"abcdef"[3] == 3["abcdef"], but why?

[William B. Tyler]

In article <781 at cc.helsinki.fi> TEITTINEN at cc.helsinki.fi writes:
>
>As I was trying to figure out what the cryptic maze program (posted to
>net some days ago) does, I ran into a very interesting feature of C. The
>program exploiting the fact that the following equation is true
>
>         "abcdef"[3] == 3["abcdef"]   (both equal to 'd')
>
>In fact, not only the values are the same. If the string "abcdef" was
>replaced by a pointer to char, the expressions would refer to the same
>memory location! This can't be a compiler-dependent feature (or bug)
>because the maze program runs correctly on various machines.
>
>Could someone explain to me what a C compiler does when it runs into
>expression 3["abcdef"]? 

This is a standard, though little-known part of the C language.
The expression   a[b]  is defined to mean the same thing as
*(a+b).  The result you obtained follows naturally from the rules
for pointer arithmetic.

Bill Tyler
-- 
Bill Tyler				...(tolerant|hpda)!procase!tyler