I see some more clever ways have been posted.
Perhaps a more interesting, general problem: detect if the input is an
arithmetic sequence at all.
Here's a few implementations to get you started.
Straightforward:
(let [diffs (map - xs (rest xs))]
(every? identity (map = diffs (rest diffs))))
Ultra-sneaky:
(= 1 (count (distinct (map - xs (rest xs)))))
Speedy Gonzales:
(let [r (next xs)
s (first r)
diff (- s (first xs))]
(loop [f s r (next r)]
(if xs
(let [s (first r)]
(if (== diff (- s f))
(recur s (next r))))
true)))
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Protege: What is this seething mass of parentheses?!
Master: Your father's Lisp REPL. This is the language of a true
hacker. Not as clumsy or random as C++; a language for a more
civilized age.
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