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. 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 (see question 20.36).
References:
Rationale Sec. 3.3.2.1
H&S Sec. 5.4.1 p. 124, Sec. 7.4.1 pp. 186-7
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