How-to pass reference to a 2D-array in C/C++?

Question

I have written a little program to outline my problem. It should print the values of the "ary" to the screen. If I use the "ary" directly, no problem. But I want to access "ary" through a reference:

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

#define NUM_X (4)
#define NUM_Y (4)

int ary[NUM_X][NUM_Y] =
{
    {11, 12, 13, 14},
    {21, 22, 23, 24},
    {31, 32, 33, 34},
    {41, 42, 43, 44},
};

void printAry(int *ppAry[])
{
    int i, j;
    for(i = 0; i < NUM_X; ++i)
    {
        for(j = 0; j < NUM_Y; ++j)
            //printf("%d\t", ary[i][j]);
            printf("%d\t", ppAry[i][j]);

        printf("\n");
    }
}

int main()
{
    int **ppAry = &(ary[0][0]);

    printAry(ppAry);

    return 0;
}

If you run the above program you get a runtime error, because ppAry is not the right reference, I guess. How do I change ppAry to make this work?

See [this answer](http://stackoverflow.com/a/7307699/912144) for why what you did is wrong and what should be done instead. — Shahbaz, Oct 17 '13 at 08:46

score 2 · Answer 1 · answered Oct 17 '13 at 09:29

Since you have constants that define the array dimensions, I would advice against using self-made indexing (the solution provided in other answers), because that is the compiler's job and it makes your program harder to read and maintain. Just change printAry declaration from:

void printAry(int *ppAry[]) { ... }

To:

void printAry(int ppAry[][NUM_Y]) { ... }

You can include the first dimension as well, but the compiler will ignore it. The compiler needs the other dimensions because ppAry[i][j] is translated to (&ppAry[0][0])[i*NUM_Y + j]

Now, instead of calling printAry with ppAry, you can call it with ary directly.

score 0 · Answer 2 · answered Oct 17 '13 at 09:03

You need to pass the address of start of the array, and increment the address through elements to get the address of an element [i, j]; then you can print the value in that location.

void printAry(int *ppAry)
{
    int i, j;
    for(i = 0; i < NUM_X; i++)
    {
        for(j = 0; j < NUM_Y; j++)
            printf("%d\t", ppAry[i * NUM_Y + j]);

        printf("\n");
    }
}

How-to pass reference to a 2D-array in C/C++?

2 Answers2