How does the expression -2[array] work?

Question

I ran into this beauty, but I can't really understand it!

#include <iostream>

using namespace std;

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

  return 0;
}

It prints -30. Thank you in advance

Edit: Not a duplicate of this question because of the "-" sign.

I remember there was a duplicate I've seen for this question. Unfortunately it's not on my favorites list. — πάντα ῥεῖ, Jan 12 '16 at 17:32
@πάνταῥεῖ, i believe, it is different because of the '-'. — SergeyA, Jan 12 '16 at 17:35
@πάνταῥεῖ, simply because you've comented saying that it was duplicate, I am trying to explain why I believe it is not. — SergeyA, Jan 12 '16 at 17:42
Writing `-2[array]` is what's evil. It's ugly and appears to do something other than what it does. — Rob K, Jan 12 '16 at 21:21

score 12 · Accepted Answer · edited May 23 '17 at 11:44

12

This is funny and simple. -array[2] is the same as -*(array + 2), which is the same as -*(2 + array) which is the same as -2[array], which is -30.

There is already a duplicate for general case of using square brackets with arrays (With arrays, why is it the case that a[5] == 5[a]?), but the quirk here is the unary - operator in front.

It might seem intuitive to assume that the actual array index would be -2, like array[-2].

But this is not happening due to operator precedence rules:

operator [] has higher precedence than unary -, and as such is applied first.

I've shown the tranformation with 'conventional' array subscription to make this more intuitive

as we do not negate the array before subscripting it, we do not negate 2 the same way.

edited May 23 '17 at 11:44

Community

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answered Jan 12 '16 at 17:30

SergeyA

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Funny thing is, I understand all the equivalent versions that you mentioned. I just don't "see" why this works. Thank you both! – Aerozeek Jan 12 '16 at 18:12
Oh.. and by the way, thank you for the link to the related question. – Aerozeek Jan 12 '16 at 18:16

How does the expression -2[array] work?

1 Answers1