Strange C++ 2D array initialization

Asked Oct 11 '19 at 19:37

Active Oct 11 '19 at 19:52

Viewed 46 times

EDIT

Thanks to @crashmstr, he showed the error. I forgot to add const to ROW and COL, VLAs were the reason for the 'errors'.

This is literally all the code:

#include <iostream>

int main()
{
    int ROW = 3;
    int COL = 4;

    int arr[ROW][COL] = { { 5 } };

    for(int row = 0; row < ROW; row++)
    {
        for(int col = 0; col < COL; col++)
        {
            std::cout << arr[row][col] << '\t';
        }
        std::cout << '\n';
    }

    return 0;   
}

This is the output:

5   4199120 4201168 0   
0   0   0   0   
0   0   0   0

And this was my expected output:

5   0   0   0   
0   0   0   0   
0   0   0   0

My Question: Why is this happening? When you initialize a 1D array int arr[3] { 5 }; the first element is set to 5 and everything else to 0, but when I do the 'same' with the 2D array I get this

edited Oct 11 '19 at 19:52

asked Oct 11 '19 at 19:37

Marat Isaw

5

`int arr[ROW][COL]` is not standard C++ unless `ROW` and `COL` are constant expressions. – crashmstr Oct 11 '19 at 19:43
My understanding is that only the first element is initialized because only one value was supplied; all other elements are not initialized. – Thomas Matthews Oct 11 '19 at 19:45
@ThomasMatthews that's the same situation with `int arr[3] { 5 }`. why all are zero in this case? – mangusta Oct 11 '19 at 19:46
@crashmstr I... I just have no words for myself. Thank you, I totally overlooked that! – Marat Isaw Oct 11 '19 at 19:48
@mangusta In your example you have a legal array. In the OP's example they have a VLA which is not legal and how it is initialized is implementation defined. – NathanOliver Oct 11 '19 at 19:48
@MaratIsaw Make sure to turn on your compiler warnings! – 0x5453 Oct 11 '19 at 19:52
@0x5453 Yeah, now I know why warnings are so important – Marat Isaw Oct 11 '19 at 19:53
In light of your edit, see https://stackoverflow.com/q/57367473/10957435 – Oct 11 '19 at 19:57

Strange C++ 2D array initialization

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