problem about using a dynamically allocated 2d array in a c program

Question

I'm having a problem on making a move on my 9 x 9 tic tac toe program.

When I enter a coordinate such as (5,5), The x is displayed correctly on the grid.

MY PROBLEM IS THAT when I enter a coordinate which has the number 7 in it such as (4,7), TWO X's ARE DISPLAYED ON THE GRID.

I previously did the program declaring my array as a global variable. All was working fine. The issue started when I switched to a dynamically allocated array and passed the array using a double-pointer. So I'm guessing that my problem is because of my array. Can someone tell me where does this issue occur and how to fix it please.

I have declared the array in my main

//previously the array was declared here
//char grid[ROW][COLUMN];

int main() 
{
    //dynamically create an array of pointers os size ROW
    char **grid = (char **)malloc(ROW * sizeof(char *));

    // dynamically allocate memory of size ROW*COLUMN and let *grid point to it
    *grid = (char *)malloc(sizeof(char) * ROW * COLUMN);

make move method

int make_move(char **grid, int x, int y,int row, int col, char letter) 
{
    if (x < 0 || x >= row || y < 0 || y >= col || grid[x][y] != ' ' )
    { 
        // checks to see if the input is valid
        return 1;
    }
    if( grid[x][y] == ' ')
    grid[x][y] = letter; 
    // sets the coordinates in the grid to the letter
    return 0;
}

update grid method


// Updates the grid accordingly every time a move is made
void update_grid(char **grid,int x, int y, int row, int col)
{
   // int counter = 1;

    //checks the input
    while  (x < 0 || x >= row || y < 0 || y >= col || grid[x][y] != ' ')
    {
        fputs("Error, Move not valid! Please reenter: ", stderr);
    scanf("%d,%d", &x, &y);
    }



    ++counter; 
    { 
        //Acts as an increment for the turns of the players
        if(counter % 2 == 0)
        { 
            //checks to see if it is player X's turn
        grid[x][y] = 'X';
        }
        if(counter % 2 != 0)
        {  
            //checks to see if it is player O's turn
            grid[x][y] = 'O';
        }

//prints grid

        printf(" ");
        for (int c = 0; c < col; c++) 
        {
            printf(" ");
        printf(" %d", c);
        }
        printf("\n");

        for (int r = 0; r < row; ++r) 
        {
            printf("%d", r);
            printf("|");
        for (int dot = 0; dot < (col*row); ++dot) 
            {

            printf("|");
            printf("%c", grid[r][dot]);
            printf(" ");

            if (dot == col - 1) 
                { 
                    // stops j from over printing
                printf("|| \n");
                break;
            }
            }
        }
    }
}

I think your mallocs are wrong, as you define only the value for the first *grid, but not the other members of your array **grid !? I was expecting a loop of mallocs over *grid... — B. Go
– B. Go, Commented Oct 25, 2019 at 19:26

nee · Accepted Answer · 2019-10-26 17:52:14Z

1

As B. Go said your malloc is wrong, here's how it should look.

//dynamically create an array of pointers os size ROW
char **grid = malloc(ROW * sizeof(char *));

for(size_t i = 0; i < ROW; i++){
    grid[i] = malloc(COLUMN);
}

you allocated ROW pointers but you only filled the first one, you need to give them each COLUMN bytes of data.

edited Oct 26, 2019 at 17:52

answered Oct 25, 2019 at 20:08

nee

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