Untitled

#include <stdio.h>

int main(int argc, char *argv[])
{
    // I'm not sure wether this array is actualy initalized if you
    // write it like that
    int primes[50];
    int num = 0;

    // Declare iteration variables like `i` inside the for-clause
    for(int i = 1; i < 100; i++) {
        // Move the definition of prime in here if that's the only place you'll
        // ever use it
        //
        // I used prime instead of not_prime, so the if-statement after the loop
        // is more readable
        int prime = 1;

        // Arrays in C start at index 0
        for(int x = 0; x < num; x++) {
            if(i % primes[x] == 0) {
                prime = 0;
                // You don't need to test with the rest of the primes
                break;
            }
        }

        // A non-zero integer is treated as true in C
        if(prime) {
            primes[num] = i;
            num++;
        }

    }

    // Arrays in C start at index 0
    for(int i = 0; i < num; i++) {
        printf("%d ",primes[i]);
    }

    return 0;
}