Example: Find Primes in Range using Optimized Function in C

// Find Primes in Range using Optimized Function
#include <stdio.h>
#include <stdbool.h> // For using bool data type
#include <math.h>    // For using sqrt() function

// Function to check if a number is prime (optimized)
bool isPrime(int num) {
    if (num <= 1) {
        return false; // Numbers less than or equal to 1 are not prime
    }
    if (num <= 3) {
        return true;  // 2 and 3 are prime
    }
    if (num % 2 == 0 || num % 3 == 0) {
        return false; // Multiples of 2 or 3 are not prime
    }

    // Check for divisibility from 5 onwards
    // Optimized: only check up to sqrt(num)
    // and only check numbers of the form 6k ± 1
    for (int i = 5; i * i <= num; i = i + 6) {
        if (num % i == 0 || num % (i + 2) == 0) {
            return false; // If divisible, it's not prime
        }
    }
    return true; // If no divisors found, it's prime
}

int main() {
    // Step 1: Declare variables for the range
    int lowerBound, upperBound;

    // Step 2: Get input for the range from the user
    printf("Enter the lower bound of the range: ");
    scanf("%d", &lowerBound);
    printf("Enter the upper bound of the range: ");
    scanf("%d", &upperBound);

    // Step 3: Ensure valid range (lower <= upper)
    if (lowerBound > upperBound) {
        int temp = lowerBound;
        lowerBound = upperBound;
        upperBound = temp;
    }

    // Step 4: Print a header for the output
    printf("Prime numbers between %d and %d are:\n", lowerBound, upperBound);

    // Step 5: Iterate through the range and check each number for primality
    for (int i = lowerBound; i <= upperBound; i++) {
        if (isPrime(i)) {
            printf("%d ", i);
        }
    }
    printf("\n"); // Newline for better formatting

    return 0;
}