you can do this using partition.
let's assume I first define a
user=> (def a [:w :n :e :s])
#'user/a
user=> (partition 2 1 (conj a (first a)))
((:w :n) (:n :e) (:e :s) (:s :w))
gives you the pairs you need.
then you just need to turn it into hash-map
by doing
(map #(apply hash-map %) (partition 2 1 (conj a (first a))))
On Mon, Aug 9, 2010 at 3:09 PM, Alan <a...@malloys.org> wrote:
> Hi all, I'm new to the group; I have some experience with both CL and
> Java, though it's been a while for each. Anyway I really like Clojure
> as a way of combining the best parts of the two languages, but I'm
> still getting the hang of it and there are often things that confuse
> me.
>
> For example, I wanted to define a ring function, which takes as input
> N objects, and returns a hash table mapping object N to object N+1
> (mod N). I intended to use this to describe a compass object:
> (ring :w :n :e :s) should result in {:w :n, :n :e, :e :s, :s :w}.
>
> I could have done this with basic recursion or as a list comprehension
> using (for), (count), and (rem), but it seemed there must be a more
> elegant solution with lazy sequences, like maybe combining cycle and
> map to gloss over the N==0 wraparound issue. What I came up with was
> frankly a monstrosity; I don't have the source with me at work, but it
> looked roughly like:
>
> (defn ring [& elts]
> (apply assoc {}
> (map #(list
> %1
> (fnext (drop-while
> (comp (partial or
>
> (partial not= %1)
>
> nil))
> (cycle elts))
> elts))))
>
> Since then I've realized I could have used nth and map-indexed to get
> a less ugly result, but I was baffled by the awkwardness of drop-
> while: is there a reason it demands nil or not-nil, instead of
> treating false and nil as logical false? Converting false to nil was a
> real bear (and retyping this from memory I'm pretty sure my syntax for
> comp/partial/or is wrong somewhere), and in my experience clojure is
> too clever make me do crap like this; what am I missing?
>
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