Write a function for malloc double pointer

Question

I need to write a function that creates a double pointer using malloc.

This is how I declared my double pointer, as usual:

double **G; //Create double pointer to hold 2d matrix

*G = malloc(numNodes * sizeof(double*));
for(i = 0; i < numNodes; i++)
{
    G[i] = malloc(numNodes*sizeof(double));
    for (j = 0; j < numNodes; j++)
    {
        G[i][j] = 0;
    }
}

Now I tried replacing it with:

double **G;
mallocDoubleArr(G, numNodes);

With function:

void mallocDoubleArr(double **arr, int size)
{
    int i, j;

    *arr = malloc(size * sizeof(double*));

    for(i = 0; i < size; i++)
    {
        arr[i]= malloc(size*sizeof(double));
        for (j = 0; j < size; j++)
        {
            arr[i][j] = 0;
        }
    }
}

Why is this not working?

Answer 1

You will need one more "indirectness", in other words, pass Gby reference as a pointer to a pointer to a pointer to a float:

void mallocDoubleArr(double ***arr, int size);

And then call him

mallocDoubleArr(&G, numNodes);

Change mallocDoubleArraccordingly, for example

(*arr)[i] = malloc(size*sizeof(double));

Answer 2

C is a call by value. IN

double **G;
mallocDoubleArr(G, numNodes);

mallocDoubleArr. , - , , mallocDoubleArr.

, G-, . . , mallocDoubleArr double **.

double **G;
G = mallocDoubleArr(numNodes);

double **mallocDoubleArr(int size)
{
    int i, j;
    double **arr;

    arr = (double **) malloc(size * sizeof(double *));

    /* continue with old code */

    return arr;
}

[edit: bstamour post . .]

Answer 3

*G = malloc(numNodes * sizeof(double*));

G = malloc(numNodes * sizeof(double*));

( , -.)

-, , .

void mallocDoubleArr(double ***arr, int size)

, .

, , , , - , - . , ints, floats .., , : , . (int, float, pointer ..) , . .

Answer 4

, .

int **inputMatrix, i, j;
Grid myGrid = *grid;

inputMatrix = (int *) calloc (myGrid.num_nodes, sizeof(int*));
for(i=0; i < myGrid.num_nodes; i++){
    inputMatrix[i] = calloc(myGrid.num_nodes, sizeof(int));
    for(j=0;j<myGrid.num_nodes;j++){
        inputMatrix[i][j] = 0;
    }
};

for(i=0; i < myGrid.num_nodes; i++){
    free(inputMatrix[i]);
}
free (inputMatrix);