比较两个程序: error: ‘i’ undeclared (first use in this function)

#include <stdio.h>
int main()
{
    int s = 0;
    for (int i = 1; i <= 10; i+=3) {
        s += i;
    }
    printf("s = %d\n", s);
    printf("i = %d\n", i);
    return 0;
}

#include <stdio.h>
int main()
{
    int s = 0;
    int i;
    for (i = 1; i <= 10; i+=3) {
        s += i;
    }
    printf("s = %d\n", s);
    printf("s = %d\n", i);
    return 0;
}