[C++]即使自第一次声明以来从未分配过变量，该值也已更改

Question

Envs

g++ (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0

Background

I practice implementing DFS.
I tried to solve this problem .

Code

sorry for dirty code

#include<iostream>
using namespace std;

int dfs(int params[4], int i, int arr[3], int sum) {
    cout << "i: " << i << endl;
    cout << "params[0]: " << params[0] << endl << endl;
    if (i == 4) {
        if (sum == 7) {
            cout << params[0];
            for (int x = 1; x < 4; x++) {
                if (arr[x]) {
                    cout << '+';
                } else {
                    cout << '-';
                }
                cout << params[x];
            }
            cout << "=7" << endl;
            exit(0);
        }
        return 0;
    }
    arr[i] = 1;
    dfs(params, i + 1, arr, sum + params[i]);
    arr[i] = 0;
    dfs(params, i + 1, arr, sum - params[i]);
}

int main() {
    string num; cin >> num;
    int A = num[0] - '0';
    int B = num[1] - '0';
    int C = num[2] - '0';
    int D = num[3] - '0';
    int params[4] = {A, B, C, D};
    int arr[3];
    dfs(params, 1, arr, A);
}

What happened

The params[0] is declared as the first number of input.
And I never changed the value.
But when I get the value, sometimes it changes.

Input and Output

input

1222

output(This is no problem)

$ ./c
1222
i: 1
params[0]: 1

i: 2
params[0]: 1

i: 3
params[0]: 1

i: 4
params[0]: 1

1+2+2+2=7

input2

0290

output2

$ ./c
0290
i: 1
params[0]: 0

i: 2
params[0]: 0

i: 3
params[0]: 0

i: 4
params[0]: 1

i: 4
params[0]: 0

i: 3
params[0]: 0

i: 4
params[0]: 1

i: 4
params[0]: 0

i: 2
params[0]: 0

i: 3
params[0]: 0

i: 4
params[0]: 1

1-2+9+0=7

input3

3242

output3

3242
i: 1
params[0]: 3

i: 2
params[0]: 3

i: 3
params[0]: 3

i: 4
params[0]: 1

i: 4
params[0]: 0

0+2+4-2=7

The value of params[0] should always stay.
Even if I change the type int to const int , the outputs didn't change.

I want to know

Why this happened?

Answer 1

The array arr has only 3 elements, so arr[3] is out-of-range and no read and write to there are allowed. It seems params is happened to be placed just after arr and writing to arr[3] is breaking data in params .

To easily fix this, allocate one more element for arr .

Better fix is to fix range of i not to cause this out-of-range access.