Strange operator[] behavior

Question

When I was writing a program, I made a typo. I have wrote i[data] instead data[i]. However the program was successful compiled and worked right.

Operator[] behavior with arrays:

#include <iostream>

using namespace std;

int main()
{
  int data[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
  
  cout << data[6] << endl; // prints 6
  cout << 6[data]; // prints 6
  return 0;
}

Similar operator[] behavior with pointers:

#include <iostream>

using namespace std;

int main()
{
  char* str = "Hello, world!";
  
  cout << str[9] << endl; //prints 'r'
  cout << 9[str]; //prints 'r'
  return 0;
}

Why data[i] equals i[data]?

Answer 1

i[data] is the same as *(i + data)

data[i] is the same as *(data + i)

And data + i is equal to i + data.