An Uncommon representation of array elements in C++ program

In this article, we will learn an uncommon representation of array elements in both C and C++.

Example of Uncommon Representation of Array Elements

Consider following code in C/C++, that is trying to print an element from an array.

#include <stdio.h>
int main() {
  int arr[2] = {0,1};
  printf("First Element = %d\n",0[arr]);
}

First Element = 0

#include <iostream>
using namespace std;

int main() {
  int arr[2] = {0,1};
  cout << "First Element = " << 0[arr] << endl;
}

First Element = 0

In the above code, we are accessing the first element of the array using the syntax 0[arr]. This is an uncommon way to access array elements, but it works because of how arrays are implemented in C/C++.

Understanding the Syntax

Normally in C and C++, the expression arr[index] will be converted to *(arr + index). Now, when you write index[arr], this will be converted as *(index + arr), just the reverse the normal syntax. Thus, both arr[index] and index[arr] refer to the same memory location and give same value as output.

// Normal syntax
arr[index]; // This is equivalent to *(arr + index)

// Uncommon syntax
index[arr]; // This is equivalent to *(index + arr)

This is a valid syntax, that is not known to many programmers. Using this types of syntax indicate the awareness of system level knowledge of arrays and pointer arithmetic of C/C++.