How is uncommenting a cout line changing the final answer?

Question

I have the following code

#include <bits/stdc++.h>

using namespace std;

int main() {

    int r = 0, c = 0;
    int best = 0;

    cin >> r >> c;
    int myGrid[r + 2][c + 2] = {};

    for (int i = 1; i < r + 1; i++) {
        for (int j = 1; j < c + 1; j++) {
            cin >> myGrid[i][j];
        }
    }

    bool stillIn = false;

    int di[] = {-1,-1, -1, 0, 0, 1, 1, 1};
    int dj[] = {-1,0, 1, -1, 1, -1, 0, 1};

    for (int i = 1; i < r + 1; i++) {
        for (int J = 1; J < c + 1; J++) {
            stillIn = false;
            for (int k = 0; k < 8; k++) {
    //        cout << myGrid[i][J] << " " << endl;
                if (myGrid[i][J] == myGrid[di[k]][dj[k]]) {
                    stillIn = true;
                }
            }
            if (stillIn == true) {
                best = myGrid[i][J];
            }
        }
    }

    cout << best;

    return 0;
}

If I run the code with the following input:

It prints 4. However, if I uncomment line 28, which is

//        cout << myGrid[i][J] << " " << endl;

Then it gives me 1, which is the correct answer. Why is this happening!? How does using cout change the final answer?

Thanks in advance for any help.

This is not allowed in standard C++ `int myGrid[r + 2][c + 2] = {};` since the size of an array must be know at compile time, not run time. Instead you should prefer `std::vector` for this case. — Cory Kramer, Apr 19 '20 at 16:46
A side note: [Don't use `bits/stdc++.h`](https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h), even though lazy teachers may tell you to. — CherryDT, Apr 19 '20 at 16:48
Ok, there were a couple of other mistakes as well, but my program works now. Thanks everyone! — Mike Smith, Apr 19 '20 at 16:58

cigien · Accepted Answer · 2020-04-19T16:51:19.287

3

You have undefined behavior(UB) because you are indexing out of bounds on this line

myGrid[di[k]][dj[k]]

because di and dj contain values of -1.

If you have UB, then anything can happen, such as a cout statement exisiting or not, changing the program in weird ways.

Also variable length arrays are not allowed in standard c++.

edited Apr 19 '20 at 16:51

answered Apr 19 '20 at 16:48

cigien

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Oh! no wonder. I forgot to put the i - and j - before those. However, I still have this question: how is the cout statement changing the final answer? – Mike Smith Apr 19 '20 at 16:51
2

Because it's undefined behavior. Undefined behavior is undefined. It can do literally anything, such as delete your harddrive or launch a nuclear attack. (That's very unlikely though, obviously .) And what exactly it does can be influenced by literally anything as well, such as whether you have a `cout` there or not. It's just that the code that ends up being generated looks a bit differently, probably using different registers or memory layout or doing different optimization, and that influences the result you see. – CherryDT Apr 19 '20 at 16:52
_Theoretically_ the out-of-bound access may turn the program into a copy of skynet, yes. As in, the C++ specs have nothing that says this can _not_ happen. (Obviously I was just trying to make a point. Although, I was actually unlucky once in such a way that I actually _did_ build code that started deleting all files on my harddisk due to undefined behavior! So it's not purely theoretical. Granted, there already existed a function in my code that was supposed to delete a folder and all its contents recursively, although it was not supposed to do that to `C:\ `) – CherryDT Apr 19 '20 at 17:03
What happened was that due to undefined behavior elsewhere, an empty string got passed into that function. And the function would check if the path already ended with `\ `, and if it didn't, add a `\ `. Now we got an empty string plus a backslash, that is _just_ a backslash - which means "root of the current drive", i.e. `C:\ `... Oops! – CherryDT Apr 19 '20 at 17:09
@CherryDT, I'm kidding, I do hope Arnold Schwarzenegger doesn't land in my living room and wrecks my furniture :). – anastaciu Apr 19 '20 at 17:09

How is uncommenting a cout line changing the final answer?

1 Answers1