Question

This:

bool grid[1280][1024];
for (int x = 0; x<1280; x++)
{
    for (int y = 0; y<1024; y++)
    {
        grid[x][y] = false;
    }
}

works fine, but

bool grid[1280][1024];
bool grid2[1280][1024];

for (int x = 0; x<1280; x++)
{
    for (int y = 0; y<1024; y++)
    {
        grid[x][y] = false;
        grid2[x][y] = false;
    }
}

gives me a segfault. Why?

Answer 1

Probably not enough stack space, your second example also crashes on my PC. Try allocating on the heap, or even better, use a proper container class:

#include <array>
#include <vector>

typedef std::array<bool, 1280> line;

int main()
{
    std::vector<line> grid(1024);
    std::vector<line> grid2(1024);

    // no initialization to false necessary
}

Note how I switched the width and the height. You probably want your elements aligned this way to ensure fast linear access.