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.

Not sure what you mean by dynamic. Please edit your question and add some clarification. At a guess ... Currently, you have BOARD_HEIGHT/BOARD_LENGTH as fixed constants either via #define or an enum. And, you want these values to be [global] int declarations (i.e. You can prompt the user for the width/height via (e.g.) scanf)??? — Craig Estey
– Craig Estey, Commented Feb 22, 2021 at 22:06
I do not get what your question is. Do you observe any problems with the shown code? Would you like to describe them? Would you like to create a minimal reproducible example which demonstrates what you describe? — Yunnosch
– Yunnosch, Commented Feb 22, 2021 at 22:06
You have a two dimensional array, so you need nested loops that iterate the rows and columns. Or you could just use memcpy(board, boardinit, sizeof(board)) in the main function. — user3386109
– user3386109, Commented Feb 22, 2021 at 22:07
it is a 8x8 array, and boardinit is a static array, it never changes. The array will always initialize as the listed values. Since i am passing the 2d array as a pointer to an array of 8 ints, and in my current code i get a error of expression must be a modifiable lvalue for pboard[i] = boardinit + i; — aaronb8
– aaronb8, Commented Feb 22, 2021 at 22:11
If everything is fixed, you can simplify the initboard prototype: void initboard(int pboard[BOARD_HEIGHT][BOARD_LENGTH]) { for (int irow = 0; irow < BOARD_HEIGHT; irow++) for (int icol = 0; icol < BOARD_LENGTH; icol++) pboard[irow][icol] = boardinit[irow][icol]; } — Craig Estey
– Craig Estey, Commented Feb 22, 2021 at 22:17

Block of Diamond · Accepted Answer · 2021-02-22 22:21:48Z

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];
}

edited Feb 22, 2021 at 22:21

answered Feb 22, 2021 at 22:13

Block of Diamond

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6 Comments

aaronb8 Over a year ago

Hello, thank you for your reply. Isn't the parameter of initboard a pointer to an array of BOARD_HEIGHT ints? Wouldn't that make indexing not work, forcing you to use pointer arithmetic since technically its not a 2D array?

Block of Diamond Over a year ago

@aaronb8 Why is it 'technically not a 2D array?'

aaronb8 Over a year ago

Well, it's a pointer to a array of [8] ints. The parameter in your function you wrote is a copy of board from main, which is what i am trying to avoid.

Block of Diamond Over a year ago

@aaronb8 Arrays are passed by reference, pointer or not, so declaring the parameter as int [BOARD_HEIGHT][BOARD_LENGTH] does not make a copy of the board in main

aaronb8 Over a year ago

Thanks. I'm new to C as you can tell.

|

ryyker · Accepted Answer · 2021-02-22 23:15:32Z

0

"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. i.e., 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;
}

answered Feb 22, 2021 at 23:15

ryyker

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3 Comments

Block of Diamond Over a year ago

Using dynamic allocation (malloc() or calloc()) when stack allocations would be fine should be avoided

ryyker Over a year ago

@BlockofDiamond - If you see anything that suggests I am promoting the use of allocated memory here, that was not my intent. The whole purpose of the first section of this answer was to point out non-contiguous blocks of memory are not always optimal. The second part of my answer was just showing how to use OP's stated desire to use "array of pointers" without non-contiguous memory. Your answer already covered the array approach. No need to cover it here.

Block of Diamond Over a year ago

Ok, I understand

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