Why is the following program print -7 instead of throwing syntax error?
Code:
#include <stdio.h>
int main (int argc , char **argv)
{
int arr[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
int *b = arr + 6;
printf("%d\n", -1[b]);
return 0;
}
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2What will really bake your noodle is `printf("%d\n", (-1)[b]);` – WhozCraig Jun 21 '21 at 05:07
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Subscription operates on *both* operands, not just one. It's also useful for code golfing... `digit["0123456789"]` – StoryTeller - Unslander Monica Jun 21 '21 at 05:08
1 Answers
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Because the index operator a[b] is just syntactic sugar, that literally translates into *(a+b) thus invoking pointer arithmetic which is commutative on its arguments. Hence it's perfectly allowed to swap the "inside" and the "outside" of …[…].
datenwolf
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2@Cosinus `(void*)a + ...` makes no sense. And in C it's true that `pointer_value + integer_value` gives the same as `integer_value + pointer_value` – Support Ukraine Jun 21 '21 at 07:03
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1@Cosinus: But this (what you've been suggesting) is *not* what the `index` does! The C specification (6.5.2.1 Array subscripting) literally states *"The definition of the subscript operator `[]` is that `E1[E2]` is identical to `(*((E1)+(E2)))`." (sic)* – Please read the spec (and then maybe delete your misleading comment). – datenwolf Jun 21 '21 at 14:05