#include <bits/stdc++.h>

using namespace std;

/**
 功能：正数+负数取模
 n=3为例
 MOD就是一直0,1,2,0,1,2这样的向下走，不会乱了规矩
 MOD     0    1     2     0     1     2     0    1    2
         @    @     @     @     @     @     @    @    @
 a      -3   -2    -1     0     1     2     3    4    5
 */
int MOD(int a, int b) {
    return (a % b + b) % b; //配合数组下标从0开始合适
}

int main() {
    int n = 5;
    for (int a = -4; a <= 2; a++) {
        cout << "a=" << a << " MOD=" << MOD(a, n) << endl;
    }
    return 0;
}