Need help/hints/tips with a method for connect 4 game.

Question

Hi!
I'm currently trying to get my "movepiece"-method to make it so that players can only drag a piece into the empty square at the bottom row, and if the square is not empty the method will move up one row and check if it's empty if not, repeat until
 6 are checked if no one is empty then move is invalid.
I've been trying out while loops and for loops but i think i might set the wrong conditions for exiting the loop...
Here the code for the method without any loops simply adds pieces.
        /// <summary>
        /// Method that performs the actual move.
        /// The old piece gets overrun by the attacking piece. 
        /// The attacking piece's position becomes null.
        /// </summary>public Piece[,] MoveAttackpiece(int[,] changearray)
int origX = changearray[0, 0];
int origY = changearray[0, 1];
int targetX = changearray[1, 0];
int targetY = changearray[1, 1];
pieceArray[targetX, targetY] = pieceArray[origX, origY]; //swap
return pieceArray;
The game is using winforms and a 2D array(pieceArray)to draw the game board
I'm using a Constants class for declaring the height and width of the board, which the pieceArray is based on, Namely const int BOARDESIZEx and BOARDERSIZEy.
And while i'm at it when i did different loops for tryng to make the piece move down/up (based on if a square in bottom row was empty or not) the program crashes and i get the "Null Reference Exception unhandled"
at this part in the game.CS (the earlier code reside in a class named Squares.cs)
if (changearray != null)
//Building string to print (split over several rows for readability but also to avoid calculations)
Piece attackpiece = pieceArray[changearray[1, 0], changearray[1, 1]];
string message = "";
if (move % 2 == 1) message += "TURN " + move / 2 + ":\r\n"; //integer division, as there are two draws per turn
message += attackpiece.Name + ": "; //Walker:
message += IntegerToChar(changearray[0, 1]); //Walker: A
message += (changearray[0, 0] + 1) + " - "; //Walker A1 -
message += IntegerToChar(changearray[1, 1]); //Walker A1 - B
message += (changearray[1, 0] + 1); //Walker A1 - B2
if (move % 2 == 0) message += "\r\n"; //blank row inserted after each completed turn
Board.PrintMove(message);
I've tried making
While(piecearray[targetX, targetY]!=null)
pieceArray[targetX, targetY]==pieceArray[targetX, targetY +1];
return array;
but that didn't work and i don't really know how to make the game start at bottom row.
I appreciate any help or tips on how to get this to work since it's the only thing left for making the game work as intended.
Cheers and thanks for reading.
PS: if there's anything you wonder just ask and i'll explain as much as i can.
Regards Gabbelino
Student at the University of Borås, Sweden

Answer 1

Let's look at what's wrong with the following code.
While(piecearray[targetX, targetY]!=null)
pieceArray[targetX, targetY]=pieceArray[targetX, targetY +1]; // I assume the "==" in your post above was a typo.
Suppose, for the sake of argument, that targetX is 2 and targetY is 3, and that pieceArray[2,3] is not null. The line inside the loop will then become...
pieceArray[2,3] = pieceArray[2, 4];
...and this line will keep being executed as long as pieceArray[2,3] is not null, which if pieceArray[2,4] is not null will be forever (infinite loop).
I suspect that your loop should look something like this...
for(int i = 0; i < BOARDSIZEy; i++)
// Look for the first available square in the target column.
// When found, set the piece on that square then break out.
if(pieceArray[targetX, i] == null)
pieceArray[targetX, i] = pieceArray[origX, origY];
break;