C prime numbers program problem

[Uni Git]

Hi there, I have a problem with a seemingly simple C program that is meant to produce prime numbers up to a desired amount.

However, the program I've made doesn't produce them past 3 - as in I run the program and set the number to 10 and only the numbers 2 and 3 are displayed.

It is meant to use the Sieve of Eratosthenes to calculate prime numbers up to (but not including) N.

This is the code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int N, i, j;
    int *a;
    printf("How much? "); scanf("%i", &N);
    a = (int*) malloc(N * sizeof(int));
    for(i=2;i<N;i++) {
        a[i] = 1;
    }
    
    for(i=2;i<N;i++)
        if(a[i] == 1)
          for(j=i+i;j<N;j++)
              a[j] = 0;
      
    for(i=2;i<N;i++)
        if(a[i] == 1)
            printf("%i ", i);
    
    printf("\n");
}

I would appreciate any help on this. Thanks.

[MakeR]

You need to increment j by i so that j is always divisible by i.

for (j=i+i; j<N; j+=i)

[ChristopherB]

Adding some comments and/or descriptive variable names would be helpful for future problems.