Accessing multi-dimensional arrays in C using pointer notation

arrays, c, multidimensional-array, pointers

Solution

here i am thinking of 2D array b as array of 1D arrays

That's the correct way to approach this, because that's indeed what it is. The C standard does not actually specify multi-dimensional arrays as some special case, but they are rather possible because an array is a type like any other. So we can have an array of arrays.

- How is the expression b[i] evaluated in case when b is 2D array? ...

- What does b evaluate to if b is a 2D array?

As for any array, `b` when used in an expression, decays into a pointer to the first element. `b[i]` is therefore equivalent to `*(b+i)`, as for any other array expression.

In this case `b` decays into an array pointer of type `int(*)[3]`.

- How is the expression b[i][j] evaluated in case when b is 2D array?

- `b` is used in an expression, so in this expression it decays into a pointer to the first element, which is an array pointer to the first array.

- `b[i]` causes pointer arithmetic to get applied to the array pointer, equivalent to `*(b+i)`. This gives array number `i`.

- At this position we have a 1D array of type `int[3]`. Since this array is part of another array, it has no identifier by itself. But for the sake of illustration, lets pretend it gets a temporary name "`tmp`". We would then have the expression `tmp[j]`, which as always decays into `*(tmp+j)`, resulting in an `int`.

Essentially the whole expression can be treated as `*(*(b+i) + j)`.

- How to access elements of a Multi-dimensional Array in C using pointers.Here i would like to know about how compiler treats a Multi-dimensional Array internally ?

As explained above, it treats it as an array of arrays. For example, you can iterate over a 2D array by using array pointers:

#include <stdio.h>

void print_array (int array[3])
{
  printf("%d %d %d\n", array[0], array[1], array[2]);
}

int main (void)
{
  int b[2][3]={ {1,2,3},{4,5,6} };
  const size_t b_size = sizeof b / sizeof *b;

  for(int(*ptr)[3] = b; ptr < b+b_size; ptr++)
  {
    print_array(*ptr);
  }
}

Problem

Original source

Related problems

• Correctly allocating multi-dimensional arrays