Dynamic array swaps columns and rows when passed by reference

Question

I have following code with methods to:

create a matrix
print it
add two matrices

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

#define VECTOR_ROWS 3
#define VECTOR_COLS 1

void add_matrix(double***, double**, int, int);
void print_matrix(double**, int, int);
double** new_matrix(int, int);

int main() {
    
    printf("First vector:\n");
    double **vector = new_matrix(VECTOR_ROWS, VECTOR_COLS);
    print_matrix(vector, VECTOR_ROWS, VECTOR_COLS);
    
    printf("\nSecond vector:\n");
    double **vector2 = new_matrix(VECTOR_ROWS, VECTOR_COLS);
    print_matrix(vector2, VECTOR_ROWS, VECTOR_COLS);
    
    printf("\nAdd vector:\n");
    add_matrix(&vector, vector2, VECTOR_ROWS, VECTOR_COLS);
    print_matrix(vector, VECTOR_ROWS, VECTOR_COLS);
    
    return 0;
}

void add_matrix(double*** matrix, double** trans_matrix, int rows, int cols)
{
    int r, c;
    
    for (r = 0; r < rows; r++)
        for (c = 0; c < cols; c++)
            *matrix[c][r] += trans_matrix[r][c];        // WHY DOES IT WORK THIS WAY?   
    
}

double** new_matrix(int row, int col)
{
    double **matrix = (double**)malloc(row * sizeof(double*));
    int r, c;
    
    for(r = 0; r < row; r++)
        matrix[r] = (double*)malloc(col * sizeof(double));
    
    for (r = 0; r < row; r++)
        for (c = 0; c < col; c++)
            scanf("%lf", &matrix[r][c]);
    
    return matrix;
}

void print_matrix(double** matrix, int rowCount, int colCount)
{
    int r, c;
    
    for (r = 0; r < rowCount; r++)
    {
        for (c = 0; c < colCount; c++)
            printf("%lf ", matrix[r][c]);
        printf("\n");
    }
}

Basically, I create vector and vector2 and attempt to add them. It only works if the first matrix in add_matrix(...) has the [r][c] swapped, as if because I passed it by reference, its rows and columns swapped. If I reference it with [r][c], I get a sef fault, but this works and I have no clue why.

This seems apropos: [**Correctly allocating multi-dimensional arrays**](https://stackoverflow.com/questions/42094465/correctly-allocating-multi-dimensional-arrays) Note that does require that your compiler support variable-length arrays, which are required by C11, but are only optional for C17. — Andrew Henle, Oct 26 '20 at 21:57
regarding: `void add_matrix(double*** matrix, double** trans_matrix, int rows, int cols)` The code is not changing where the pointers in the `matrix` point to, so no need to take the address of `matrix` — user3629249, Oct 28 '20 at 09:23
You might want to read: [three star programmer](https://wiki.c2.com/?ThreeStarProgrammer) — user3629249, Oct 28 '20 at 09:25

Vlad from Moscow · Accepted Answer · 2020-10-26T22:03:25.830

The left operand in this assignment

*matrix[c][r] += trans_matrix[r][c];

is incorrect.

That is the left operand is equivalent to

*( matrix[c][r] )

Write

( *matrix )[r][c] += trans_matrix[r][c];

or

matrix[0][r][c] += trans_matrix[r][c];

Actually there is no sense to pass the pointer to the first matrix by reference because the pointer itself is not changed within the function.

The function could be declared like

void add_matrix(double** matrix, double** trans_matrix, int rows, int cols);

Dynamic array swaps columns and rows when passed by reference

1 Answers1