Step 3: Loop through possible divisors
    for i in range(2, int(math.sqrt(number)) + 1):
        if number % i == 0:  # Check for a divisor
            is_prime = False  # Found a divisor, not prime
            break  # Exit the loop

    # Step 4: Print the result
    if is_prime:
        print(f"{number} is a prime number.")
    else:
        print(f"{number} is not a prime number.")

Conclusion

This simple Python program allows users to check if a number is prime. By following these steps, you can easily understand how to determine the primality of a number using basic programming concepts. Whether you’re new to coding or looking to brush up on your skills, this example is a great start!