How do I fix 'cannot initialize a variable of type int*' error when using 2D arrays in C++?

I am getting this error for this code:

> Cannot initialize a variable of type 'int *' with an lvalue of type 'int[3][3]'

#include <iostream>
using namespace std;

int main(){
    int mat_1[3][3]={1,2,3,4,5,6,7,8,9};
    int mat_2[3][3]={10,11,12,13,14,15,16,17,18};
    int dest_mat[3][3];
    int *ptr_1 = mat_1;
    int *ptr_2 =mat_2;
    int *ptr_3=dest_mat;

I am expecting to just create a pointer for each of the following. I even tried placing the ampersand (&) before the matrix names, but still no different.

In most contexts, when you refer to an array by its name alone, it decays into a pointer to its 1st element.

mat_1 is an array of 3 elements, where each element is an array of 3 ints. So, when you do:

int *ptr_1 = mat_1;

mat_1 decays into a pointer to its 1st element, ie a pointer to an int[3] array (aka int(*)[3]). A pointer to an array is a very different type than a pointer to a single int, thus the error you are seeing.

Same with ptr_2 and ptr_3.

Depending on what you are attempting to accomplish, you need to either:

• assign ptr_1 (and ptr_2 and ptr_3) the address of the 1st int of the 1st array element, eg:

  int *ptr_1 = &mat_1[0][0];

  which can be simplified as follows (letting the inner array decay):

  int *ptr_1 = mat_1[0];

• change the type of ptr_1 (and ptr_2 and ptr_3) to be a pointer to an array of ints, not a pointer to a single int, eg:

  int (*ptr_1)[3] = mat_1;