Basic and Advance C Question:

I came across some ``joke'' code containing the ``expression'' 5["abcdef"] . How can this be legal C?

Yes, Virginia, array subscripting is commutative in C. This curious fact follows from the pointer definition of array subscripting, namely that a[e] is identical to *((a)+(e)), for any two expressions a and e, as long as one of them is a pointer expression and one is integral. The ``proof'' looks like

a[e]
*((a) + (e)) (by definition)
*((e) + (a)) (by commutativity of addition)
e[a] (by definition)

This unsuspected commutativity is often mentioned in C texts as if it were something to be proud of, but it finds no useful application outside of the Obfuscated C Contest .
Since strings in C are arrays of char, the expression "abcdef"[5] is perfectly legal, and evaluates to the character 'f'. You can think of it as a shorthand for
char *tmpptr = "abcdef";

... tmpptr[5] ...