Could someone explain this code and its output?

Question

Here's the code, written in C++:
"#include <iostream>
using namespace std;
int main() {
int a[]={1, 2, 3, 4};
for (int i=0; i<3; i++) {
a[3]-=a[i];
}
cout<<a[2] % a[3]<<"\t";
cout<<a[3];
return 0;
}"
This code outputs:
"1 -2"
Evidently the operation "a[3]-=a[i]" will get you "-2." How is this possible?

Answer 1

1st iteration: a[3] = a[3]-a[0] = 4-1 = 3
2nd iteration: a[3] = a[3]-a[1] = 3-2 = 1
3rd iteration: a[3] = a[3]-a[2] = 1-3 = -2

Answer 2

Wow I feel dull...I was only counting what I saw instead of changing the value of a[3] in my head. Thanks a bunch Shamima Yasmin.