C Program To Find Prime Numbers Between Given Range

// C Program to Find Prime Numbers in a Given Range using
// Trial Division method
#include <stdbool.h>
#include <stdio.h>

bool isPrime(int n) {

    // Checking primality by finding a complete division
  	// in the range 2 to n-1
    if (n <= 1)
        return false;
    for (int i = 2; i < n; i++) {
        if (n % i == 0)
            return false;
    }
    return true;
}

void findPrimes(int l, int r) {

    // Flag to check if any prime numbers are found
    bool found = false;
    for (int i = l; i <= r; i++) {

        // Checking if the number is prime
        if (isPrime(i)) {
            printf("%d ", i);
            found = true;
        }
    }
    if (!found) {
        printf(
            "No prime numbers found in the given range.");
    }
}

int main() {
    int l = 10, r = 30;
	
  	// Finding and printing the prime between [l, r]
    findPrimes(l, r);
  
    return 0;
}

// C Program to Find Prime Numbers in a Given Range using
// Simple Iterative Approach
#include <math.h>
#include <stdbool.h>
#include <stdio.h>

bool isPrime(int n) {

    // Checking 1 and 2 for primality
    if (n <= 1)
        return false;
    if (n == 2)
        return true;

    // Checking for divisiblity by 2
    if (n % 2 == 0)
        return false;

    // Checking for divisiblity from 3 to sqrt(n)
    for (int i = 3; i * i <= n; i += 1)
        if (n % i == 0)
            return false;
    return true;
}

void findPrimes(int l, int r) {

    // Flag to check if any prime numbers are found
    bool found = false;
    for (int i = l; i <= r; i++) {
        if (isPrime(i)) {
          
            // Print the prime number
            printf("%d ", i);
            found = true;
        }
    }
    if (!found) {
        printf("No prime numbers found in the given "
               "range.");
    }
}

int main() {
    int l = 10, r = 30;

    // Finding and printing prime number between [l, r]
    findPrimes(l, r);

    return 0;
}

// C Program to Find Prime Numbers in a Given Range using
// Sieve of Eratosthenes
#include <math.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

int* sieve(int n, int* size) {

    // Allocate array to mark the multiples
    bool* isPrime = (bool*)malloc((n + 1) * sizeof(bool));
  
  	// Initially, mark all the multiples as true
    for (int i = 0; i <= n; i++) {
        isPrime[i] = true;
    }

    // 0 and 1 are not prime numbers so mark
  	// them seperately
    isPrime[0] = isPrime[1] = false;

    // Sieve of Eratosthenes algorithm
    for (int i = 2; i * i <= n; i++) {
        if (isPrime[i]) {
            for (int j = i * i; j <= n; j += i) {

                // Mark multiples of i as non-prime
                isPrime[j] = false;
            }
        }
    }

    // Count the number of prime numbers
    int count = 0;
    for (int i = 2; i <= n; i++) {
        if (isPrime[i]) {
            count++;
        }
    }

    // Store all the prime numbers in seperate array
    int* primes = (int*)malloc(count * sizeof(int));
    int pi = 0;
    for (int i = 2; i <= n; i++) {
        if (isPrime[i]) {
            primes[pi++] = i;
        }
    }

    // Set the size of the primes array
    *size = count;

    free(isPrime);
    return primes;
}

void findPrimes(int l, int r) {
    int size;

    // Get all primes up to r
    int* primes = sieve(r, &size);

    // If there are no prime numbers in the range
    if (size == 0) {
        printf("There are no prime numbers between the "
               "given range!\n");
        return;
    }

    // Starting from the prime number just after the l
    int start = 0;
    while (primes[start] < l) {
        start++;
    }

    // Printing all prime numbers in the range
    for (int i = start; i < size; i++) {
        printf("%d ", primes[i]);
    }

    free(primes);
}

int main() {
    int l = 10, r = 30;
  	
  	// Finding and printing prime numbers in range [l, r]
    findPrimes(l, r);

    return 0;
}