Using a 1D array of pointers to a 2D array?

Question

So I was working on a Tic-Tac-Toe game and for my input function I was getting the move the player made to store as an integer in a 2d array, the input is gotten using a reference to a 1D array of pointers to positions in the 2D array.

However my problem is, when I seem to set the value of the multi-dimensional array's square to something by using a pointer, nothing happens.

Here is the input function:

void Game::input(Board b){
int *spots[9]; // Possible spots for the input
bool validInput = false;
spots[0] = &b.board[2][0];
spots[1] = &b.board[2][1];
spots[2] = &b.board[2][2];
spots[3] = &b.board[1][0];
spots[4] = &b.board[1][1];
spots[5] = &b.board[1][2];
spots[6] = &b.board[0][0];
spots[7] = &b.board[0][1];
spots[8] = &b.board[0][2];
redo:
    cout << ">> " << endl;
    int input; // Input
    cin >> input; // Get the input
    validInput = cin;
    if(!validInput){
        cout << "Numbers only please!" << endl;
        cin.clear();
        while(cin.get() != '\n');
        goto redo;
    }
    if(input > 9 || input <= 0){
        cout << "Invalid move!" << endl;
        goto redo;
    }
    input--; // Subtract 1 for array location
    if(*spots[input] != 0){
        cout << "Square is already being used!" << endl;
        goto redo;
    }
    *spots[input] = 1;
}

Now, say I input the number 7. It should set b.board[0][0] to 1. However this doesn't seem to happen. When I run a unit case afterwards, the board[0][0] doesn't seem to be set to 1, and it doesn't reflect in my array. Am I messing something up about pointers here?

You should get into the habit of using loops over goto. Also, you're not passing the board in by reference, you're passing in some random copy. — Qaz
– Qaz, Commented Jul 27, 2012 at 20:38
Do you have the option of using a vector , map, or for-loop? — andre
– andre, Commented Jul 27, 2012 at 20:41
Never-mind, this has been solved, can't believe I missed the reference thing. — Rivasa
– Rivasa, Commented Jul 27, 2012 at 20:41

mathematician1975 · Accepted Answer · 2012-07-27 20:41:33Z

6

The argument to your function is passed by value and therefore any changes you make to it are not recognized since pass by value creates a copy of the argument. Consider passing by pointer or reference instead.

answered Jul 27, 2012 at 20:41

mathematician1975

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1 Comment

Rivasa Over a year ago

Can't believe I missed that :p Thank you very much! Accepted. (In 5 minutes)

Remy Lebeau · Accepted Answer · 2012-07-27 20:46:13Z

You are passing in a Board instance by value to input() instead of by pointer/reference. Any changes you make to that Board instance will not be reflected in the original Board instance that was passed to input().

BTW, paranthesis and loops are your friend:

void Game::input(Board &b)
{ 
    int* spots[9]; // Possible spots for the input 
    bool validInput; 
    int input;
    spots[0] = &(b.board[2][0]);
    spots[1] = &(b.board[2][1]); 
    spots[2] = &(b.board[2][2]); 
    spots[3] = &(b.board[1][0]); 
    spots[4] = &(b.board[1][1]); 
    spots[5] = &(b.board[1][2]); 
    spots[6] = &(b.board[0][0]); 
    spots[7] = &(b.board[0][1]); 
    spots[8] = &(b.board[0][2]); 
    do
    {
        std::cout << ">> " << std::endl; 
        std::cin >> input; // Get the input 
        validInput = std::cin; 
        if (!validInput)
        { 
            std::cout << "Numbers only please!" << std::endl; 
            std::cin.clear(); 
            while (std::cin.get() != '\n'); 
            continue; 
        } 
        if ((input > 9) || (input <= 0))
        { 
            std::cout << "Invalid move!" << std::endl; 
            continue; 
        } 
        --input; // Subtract 1 for array location 
        if (*spots[input] != 0)
        { 
            std::cout << "Square is already being used!" << std::endl; 
            continue; 
        } 
        *spots[input] = 1; 
        break;
    }
    while (true);
}

Wolfgang Ziegler · Accepted Answer · 2012-07-27 20:43:23Z

2

Change your method signature to

void Game::input(Board& b)

that way you will actually see the changes you made to it reflected. At the moment you are calling the method by value.

answered Jul 27, 2012 at 20:43

Wolfgang Ziegler

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Comments

tweaksp · Accepted Answer · 2012-07-28 00:45:22Z

2

Another thing you could try is a little math to pick the move. Say the user inputs 7. 7/3 = 2, the row. 7 % 3 = 1, the column. So 7 refers to board[2][1], which is the correct spot.

edited Jul 28, 2012 at 0:45

answered Jul 27, 2012 at 20:43

tweaksp

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4 Comments

Rivasa Over a year ago

Tried that, didn't work, thanks for trying though. Oh, and board[0][0] is #7 due to subtracting from input by 1 for array indice.

Rivasa Over a year ago

actually, I'll try this as it seems much easier and more elegant.

tweaksp Over a year ago

You still need to pass the Board object by reference as suggested (Board &b).

Rivasa Over a year ago

I know, but this is much easier for me as well :)

Code-Apprentice · Accepted Answer · 2012-07-27 20:45:52Z

Your input function is declared as:

void Game::input(Board b)

When you call this from another function, such as

Board tictactoe;
Game g;

g.input(tictactoe);

b gets a copy of tictactoe. Any of the changes you make in your input() function to b will not be reflected in tictactoe.

You have at least two solutions:

1) Pass the board by reference:

void Game::input(Board& b)

2) Or pass a pointer to the board:

void Game::input(Board* b)

There are also other solutions which would require some redesign of your code. These two are probably the simplest and most direct ways to fix the current problem.

Also, the way you map your indices from a 1D array to a 2D array seems a little strange to me. I think it would be easier to keep track of what is going on if you had

spots[0] = &(b.board[0][0]); 
spots[1] = &(b.board[0][1]);  
spots[2] = &(b.board[0][2]);  
spots[3] = &(b.board[1][0]);  
spots[4] = &(b.board[1][1]);  
spots[5] = &(b.board[1][2]);  
spots[6] = &(b.board[2][0]);  
spots[7] = &(b.board[2][1]);  
spots[8] = &(b.board[2][2]);

In fact, as others have noted, this is unnecessary since you can calculate the row and col index from the input. Alternatively, you could ask the user to input two numbers rather than just one.

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