Question

I’m new to C++, and I think there’s some concept I’m missing because all I’m trying to do is set a value in a 2d array and it’s messing up. Instead of just setting it once in the location I specify, it sets it in multiple locations. The code makes perfect sense in other languages, so it must be some quirk of C++ that I don’t know?

#include <iostream>
#include <string>

int main()
{
const int width=26;
const int height=10;
char arr[width][height];
//fill with dots
std::string empty=".";
for (int i=0;i<height;i++)
{
    for (int j=0;j<width;j++)
    {
        arr[i][j]=empty[0];
    }
}
std::string msg="test";
//set location row=5, col=3 to "t"
arr[5][3]=msg[0];

//print the array
for (int i=0;i<height;i++)
{
    for (int j=0;j<width;j++)
    {
        std::cout<<arr[i][j];
    }
    std::cout<<std::endl;
}
}

The result is:

..........................
..........................
..........................
.......................t..
............t.............
...t......................
..........................
..........................
..........................
..........................

There should only be 1 t in the middle. I don’t understand why it’s putting the other 2 t’s.

Answer 1

By the shape of the output, you’re obviously using height of 10 and width of 26.

You are referencing the array as [i][j] where i is vertical and j is horizontal, but in that case the array is defined with 26 rows and 10 columns. So you are overflowing it when you print out. The array is just a contiguous block of memory, so this isn’t overflowing that block. Otherwise you’d have a crash. But it is incorrect.

Your rows are currently 10 chars long but you are accessing as if they are 26. This means you take values from the next 2 rows. The extra ‘t’ values in the output are the result of overflowing in the rows before them.

Hope this makes sense.

Either change the accesses to arr[j][i], or the definition to char arr[10][26].