Cfrgtkky

I'm having a rough time with pointers, can someone help me a little bit?

I'm trying to initialize a double (double) pointer inside a struct array, but I'm somehow doing it wrong.. Example:

struct MyStruct

{

    double **matrix;

};



double **CalculateMatrix(...)

{

    double **matrix = (double**)malloc(ROWS * sizeof(double*));

    for (int i = 0; i < ROWS; i++)

        matrix[i] = (double*)malloc(COLS * sizeof(double));

    // + assign some values

    return matrix;

}



void Initialize(struct MyStruct *structs, int size)

{

    for (int i = 0; i < size; i++)

        // Here structs[i].matrix holds the correct matrix values

        structs[i].matrix = CalculateMatrix(...);

}



int main()

{

    struct MyStruct *structs = (struct MyStruct*)malloc(SIZE * sizeof(struct MyStruct));

    Initialize(structs, SIZE);

    // but once the function returns, matrix values are completely different

}

Sorry if it's duplicated, I couldn't find anything

You do not need global variables here. So, you could declare, define and initialize the size of your struct array, as well as the dimensions of your matrices in the main method.

Moreover, your methods' names are misleading, I changed them to something that communicates to the author what every function's purpose is. Or, more accurately your methods do more than one tasks. It's good to divide tasks for reusability.

Here is a minimal example to get you started:

#include <stdlib.h>

#include <stdio.h>



struct MyStruct

{

    double **matrix;

};



double **allocate_matrix(int rows, int cols)

{

    double **matrix = malloc(rows * sizeof(double*));

    for (int i = 0; i < rows; i++)

        matrix[i] = malloc(cols * sizeof(double));

    return matrix;

}



void allocate_matrices(struct MyStruct *structs, int size, int rows, int cols)

{

    for (int i = 0; i < size; i++)

        structs[i].matrix = allocate_matrix(rows, cols);

}



void fill_matrices(struct MyStruct *structs, int size, int rows, int cols)

{

    for (int i = 0; i < size; i++)

        for(int j = 0; j < rows; j++)

            for(int z = 0; z < cols; z++)

                structs[i].matrix[j][z] = -1.2;

}



void print_matrices(struct MyStruct *structs, int size, int rows, int cols)

{

    for (int i = 0; i < size; i++)

        for(int j = 0; j < rows; j++)

            for(int z = 0; z < cols; z++)

                printf("%fn", structs[i].matrix[j][z]);

}



void free_matrices(struct MyStruct *structs, int size, int rows) {

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

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

            free(structs[i].matrix[j]);

        }

        free(structs[i].matrix);

    }

}





int main()

{

    int rows = 3, cols = 4, size = 2;

    struct MyStruct *structs = malloc(size * sizeof(struct MyStruct));

    structs[0].matrix = NULL;

    structs[1].matrix = NULL;

    if(structs[0].matrix == NULL)

        printf("nulln");

    allocate_matrices(structs, size, rows, cols);

    if(structs[0].matrix == NULL)

        printf("nulln");

    fill_matrices(structs, size, rows, cols);

    print_matrices(structs, size, rows, cols);

    free_matrices(structs, size, rows);

    free(structs);

}

Output:

null

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

-1.200000

Inspired from my 2D dynamic array (C).

Extending gsamaras' answer:

'You do not need global variables here' is truth. Still, having constants ROWS and COLS can be meaningful as well. It depends on the use case!

gsamaras' approach allowes you to define individual sizes for each matrix. If you want to enforce same size for all matrices created, it is less error prone not to have parameters for but constants instead.