How to pass a 2d array through pointer in c

Question

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Passing a pointer representing a 2D array to a function in C++

I am trying to pass my 2-dimensional array to a function through pointer and want to modify the values.

#include <stdio.h>

void func(int **ptr);

int main() {
    int array[2][2] = {
        {2, 5}, {3, 6}
    };

    func(array);

    printf("%d", array[0][0]);
    getch();
}

void func(int **ptr) {
    int i, j;
    for (i = 0; i < 2; i++) {
        for (j = 0; j < 2; j++) {
            ptr[i][j] = 8;
        }
    }
}

But the program crashes with this. What did I do wrong?

Use proper types, `void func(int (*ptr)[2])`, or pass an array of pointers. — Daniel Fischer, Dec 23 '12 at 15:10
[How do I use arrays in C++?](http://stackoverflow.com/q/4810664/1202636) — effeffe, Dec 23 '12 at 15:13
Arrays are not pointers. Pointers are not arrays. What you did wrong is not having repeated this to yourself until you stopped doubting it. — Kerrek SB, Dec 23 '12 at 15:16

Joseph Mansfield · Answer 1 · 2012-12-23T15:18:42.077

Your array is of type int[2][2] ("array of 2 array of 2 int") and its name will decay to a pointer to its first element which would be of type int(*)[2] ("pointer to array of 2 int"). So your func needs to take an argument of this type:

void func(int (*ptr)[2]);
// equivalently:
// void func(int ptr[][2]);

Alternatively, you can take a reference to the array type ("reference to array of 2 array of 2 int"):

void func(int (&ptr)[2][2]);

Make sure you change both the declaration and the definition.

score 3 · Accepted Answer · answered Dec 23 '12 at 15:14

3

It crashes because an array isn't a pointer to pointer, it will try reading array values as if they're pointers, but an array contains just the data without any pointer.
An array is all adjacent in memory, just accept a single pointer and do a cast when calling the function:

func((int*)array);

...

void func(int *ptr) {
    int i, j;
    for (i = 0; i < 2; i++) {
        for (j = 0; j < 2; j++) {
            ptr[i+j*2]=8;
        }
    }
}

answered Dec 23 '12 at 15:14

Ramy Al Zuhouri

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Thanks. You made my day. But how to set the i and j if I want to store different things in an array ?? – Sayed Hussain Mehdi Dec 23 '12 at 17:38
I haven't understood what you mean. – Ramy Al Zuhouri Dec 23 '12 at 17:42

How to pass a 2d array through pointer in c

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