How are vectors different from arrays when used in recursive calls? - c++

Question

I have 2 very similar programs based on this challenge here. One uses arrays, one uses vectors. But their outputs are different. Even though algorithm is the same. Please explain?
Here is the code.
Array version (output is 2, which is incorrect):
#include <iostream>
#define SIZE 4
int step(int grid[][SIZE], int i, int j)
{
if (i < 0 || j < 0 || i > 3 || j > 3)
return 0;
if (grid[i][j] == 1)
return 0;
if ( i == 3 && j == 3)
return 1;
grid[i][j] = 1;
return step(grid, i - 1, j) + step(grid, i + 1, j) + step(grid, i, j - 1) + step(grid, i, j + 1);
}
int main()
{
int grid[][SIZE] = {{0,0,0,0},{0,0,0,0},{0,0,0,0},{0,0,0,0}};
std::cout<<step(grid, 0, 0)<<std::endl;
return 0;
}
And the vector version (output 184, correct):
#include <iostream>
#include <vector>
#define SIZE 4
int step(std::vector<std::vector<int> > grid, int i, int j)
{
if (i < 0 || j < 0 || i > 3 || j > 3)
return 0;
if (grid[i][j] == 1)
return 0;
if ( i == 3 && j == 3)
return 1;
grid[i][j] = 1;
return step(grid, i - 1, j) + step(grid, i + 1, j) + step(grid, i, j - 1) + step(grid, i, j + 1);
}
int main()
{
std::vector<std::vector<int> > grid;
std::vector<int> column1(4,0);
for (int i = 0; i < SIZE; ++i)
grid.push_back(column1);
std::cout<<step(grid, 0, 0)<<std::endl;
return 0;
}

Answer 1

The first version passes array by reference because it is equivalent to passing pointer to array. The second version passes vector by value which means that each recursive call vector is created on stack and you change only local copy. This change disappears immediately when you return from function.
You should use the following signature to pass vector by reference:
int step(std::vector<std::vector<int> >& grid, int i, int j)