Trouble with moving tiles in Game of Fifteen - c

Question

Below is a function in the game of fifteen which should search for the "0" tile and once found, allow adjacent tiles to move into its place. It works for the first few moves but then doesn't allow moves that it should permit when the zero tile is at the top row (it starts on the bottom right). Note below the "0" tile is drawn as an underscore. For example:
8 7 6
5 4 3
2 1 _
Tile to move: 3
8 7 6
5 4 _
2 1 3
Tile to move: 6
8 7 _
5 4 6
2 1 3
Tile to move: 7
Illegal move.
Here is the code:
bool move(int tile)
{
int blankrow;
int blankcol;
for (int i = 0; i < d; i++)
{
for (int j = 0; j < d; j++)
{
if (board[i][j] == 0)
{
blankrow = i;
blankcol = j;
}
}
}
for (int i = 0; i < d; i++)
{
for (int j = 0; j < d; j++)
{
if ((board[i][j] == tile) && ((board[i+1][j] == 0 || board[i-1][j] == 0) &&
(board[i][j+1] == 0 || board[i][j-1] == 0)))
{
int swapped = board[i][j];
board[blankrow][blankcol] = swapped;
board[i][j] = 0;
return true;
}
}
}
return false;
}

Answer 1

Just by looking, I'm pretty sure you need to change:
((board[i+1][j] == 0 || board[i-1][j] == 0) &&
(board[i][j+1] == 0 || board[i][j-1] == 0))
To:
(board[i+1][j] == 0 || board[i-1][j] == 0 ||
board[i][j+1] == 0 || board[i][j-1] == 0)
EDIT: I agree with the user comment below. Better code would look something like:
bool move(int tile)
{
int blankrow, blankcol, tilerow, tilecol;
for (int i = 0; i < d; i++) {
for (int j = 0; j < d; j++) {
if (board[i][j] == 0) {
blankrow = i;
blankcol = j;
}
else if (board[i][j] == tile) {
tilerow = i;
tilecol = j;
}
}
}
if( (blankrow == tilerow && abs(blankcol - tilecol) == 1) ||
(blankcol == tilecol && abs(blankrow - tilerow) == 1) ) {
board[blankrow][blankcol] = board[tilerow][tilecol];
board[tilerow][tilecol] = 0;
return true;
}
return false;
}

Answer 2

You found the blankrow and blankcol. Do the same thing to find the tilerow and tilecol. Then verify either
((blankrow == tilerow) && (abs(blankcol - tilecol) == 1))
or
((blankcol == tilecol) && (abs(blankrow - tilerow) == 1))
Swap if either of those conditions is met. The problem with the existing code is that you can have array accesses out-of-bounds. For example, if i is 0, then board[i-1][j] is an out-of-bounds access.

Answer 3

I think you should change the if statement in the second loop from
if ((board[i][j] == tile) &&
((board[i+1][j] == 0 || board[i-1][j] == 0) &&
(board[i][j+1] == 0 || board[i][j-1] == 0)))
to
if ((board[i][j] == tile) &&
((i+1==blankrow && j==blankcol) ||
(i-1==blankrow && j==blankcol) ||
(i==blankrow && j+1==blankcol) ||
(i==blankrow && j-1==blankcol)))
In the original you are requiring a row offset of one and a column offset of zero and a row offset of zero and a column offset of one simultaneously. Also you're potentially taking a negative index into the array or reading past its end.