C++ Connect 4 Checking Next Step Error - c++

Question

I am writing a Connect 4 game with minimax, but my next-step checking function is sometimes giving me unexpected results. Can you please tell me if my algorithm is correct?
For example if my board looks like this
0000000
0000000
0000000
0000000
1000000
2002120
it will return column 0 as true for player 2.
bool Board::check2(int player, int& bestMove)
{
for (int i=0; i<WIDTH; i++)
{
if(addToColumn(i, player))
{
if (checkNext(2, i, player))
{
bestMove=i;
removeFromColumn(i, player);
return true;
}
removeFromColumn(i, player);
}
}
return false;
}
bool Board::checkNextVertical(int size, int column, int player1)
{
int counter=0;
int player2;
if (player1==1)
{
player2=2;
}
else
player2=1;
for (int i=0 ; i<DEPTH; i++)
{
if (arrBoard[column][i]==player1)
{
counter++;
}
if (arrBoard[column][i]==player2)
{
return false;
}
if (counter==size)
{
return true;
}
}
return false;
}
bool Board::checkNextHorizontal(int size, int column, int player1)
{
int counter=0;
int player2;
if (player1==1)
{
player2=2;
}
else
player2=1;
for (int i=0 ; i<DEPTH; i++)
{
if (arrBoard[i][column]==player1)
{
for (int j = 0; j<WIDTH; j++)
{
if (arrBoard[i][j]==player1)
{
counter++;
}
if (arrBoard[i][j]!=player1)
{
counter=0;
}
if (counter==size)
{
return true;
}
}
}
}
return false;
}
bool Board::checkNext(int size, int column, int player)
{
if (checkNextVertical(size, column, player))
{
// printBoard();
return true;
}
if (checkNextHorizontal(size, column, player))
{
// printBoard();
return true;
}
return false;
}

Answer 1

Welcome to the forum.
There are a few problems with the code you posted:
Your checkNextVertical function appears to be attempting to check horizontally, and your checkNextHorizontal function appears to attempting to check both horizontally and vertically.
If you notice you use both arrBoard[column][i] and arrBoard[i][column]. I'm sure you'll agree only one of these can be correct. It's important to understand which is correct, or else your code will end up attempting to access locations in the array which are not valid, and you will get unexpected behaviour, for example your j loop in the checkNextHorizontal function is currently doing this.
It should be used as array[y / depth / row][x / width / column] - or whatever you will remember.
Personally, this code seems confusing:
int player2;
if (player1==1)
{
player2=2;
}
else
player2=1;
player2=1 seems like trying to push a square peg in a round hole. Could you use int player and set it to either 1 or 2 to make it easier to read?
I totally agree with Joachim - if you have these sorts of problems, it's always a great idea to fill the array with some data, then use the debugger to step through your code and check that the data being accessed is the data you expect to be accessed.
Alternatively, since it's a connect4 game, I assume at some point you know the column which the last move was made, in which case you can use this function to check if it was a winning move. You just need to tell it which column the last move was, and the required 'size' to win. If you do use it, I would still recommend stepping through it with the debugger so you can understand the array access. NB: your code wasn't checking diagonally - so neither does this. Some extra logic required if you want to do that:
bool winningMove(int column, int size)
{
bool winnerWinnerChickenDinner = false;
int player = 0;
int row = 0;
// Who was the last player to go in this column
// i.e. find the top non-zero entry
for (int i = 0; i < DEPTH; i++)
{
if (arrBoard[i][column] != 0)
{
player = arrBoard[i][column];
row = i;
break;
}
}
// If we found a player, check if it was a winning move
if (player != 0)
{
int count = 0;
// Loop twice, first horizontally, then vertically
for (int i = 0; i < 2 && !winnerWinnerChickenDinner; i++)
{
bool horizontal = (i == 0);
for (int j = 0; j < (horizontal ? WIDTH : DEPTH); j++)
{
// Check if we have 'size' consecutive entries by the same player
// (When we check horizontally, use arrBoard[row][j] to check the row)
// (When we check vertically, use arrBoard[j][column] to check the column)
if (arrBoard[(horizontal ? row : j)][(horizontal ? j : column)] == player)
{
if (++count == size)
{
winnerWinnerChickenDinner = true;
break;
}
}
else
{
count = 0;
}
}
}
}
return winnerWinnerChickenDinner;
}
The old games are the best - Connect4 is awesome, so good luck.