How do you access the elements of double dimension arrays?

Answer 1

Working with two-dimensional arrays is really no different to working with a one-dimensional array. A two-dimensional array is really just a one-dimensional array where every element is a one-dimensional array.

int x[4];

Here, x is a one-dimensional array of 4 elements of type int.

int y[3][4];

Here, y is two-dimensional, however it's actually a one-dimensional array of 3 elements where each element is exactly the same type as x; an array of 4 elements of type int.

The biggest problem with multi-dimensional arrays is when we pass arrays into functions. An array implicitly decays to a pointer so we lose all information regarding the dimensions when we pass them to functions. Thus we have to pass the dimensions as separate arguments. However, a two-dimensional array does just decay to a point it decays to a pointer-to-pointer. That is, each dimension adds another level of indirection.

The following example demonstrates how to pass a multi-dimensional array to a function:

#include<stdio.h>

void print2d (int* a, unsigned rows, unsigned cols) {

for (unsigned r=0; r<rows; ++r) {

for (unsigned c=0; c<cols; ++c)

printf ("%d\t", a[r * cols + c]);

printf ("\n");

}

}

int main()

{

int x[3][4] = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}};

print2d (&x[0][0], 3, 4);

return 0;

}

Note that we don't have this problem in C++ because we can use std::array and std::vector objects, which are much easier to work with and just as efficient as any built-in array.