Access multidimensional array elements with a pointer in C++

Question

I'm trying to access the elements of a multidimensional array with a pointer in C++:

#include<iostream>

int main() {

  int ia[3][4] = {
    {0, 1, 2, 3},
    {4, 5, 6, 7},
    {8, 9, 10, 11}
  };

  int (*pia)[4] = &ia[1];
  std::cout << *pia[0] 
    << *pia[1]
    << *pia[2]
    << *pia[3]
    << std::endl;

  return 0;
}

I'm expecting *pia to be the second array in ia and therefore the output to be 4567.

However the output is 4814197056, so I'm obviously doing it wrong. How do I the access the elements in the rows correctly?

No warnings with ´-std=c++11 -Wall -Wextra -pedantic´. @i486: ´error: invalid type argument of unary ‘*’ (have ‘int’) std::cout << *pia[0]´ — Christoph90
– Christoph90, Commented Nov 28, 2014 at 10:38

Wintermute · Accepted Answer · 2014-11-28 10:36:27Z

5

As it stands, you would have to write

std::cout << (*pia)[0] ...

because [] binds more strongly than *. However, I think what you really want to do is

int *pia = ia[1];
std::cout << pia[0] 
          << pia[1]
          << pia[2]
          << pia[3]
          << std::endl;

Addendum: The reason you get the output you do, by the way, is that *pia[i] is another way of writing pia[i][0]. Since pia[0] is ia[1], pia[1] is ia[2], and pia[2] and beyond are garbage (because ia is too short for that), you print ia[1][0], ia[2][0] and then garbage twice.

answered Nov 28, 2014 at 10:36

Wintermute

44.3k5 gold badges84 silver badges85 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

moooeeeep Over a year ago

For reference: C++ Operator Precedence

Christoph90 Over a year ago

Thank you! I wasn't aware that the index operator can be used on pointers that way. So in the original code (with (*pia)[i]) I used a mixture of a pointer to int[] and array indexing whereas your suggestion relies on pointers alone (and row-major order). Are there any advantages in using "plain" pointer arithmetic here or is it a matter of style?

Wintermute Over a year ago

It looks clearer and more common. And anyway it is exactly the same as array indexing. This is because arrays decay into pointers at the drop of a hat; whenever you write arr[i], the array decays into a pointer on which pointer arithmetic then proceeds to happen.

vmcloud · Accepted Answer · 2014-11-28 10:48:25Z

1

I used the below way to print, It works well.

std::cout << (*pia)[0]
         << (*pia)[1]
         << (*pia)[2]
         << (*pia)[3] 
         << std::endl;

In precedence table of C++, [] has higher priority than *.

answered Nov 28, 2014 at 10:48

vmcloud

6524 silver badges13 bronze badges

Comments

Exceptyon · Accepted Answer · 2014-11-28 10:32:28Z

0

  int *pia[] = { &ia[1][0], &ia[1][1], &ia[1][2], &ia[1][3] };

or

    int* pia = static_cast<int*>(&ia[1][0]);
  std::cout << pia[0] << " "
    << pia[1] << " "
    << pia[2] << " "
    << pia[3] << " "
    << std::endl;

answered Nov 28, 2014 at 10:32

Exceptyon

1,60217 silver badges23 bronze badges

Collectives™ on Stack Overflow

Access multidimensional array elements with a pointer in C++

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related