Question

I’m having trouble with arrays. I copied this code from a book:

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

int main (void)
{
    int     p, i, primes[50], primeIndex = 2;
    bool    isPrime;

    primes[0] = 2;
    primes[1] = 3;

    for (p = 5; p <= 50; p = p + 2) {
        isPrime = true;

        for (i = 1; isPrime && p / primes[i] >= primes[i]; ++i)
            if (p % primes[i] == 0)
                isPrime = false;

        if (isPrime == true) {
            primes[primeIndex] = p;
            ++primeIndex;
        }
    }

    for (i = 0; i < primeIndex; ++i)
        printf ("%i ", primes[i]);

    printf ("\n");

    return 0;
}

In particular, I’m having trouble understanding the difference between the primeIndex and the i variables. The primeIndex refers to the array number and i refers to the number placed into the array. Right?

Answer 1

primeIndex is the place where the next found prime is written in the prime array, and also the number of primes known so far. i is the index of the prime used for trial division. For each candidate, i loops from 1 (we don’t need to try out primes[0] = 2 because only odd numbers are checked) to the index of the first prime larger than the square root of the candidate.