求第n个纯粹合数。求大佬指教我哪里有错

#include <stdio.h>
#include <math.h>
int isnum(int n)
{
    int i;
    if(n==0) return 1;
    if(n==1||n==2||n==3) return 0;
    for(i = 2; i <= sqrt(n); i++)
      if(n%i == 0)
        return 1;
    return 0;
}
int main()
{
    int n, i, j, p, q;
    int a[100]={0};
    while(scanf("%d", &n)==1)
    {
        for(i = 1, j = 100; i <= 100; j++)
        {
            p = j%100;
            q = p%10;
            if(isnum(j)&& isnum(p) && isnum(q))
            {
                a[i] = j;
                i++;
            }
        }
        printf("%d\n", a[n]);
    }
    return 0;
}

[此贴子已经被作者于2016-12-19 15:09编辑过]