如何遍历并在C中复制双指针

Question

I've created a two dimensional integer array in C, initialised it with values, and then cast it to an int** (I have to do that because it's for homework).
I've managed to iterate through it and set all of the values to 0. However, when I iterate through it again and print the values it, the output is not all zeros.

Here is a minimum working example:

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

#define ROWS    3
#define COLS    2

int main(void)
{
    /* Create array and convert to double pointer */
    int c[ROWS][COLS] = {{1,2},{3,5},{8,13}};
    int **ptr = (int **)c;

    /* Loop through entire array and print then double each value. */
    int *temp[ROWS];
    for(int i = 0; i<ROWS; i++){
        temp[i] = (int*)(ptr+i);
        for(int j = 0; j<COLS; j++){
            printf("Before setting: %i\n", temp[i][j]);
            temp[i][j] = temp[i][j]*2;
        }
    }

    /* Copy temp back into ptr */
    memcpy(ptr, temp, sizeof(ptr));

    /* Loop through array and print values */
    int *temp2[ROWS];
    for(int i = 0; i<ROWS; i++){
        temp2[i] = (int*)(ptr+i);
        for(int j = 0; j<COLS; j++){
            printf("After setting: %i\n", temp2[i][j]);
        }
    }

}

The issue is that the results are not what I would expect. One time I ran it, this was the output:

Before setting: 1
Before setting: 2
Before setting: 3
Before setting: 5
Before setting: 8
Before setting: 13
After setting: -1193330832
After setting: 32764
After setting: 6
After setting: 10
After setting: 16
After setting: 26

The value 32764 is the same every time the program is run, but the value -1193330832 changes each time (I assume it is the memory address of the array).

The output I was expecting is:

Before setting: 1
Before setting: 2
Before setting: 3
Before setting: 5
Before setting: 8
Before setting: 13
After setting: 1
After setting: 4
After setting: 6
After setting: 10
After setting: 16
After setting: 26
because the values in the first loop have been doubled.

What have I done wrong? Why are the values changing and how should I actually go about fixing this?

(PS the homework doesn't involve finding a way to iterate through the double pointer, but I need to be able to do it to complete the actual task)

Answer 1

int **ptr = (int **)c; isn't a valid pointer conversion, since you cannot use a pointer-to-pointer to point at a 2D array. Because it has nothing to do with 2D arrays.

Instead you can use a pointer to a 2D array, int (*)[ROWS][COLS]; . The most convenient is however to use a pointer to a 1D array and have it point at the first element of the 2D array:

int (*ptr)[COLS] = &c[0];

...

ptr[i][j] = ...;

Fixed example:

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

#define ROWS    3
#define COLS    2

int main(void)
{
    /* Create array and convert to double pointer */
    int c[ROWS][COLS] = {{1,2},{3,5},{8,13}};
    int (*ptr)[COLS] = &c[0];

    /* Loop through entire array and print then double each value. */
    for(int i = 0; i<ROWS; i++){
        for(int j = 0; j<COLS; j++){
            printf("Before setting: %i\n", ptr[i][j]);
            ptr[i][j] = ptr[i][j]*2;
        }
    }

    /* Loop through array and print values */
    for(int i = 0; i<ROWS; i++){
        for(int j = 0; j<COLS; j++){
            printf("After setting: %i\n", ptr[i][j]);
        }
    }
}

---

(A matter of style, but your order of ROWS and COLS is a bit weird, it is more common to do int[COLS][ROWS] then for(i=0; i<COLS; i++) )