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carraysvariablesintegerpuzzle

Puzzle: "[] is symmetric"?

I am trying to solve this enigma:

int direction = 1;
char direction_name = direction["nsew"];

Like why does it compile on a first place, what exactly is it trying to prove and what is it used for ?

I spotted this code in a site with no relevant explanation other than "[] is symmetric". I mean what am I supposed to do with this information..

Solution

  • This code is equivalent to:

    #include <stdio.h>

int main(void) {
    char direction_name = 1["nsew"];
    printf("[%c]\n",direction_name);
    return 0;
}

    which will print [s], because 

    char direction_name = 1["nsew"]; // "nsew" is string literal, i.e. an array
                                 // of 5 chars 'n' 's' 'e' 'w' '\0'

    is equal to 

    char direction_name = "nsew"[1];

    and also to

    char direction_name = *("nsew" +1);

    Therefore [] is symmetric in a sense that

    x[y] == y[x] // if conditions given in C standard 6.5.2.1/2 are met

    You can think of [] as an symmetric relation in algebra (under assumptions from 6.5.2.1 § 2):

    enter image description here

    Or you may think about [] as a linear mapping (transformation, functional) between reflexive (Banach) linear spaces V and V** if you wish:

    x[f] = [f,x] = f[x]

    C standard n1124 6.5.2.1 § 2 Array subscripting (emphasis main)

    A postfix expression followed by an expression in square brackets [] is a subscripted designation of an element of an array object. The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).