CODEWITHSUNDEEP
carraysfunctionmalloc2d
user3570201
user3570201

Reputation: 15

Initializing two-dimensional array in a function with malloc()

I've got a problem. I need to write a function which will allocate any 2D array with malloc() but I'm lost and have no idea what might be wrong. Here is what I wrote so far:

void matrix_ini(int **arr, int SIZE_X, int SIZE_Y);

int main() {

        int **arr;
        matrix_ini(arr, 2, 3);

        return 0;
}

void matrix_ini(int **arr, int SIZE_X, int SIZE_Y) {

        srand(time(NULL));

        arr = malloc(SIZE_X * sizeof *arr);

        for (int i = 0; i < SIZE_X; i++) {

                arr[i] = malloc(SIZE_Y * sizeof arr);
        }   

        //initializing array with some numbers:

        for (int i = 0; i < SIZE_X; i++) {

                for (int j = 0; j < SIZE_Y; j++) {

                        arr[i][j] = rand()%10;

                }   
        }   
}

What exactly am I doing wrong? Please be gentle, I just started learning. Any tips are welcome.

Upvotes: 0

Views: 1652

Answers (1)

barak manos
barak manos

Reputation: 30136

Problem #1:

This:

arr = malloc(SIZE_X * sizeof(*arr));

Is equivalent to this:

arr = malloc(SIZE_X * sizeof(int*));

Which is OK for your purpose.

But this:

arr[i] = malloc(SIZE_Y * sizeof(arr));

Is equivalent to this:

arr[i] = malloc(SIZE_Y * sizeof(int**));

Which is not OK for your purpose.

So change it to this:

arr[i] = malloc(SIZE_Y * sizeof(int));

Problem #2:

If you want a function to change the value of a variable that you call it with, then you have to call it with the address of that variable. Otherwise, it can change the value of that variable only locally (i.e., within the scope of the function). This pretty much forces you to change the entire prototype, implementation and usage of function matrix_init:

void matrix_init(int*** arr, int SIZE_X, int SIZE_Y)
{
    int** temp_arr;

    temp_arr = malloc(SIZE_X * sizeof(int*));
    for (int i = 0; i < SIZE_X; i++)
    {
        temp_arr[i] = malloc(SIZE_Y * sizeof(int));
        for (int j = 0; j < SIZE_Y; j++)
        {
            temp_arr[i][j] = rand()%10;
        }
    }

    *arr = temp_arr;
}

Then, in function main, you should call matrix_init(&arr,2,3).

Problem #3:

You should make sure that you release any piece of memory which is dynamically allocated during runtime, at some later point in the execution of your program. For example:

void matrix_free(int** arr, int SIZE_X)
{
    for (int i = 0; i < SIZE_X; i++)
    {
        free(arr[i]);
    }
    free(arr);
}

Then, in function main, you should call matrix_free(arr,2).

Upvotes: 4

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