sieve.c

#include <stdio.h>
#include <stdbool.h>

void sieve(int n, bool primes[]);

int main() {
    int n;
    printf("Enter the maximum number ");
    scanf("%d", &n);
    printf("Prime numbers are: ");
    bool primes[n+1];
    // initalize truth table of primes, index 0, 1 is not used
    for (int i = 2; i <= n; i++)
        primes[i] = true;
    sieve(n, primes);
    for (int i = 2; i <= n; i++)
        if (primes[i])
            printf("%d ", i);
    printf("\n");
    return 0;
}

void sieve(int n, bool primes[]) {
    for (int i = 2; i <= n; i++) {
        if (primes[i])
            for (int j=2*i; j <= n; j+=i)
                primes[j] = false;
    }
}