Pointer to existing 2d array of unknown size?

Question

I have the following:

struct matrix {
    int h;
    int w;
    int** data;
};

int m1[2][2] = {
    {1, 2},
    {3, 4}
};

int m2[3][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

struct matrix matrix1 = {2, 2, m1};
struct matrix matrix2 = {3, 3, m2};

This gives the error 'initialisation from incompatible pointer type'. What pointer type should I be using?

Answer 1

A matrix, as you declared, is not a pointer to pointer. Use a simple pointer to point its element.

#include <stdio.h>

struct matrix {
    int h;
    int w;
    int* data;
};

int m1[2][2] = {
    {1, 2},
    {3, 4}
};

int m2[3][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

struct matrix matrix1 = {2, 2, &m1[0][0]};
struct matrix matrix2 = {3, 3, &m2[0][0]};

int main(int argc, char **argv)
{
   int i, j;

   for (i=0; i<matrix2.h; i++)
      for(j=0; j<matrix2.w; j++)
         printf("%d ", matrix2.data[(i*matrix2.h)+j]);

    printf("\n");
}

EDIT

To answer to the comment, you can use compound literal as below. In this way you could access it with [i][j]

#include <stdio.h>

struct matrix {
    int h;
    int w;
    int** data;
};

int *m2[3] = {
    (int[]){1, 2, 3},
    (int[]){4, 5, 6},
    (int[]){7, 8, 9}
};

struct matrix matrix2 = {3, 3, m2};

int main(int argc, char **argv)
{
    int i, j;

    for (i=0; i<matrix2.h; i++)
        for(j=0; j<matrix2.w; j++)
            printf("%d ", matrix2.data[i][j]);

    printf("\n");
}

Answer 2

2d array is not a pointer to pointer.

How to use the void *.

#include <stdio.h>

struct matrix {
    int h;
    int w;
    void *data;
};

int m1[2][2] = {
    {1, 2},
    {3, 4}
};

int m2[3][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

struct matrix matrix1 = {2, 2, m1};
struct matrix matrix2 = {3, 3, m2};

int main(void){
    int i, j;

    int (*matp)[matrix2.w] = matrix2.data;//Back from the void* to pointer to array

    for (i=0; i<matrix2.h; i++){
        for(j=0; j<matrix2.w; j++)
            printf("%d ", matp[i][j]);
        puts("");
    }
    printf("\n");

    return 0;
}

Answer 3

You may be interested in the variable length arrays of C99. This solution does not directly answer your question about how to initialize the structure with a properly typed data (one can't); but you can use a simple pointer to store the array's address and then cast to the variable length array pointer when using struct matrix.

The user side would just call functions like printFroMat() which receive a single argument of type struct matrix; the code inside these functions (so to speak, the library implementation) would perform the somewhat unsightly casts, as demonstrated. The typedef makes the cast perhaps a little more understandable because it demonstrates where the variable name in a declaration would go.

Note that the funny sizeof(m2)/sizeof(*m2) etc. are not strictly necessary, you can just say 3. But the sizeof expression automatically stays in sync with the actual matrix size, which quickly becomes a real asset.

You can pass "arrays" (in fact: still just addresses, but of a known array type) together with their dimensions as parameters to functions, and index them the normal way (below in printMatrix). Example:

#include<stdio.h>
#include<string.h>


struct matrix {
    int h;
    int w;
    int *data; // first element of matrix
};

int m2[4][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9},
    {10, 11, 12}
};

void printMatrix(int dim1, int dim2, int mat[][dim2] )
{
    printf("Address of matrix: %p\n", (void *)mat);

    for(int i1=0; i1<dim1; i1++)
    {
        for(int i2=0; i2<dim2; i2++)
        {
            printf("%d ", mat[i1][i2]);
        }
        putchar('\n');
    }
}

void printFromMat(struct matrix mat)
{
    printMatrix(mat.h, mat.w, (int (*)[mat.w])mat.data);

    // or:
    typedef int (*mT)[mat.w];
    printMatrix(mat.h, mat.w, (mT)mat.data);
}

int main() 
{

    printMatrix(   sizeof(m2) /sizeof(*m2),   // number of highest-order elements
                   sizeof(*m2)/sizeof(**m2),  // number of second-order elements per highest-order
                   m2  );                     // address of the first sub-array

    struct matrix mat = { sizeof(m2) /sizeof(*m2), sizeof(*m2)/sizeof(**m2), *m2 };

    printFromMat(mat);


    return 0;
}

Sample session:

$ gcc -std=c99 -Wall -o 2d-matrix 2d-matrix.c && ./2d-matrix
Address of matrix: 0x100402020
1 2 3
4 5 6
7 8 9
10 11 12
Address of matrix: 0x100402020
1 2 3
4 5 6
7 8 9
10 11 12
Address of matrix: 0x100402020
1 2 3
4 5 6
7 8 9
10 11 12

Answer 4

You may not initialize the structure such a way like

struct matrix matrix1 = {2, 2, m1};
struct matrix matrix2 = {3, 3, m2};

because there is no conversion from types int ( * )[2] and int ( * )[3] to type int **

I suggest to allocate memory for copies of the arrays dynamically.

The approach can look the following way as it is shown in the demonstrative program

#include <stdlib.h>
#include <stdio.h>

struct matrix 
{
    size_t h;
    size_t w;
    int **data;
};

int m1[2][2] = 
{
    { 1, 2 },
    { 3, 4 }
};

int m2[3][3] = 
{
    { 1, 2, 3 },
    { 4, 5, 6 },
    { 7, 8, 9 }
};

struct matrix init( size_t h, size_t w, int a[h][w] )
{
    struct matrix matrix = { 0 };

    matrix.data = malloc( h * sizeof( int * ) );

    if ( matrix.data )
    {
        matrix.h = h;
        matrix.w = w;
        for ( size_t i = 0; i < h; i++ ) 
        {            
            matrix.data[i] = malloc( w * sizeof( int ) );
            if ( matrix.data[i] )
            {                
                for ( size_t j = 0; j < w; j++ ) matrix.data[i][j] = a[i][j];
            }
        }
    }        

    return matrix;
}

int main( void ) 
{
    struct matrix matrix1 = init( 2, 2, m1 );
    struct matrix matrix2 = init( 3, 3, m2 );

    for ( size_t i = 0; i < matrix1.h; i++ )
    {
        for ( size_t j = 0; j < matrix1.w; j++ ) printf( "%d ", matrix1.data[i][j] );
        printf( "\n" );
    }
    printf( "\n" );

    for ( size_t i = 0; i < matrix2.h; i++ )
    {
        for ( size_t j = 0; j < matrix2.w; j++ ) printf( "%d ", matrix2.data[i][j] );
        printf( "\n" );
    }
    printf( "\n" );

    // free allocated arrays of matrix1 and matrix2
}    

The program output is

1 2 
3 4 

1 2 3 
4 5 6 
7 8 9 

You need also to write a function that will free the allocated memory for the structure.

The compiler must support variable length arrays.