Characters duplicate in a multidimensional array

Question

I have created an empty char multidimensional array, but when I try to change a specific value, it sometimes duplicates to another space in the array.

Example:

#include <iostream>
using namespace std;

char arr[2][2] = { 0 };

int main ()
{
    arr[2][0] = 'A';
    for(int i = 0; i < 3; ++i)
    {
        for(int j = 0; j < 3; ++j)
        {
            cout << "arr[" << i << "][" << j << "] = " << arr[i][j] << endl;
        }
    }
}

Output:

arr[0][0] =
arr[0][1] =
arr[0][2] =
arr[1][0] =
arr[1][1] =
arr[1][2] = A
arr[2][0] = A
arr[2][1] =
arr[2][2] =

The character A should only appear in [2][0] but it also appears in [1][2]. This happens only in these spaces:

[1][0], [2][0], [0][2], [2][2]

I was able to recreate this with a bigger array, but I can't say the specific values.

I have tried defining inside the main() function but it created another problem, random characters started appearing in random locations of the array.

I tried initializing the array with char arr[2][2] = { 0 }, but it didn't help.

Unrelated to your problem, but this is a very well written first question. You've given us a minimal verifiable complete example and what you're expecting vs. your actual output. I look forward to seeing more of your contributions here! — scohe001, Oct 19 '18 at 14:35
the C++ idiom to [initialize](https://stackoverflow.com/a/31114758/1132334) a multi-dimensional, constant-sized array is `char arr[2][2] = { };`. another option is `std::fill` — Cee McSharpface, Oct 19 '18 at 14:43

Kon · Accepted Answer · 2018-10-19T14:47:32.960

6

When you declare char arr[2][2] = { 0 };, that's a 2x2 array. Which means it's indices go from 0 to 1. You're writing into index 2, which is outside of array bounds.

edited Oct 19 '18 at 14:47

answered Oct 19 '18 at 14:34

Kon

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Not could be, it is. – Samer Tufail Oct 19 '18 at 14:36

score 2 · Answer 2 · answered Oct 19 '18 at 14:42

There is probably some memory bashing happening. When you declare an array as char arr[2][2] = { 0 }; array of two arrays of size two of char. Which means 4 char elements.

You can access them as

arr[0][0];
arr[0][1];
arr[1][0];
arr[1][1];

For your code to work you need to buff up the size of the array to char arr[3][3] = { 0 };

To answer your question why are they duplicated. The memory continues when you allocate char arr[2][2] it will allocate space for 4 elements and it might be the same as char arr[4].
When you try to access an element out of the bounds of your array the behavior is undefined at it resulted in accessing the memory of the second array.

score 0 · Answer 3 · answered Oct 19 '18 at 14:34

That's because you're going past your array. An array of char arr[2] only goes from 0 to 1 (that's 2 values). Similarly, for your arr[2][2] you can also only use 0 and 1 as array index.

Try this instead:

char arr[3][3] = { 0 };

Now the array index can go from 0 to 2.

Characters duplicate in a multidimensional array

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