Example: Optimized Prime Number Finder in C

// Optimized Prime Number Finder
#include <stdio.h>
#include <stdbool.h> // For boolean type
#include <math.h>    // For sqrt() function

int main() {
    int lower_bound, upper_bound;

    // Step 1: Get user input for the range
    printf("Enter the lower bound of the range: ");
    scanf("%d", &lower_bound);
    printf("Enter the upper bound of the range: ");
    scanf("%d", &upper_bound);

    printf("Prime numbers between %d and %d are: ", lower_bound, upper_bound);

    // Step 2: Iterate through each number in the range
    for (int num = lower_bound; num <= upper_bound; num++) {
        // Step 3: Handle special cases: numbers less than 2 are not prime
        if (num <= 1) {
            continue;
        }
        // Handle 2 separately, as it's the only even prime
        if (num == 2) {
            printf("%d ", num);
            continue;
        }
        // Skip even numbers greater than 2 immediately
        if (num % 2 == 0) {
            continue;
        }

        // Step 4: Check for divisibility from 3 up to sqrt(num), only odd divisors
        bool is_prime = true;
        // Optimization: check divisors only up to sqrt(num)
        int limit = (int)sqrt(num); 
        for (int i = 3; i <= limit; i += 2) { // Check only odd divisors
            if (num % i == 0) {
                is_prime = false;
                break;
            }
        }

        // Step 5: If no divisors were found, print the number
        if (is_prime) {
            printf("%d ", num);
        }
    }
    printf("\n");

    return 0;
}