C Interview Questions and Answers


How can this be legal C?

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)) (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

char *tmpptr = "abcdef";

... tmpptr[5] ...

Posted by:Richards