unable to use a 2d array via function

Question

#include <stdio.h>
void array(int *ar,int row,int col);
int main(void)
{
    int a[][2] = {
                    {1,2},
                    {3,4},
                    {5,6}
                 };
    array(&a,3,2);
}
void array(int *ar, int row, int col)
{
    for(int i=0;i<row;i++)
    {
        for(int j=0;j<col;j++)
        {
            printf("%d",*(ar+i*col+j));
        }
    }
}

What is the error ? I'm trying to pass an 2D array via function, and using the base address want to print the elements of the array?

Answer 1

A function that takes a 2D-array as an argument may have the following prototype

void foo(int arr[][<cols>]);

where <cols> is the number of columns in the array to be passed. For example

void foo(int arr[][3]);

declares a function foo that takes a 2D int array arr that has 3 columns and any number of rows in it.

As the error message shows, your function expects an int * while you are trying to pass a value of type int (*)[3][2]. These are two different types. The former is a pointer to an int while the latter is a pointer to 2D-array of ints (i.e., the type of &foo).

The following piece of code passes a 2D-array to foo (declared above)

int bar[3][3];
foo(bar);

Notice that I'm not passing &bar because that would be a pointer to my 2D-array not my 2D-array itself.

Accessing the elements of the array within foo would be as follows

arr[0][0]; // accesses the element in the zeroth row and the zeroth column

If you want your function to take a pointer to a 2D-array of ints, it may have the following prototype

void baz(int (*arr_ptr)[][<cols>]);

where <cols> is the number of columns that are in the array that is pointed to by arr_ptr. For example

void baz(int (*arr_ptr)[][3]);

declares a function baz that takes a pointer to a 2D int array arr_ptr as an argument.

The following code passes a pointer to a 2D int array to baz

int bar[3][3];
baz(&bar);

Accessing the elements of the array within baz would be as follows

(*arr_ptr)[0][0]; // accesses the element at the zeroth row and the zeroth column

Notice that I had to dereference arr_ptr first to get the array back, then I accessed the element at [0][0].