How to use second rank pointer rightly? - c

Question

I have a 2D array called matrix, now I would like to add one to every element.
#include <stdio.h>
#include <stdlib.h>
int **add_one(int **matrix, int m, int n) {
int i, j;
int **new_mat;
new_mat = (int **) malloc(m * n *sizeof(int));
for (i = 0; i < m; i++) {
for (j = 0; j < n; j++) {
new_mat[i][j] = matrix[i][j] + 1;
}
}
//return the 2nd rank pointer
return new_mat;
}
int main() {
int matrix[3][2] = {1, 2, 3, 4, 5, 6};
int **new_mat;
int i, j;
new_mat = add_one(matrix, 3, 2);
for (i = 0; i < 3; i++) {
for (j = 0; j < 2; j++) {
printf("%d ", new_mat[i][j]);
}
printf("\n");
}
free(new_mat);
}
However, the compiler told me that
[Error] cannot convert 'int (*)[2]' to 'int**' for argument '1' to 'int** add_one(int**, int, int)'

Answer 1

It works a bit differently from what you thought
#include <stdio.h>
#include <stdlib.h>
// you can only pass what you have
int **add_one(int matrix[3][2], int m, int n)
{
int i, j;
int **new_mat;
// allocate m pointers (the rows)
new_mat = malloc(m * sizeof(*new_mat));
for (i = 0; i < m; i++) {
// allocate space for n ints
new_mat[i] = malloc(n * sizeof(int));
for (j = 0; j < n; j++) {
new_mat[i][j] = matrix[i][j] + 1;
}
}
//return the 2nd rank pointer
return new_mat;
}
int main()
{
// you forgot to encapsulate the rows, too
int matrix[3][2] = { {1, 2}, {3, 4}, {5, 6} };
int **new_mat;
int i, j;
new_mat = add_one(matrix, 3, 2);
for (i = 0; i < 3; i++) {
for (j = 0; j < 2; j++) {
printf(" %d ", new_mat[i][j]);
}
printf("\n");
}
free(new_mat);
}
If you want to pass the two-star matrix as a two-star matrix you need to build it like the new_mat in the function add_one.