script_25.py - Prime Number Checker (Simple)

Code

#!/usr/bin/env python3
"""Prime number checker (simple)"""

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

if num < 2:
    print(f"{num} is not prime")
elif num == 2:
    print(f"{num} is prime")
elif num % 2 == 0:
    print(f"{num} is not prime (divisible by 2)")
else:
    is_prime = True
    for i in range(3, int(num**0.5) + 1, 2):
        if num % i == 0:
            is_prime = False
            break
    if is_prime:
        print(f"{num} is prime")
    else:
        print(f"{num} is not prime")

Explanation

Line 4: num = int(input("Enter a number: ")) - int() converts to integer type - num stores integer value

Line 6: if num < 2: - Numbers less than 2 (0, 1, negatives) are not prime by definition - Quick elimination of small numbers

Line 8: elif num == 2: - 2 is the only even prime number - Special case handling

Line 10: elif num % 2 == 0: - % modulo operator checks remainder - All even numbers (except 2) are not prime - Eliminates half of all numbers quickly

Line 13: is_prime = True - Boolean variable initialized to True - Assumption: number is prime until proven otherwise - Will be set to False if divisor found

Line 14: for i in range(3, int(num**0.5) + 1, 2): - for loop (not covered in if/ category, but necessary here) - range() generates sequence of numbers - 3 is start value (already checked 2) - num**0.5 is square root of num (float result) - int() converts float to integer - + 1 includes the square root in range - 2 is step (only check odd numbers: 3, 5, 7, 9...) - Optimization: only need to check up to √num

Line 15: if num % i == 0: - Nested if inside for loop - Checks if current number i divides num evenly - If divisible, num is composite (not prime)

Line 16: is_prime = False - Reassigns boolean variable to False - Found a divisor, so number is not prime

Line 17: break - Exits the for loop immediately - No need to check further if divisor already found - Optimization keyword

Line 18: if is_prime: - After loop, check final boolean value - True means no divisors were found