Program not identifying correctly 2D array values

Question

Hello everyone!

I am writing a program to play the Game of Fifteen .

So, I am writing the code that moves the tiles within the board. I'll copy it herein and explain it afterwards, step by step.

bool move(int tile)
{
    for (int i=0; i<d; i++)
    {
        for (int j=0; j<d; j++)
        {
            if (tile == board[i][j])
            {
              if (board[i-1][j] == 0)
              {
                  int temporary = board[i][j];
                  board[i][j] = board[i-1][j];
                  board[i-1][j] = temporary;
                  return true;
              }

              else if (board[i][j-1] == 0)
              {
                  int temporary = board[i][j];
                  board[i][j] = board[i][j-1];
                  board[i][j-1] = temporary;
                  return true;
              }

              else if (board[i+1][j] == 0)
              {
                  int temporary = board[i][j];
                  board[i][j] = board[i+1][j];
                  board[i+1][j] = temporary;
                  return true;
              }

              else if (board[i][j+1] == 0)
              {
                  int temporary = board[i][j];
                  board[i][j] = board[i][j+1];
                  board[i][j+1] = temporary;
                  return true;
              }
              return false;
            }

        }
    }
    return false;
}

What is the purpose of the code?

1. It receives as an input -via int tile = get_int(); - the tile that the user wants to move.
2. It finds the int within the 2D array that has the inputted value tile .
3. Then, it checks if, surrounding that tile, there is a blank space (the only blank space that there can be in the game).
4. If there is a blank space, it switches the tiles positions. Et voilà .

What goes wrong?

What appears to be not working is the identification of that blank space.

See below the initialized and drawn board.

 8| 7| 6
 5| 4| 3
 2| 1| _
Tile to move: 

If I input 3, this is what happens and what should always happen: It finds the tile "3", checks if there is a blank space surrounding it (up, down, left, right), and switches positions with that blank space:

8| 7| 6
5| 4| _
2| 1| 3
Tile to move: 

Not a problem right? Well, it does not work as well as it might seem.

See below, how it works with a number not surrounded by the blank space: I have inputted 8 , and although not being surrounded by the blank space, it has converted it into a new blank space.

 _| 7| 6
 5| 4| _
 2| 1| 3
Tile to move:

...I'll do it now with the 6 ; oh, surprise!:

 _| 7| _
 5| 4| _
 2| 1| 3
Tile to move:

Why it should not happen?

I have included this code, wich in my opinion is well written and should avoid all these problems (comments inside):

for (int i=0; i<d; i++)             //For every row and every column in the board,...
{
    for (int j=0; j<d; j++)        //check if the inputted tile is within the boards' values.
    {
        if (tile == board[i][j])   //If it is, check "up/down/left/right" for a 0 (blnk spc)
        {
          if (board[i-1][j] == 0)  //If there is, switch the values of those tiles.
          {
              int temporary = board[i][j];
              board[i][j] = board[i-1][j];
              board[i-1][j] = temporary;
              return true;
          }

This code is just a piece of the one at the beginning of the post. In the one in here, it can be seen how it only checks the i-1 'th position (above) for a blank space (or 0); the full code at the beginning has not only the i-1 'th but also the i+1 'th (below), j-1 'th (left), and j+1 'th (right) positions.

So, when I input 8, the code should go to the position [0][0] and see that there is a tile with that value (which it correctly does).

But after that, it should check if there is a 0 in the positions "[-1][0]", "[0][-1]", [1][0], or [0][1] (which it incorrectly does not).

At this point is where I try a hundred different combinations of code, none of which works, and finally decide to ask in Stackoverflow.

Any help would be much appreciated! Go ahead with any requests for clarifications -I have been working on this for a long time and take for granted many things, but they might not be completely clearly explained (sorry about that if that's the case)-!

The whole code is much longer, and I just didn't want to scare anybody. If you still want to see it, here you have it: https://pastebin.com/FZPq1F7b Thanks again!

Answer 1

You need to check that the index lies within the array or something unexpected like what happens, will happen.

For example,

if (board[i-1][j] == 0)

would be:

if ((i > 0) && (board[i - 1][j] == 0))

likewise

else if (board[i+1][j] == 0)

would be

else if ((i < d - 1) && (board[i + 1][j] == 0))

And please , do not use d as a variable name. Use something with a meaning like board_size and since it's apparently a constant BOARD_SIZE would be much better. It might seem like an unimportant detail, but trust me, it's very important.

Oh, and about this comment

At this point is where I try a hundred different combinations of code, none of which works, and finally decide to ask in Stackoverflow.

Coding is not a guessing game, you should not try hundreds of combinations of code. If your code does not do what you think it does there are 2 things to do

1. Execute it in your head, you would have noticed that board[-1] was a problem immediately.
2. Use a debugger, you would have seen board[-1] immediately too.