Initialize 2D array in C using array of pointers

Question

I have a static array that I want to use to initialize a dynamic array using pointers.

Right now I have this:

boardinit[BOARD_HEIGHT][BOARD_LENGTH] = {
        {-4, -2, -3, -5, -6, -3, -2, -4},
        {-1, -1, -1, -1, -1, -1, -1, -1},
        {0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0},
        {1, 1, 1, 1, 1, 1, 1, 1},
        {4, 2, 3, 5, 6, 3, 2, 4},
    };
int main()
{
    int board[BOARD_HEIGHT][BOARD_LENGTH]; 
    initboard(board);

    for (int j = 0; j < BOARD_HEIGHT; j++)
    {
        for (int i = 0; i < BOARD_LENGTH; i++)
            printf("%d ", board[j][i]);
        printf("\n");
    }
    return 0;
}

void initboard(int (*pboard)[BOARD_HEIGHT]) 
{
    for(int i = 0;i<BOARD_HEIGHT;i++)
        pboard[i] = boardinit + i;

}

I want board[BOARD_HEIGHT][BOARD_LENGTH] to initialize as boardinit[BOARD_HEIGHT][BOARD_LENGTH] by passing board to initboard() using pointers, but can't seem to get it to go.

Answer 1

To copy the contents boardinit to the value of board , just use this:

memcpy(board, boardinit, sizeof(board));

If that does not work, try this:

void initboard(int pboard[BOARD_HEIGHT][BOARD_LENGTH]) {
    for (int i = 0; i < BOARD_HEIGHT; i++)
        for (int j = 0; j < BOARD_LENGTH; j++)
            pboard[i][j] = boardinit[i][j];
}

Answer 2

"I have a static array that i want to use to initialize a dynamic array using pointers."

The problem with arrays of pointers is that each call to [m][c]alloc() is not guaranteed (in fact is not likely) to provide a single contiguous block of memory. ie, this is a common approach that provide many little areas of memory:

int **array = malloc(sizeof(width * sizeof(int *));
{
     for(int i=0; i<width; i++)
     {
          array[i] = malloc(height*sizeof(int));
     }
}

Thus, the initializer would need to be broken into several memory block locations.

A simpler and more efficient way to do this is with a single dynamic memory allocation call, guaranteeing a single contiguous block of memory, then a single memcpy() is possible using the initializer into the pointer location returned from [m][c]alloc() . The only problem then is how to access the memory area with array notation. The example below illustrates how this can be done using *(ptr + index) notation...

//assuming your other code above
int main(void) 
{   
    int elements = BOARD_HEIGHT*BOARD_LENGTH;
    int *pArray = calloc(elements, sizeof(int));
    if(pArray)
    {
        memcpy(pArray, boardinit, sizeof boardinit);
        for(int i=0; i<BOARD_HEIGHT;i++)
             for(int j = 0;j<BOARD_LENGTH;j++)
                printf("%d\n", *(pArray + (i + j)));
        free(pArray);
    }
 
    return 0;
}