Indexing an int? How does this work?

Question

The following code

#include<stdio.h>

int main()
{
    int arr[] = {10,20,30};
    cout << -2[arr];
    return 0;
}

prints -30. How? Why?

Answer 1

In your case,

 cout<<-2[arr];

gets translated as

cout<<-(arr[2]);

because,

1. array indexing boils down to pointer arithmatic, so, the position of array name and the index value can be interchanged in notation.

   ^{The linked answer is in C, but valid for C++ also.}

2. regarding the explicit ()s, you can check about the operator precedence here.

Answer 2

Look at this statement

cout << -2[arr];

First, know that even though it looks strange the following is true

2[arr] == arr[2]

That being said operator[] has higher precedence than -. So you are actually trying to invoke

-(arr[2])

Answer 3

In C and C++, 2[arr] is actually the same thing as arr[2].

Due to operator precedence, -2[arr] as parsed as -(2[arr]). This means that the entire expression evaluates to negating the 3rd element of arr, i.e. -30.