C语言判断素数代码

#include<stdio.h>
long long a[9999999] = {0};
void prime(int n)
{
    for(int i = 2;i*i <= n;i++)
        if(n % i == 0)
            return;
    a[n] = 1;
    return;
}
int main(int argc,char **argv)
{
    a[2] = 1;
    for(int i=3;i<=9999999;i++)
        prime(i);
    for(int j = 2;j<=9999999;j++)
    {
        printf("%d:",j);
        if(a[j] == 1)
            printf("是\n");
        else
            printf("否\n");
    }
    return 0;
}