Question

I have a strange problem. I have the following piece of code in C++:

int grid[h][w]; int dp[h][w]; int p[h][w];

for(int y = 0; y < h; y++)
    for(int x = 0; x < w; x++)
        cin >> grid[y][x];

// base case
for(int y = 0; y < h; y++) dp[y][0] = grid[y][0];


// fill rest
for(int x = 1; x < w; x++)
{
    for(int y = 0; y < h; y++)
    {
        dp[y][x] = min(dp[y][x-1], min(dp[(y-1)%h][x-1], dp[(y+1)%h][x-1])) + grid[y][x];
    }
}

cout << "dp: " << endl;
for(int y = 0; y < h; y++) cout << dp[y][w-1] << endl;

As you can see, in the last lines I’m printing the last column of the dp array (in which I’m interested). When I add the following statement, just below // base case:

p[0][0] = 3;

My dp array changes and I don’t know why. I’ve only added that statement and I wonder why the dp array is changing and how I can prevent this.

Could someone explain this to me why this happens?

Thanks!

Answer 1

Your code has undefined behaviour. Consider what happens inside the following loop when y = 0:

for(int y = 0; y < h; y++)
{
    dp[y][x] = min(dp[y][x-1], min(dp[(y-1)%h][x-1], dp[(y+1)%h][x-1])) + grid[y][x];
                                   ^^^^^^^^^^^ out of bounds since -1%h equals -1

Did you mean to say (y+h-1)%h instead of (y-1)%h?