Clojure swapping elements in an input sequence

Question

How do you swap adjacent elements in an input sequence using clojure. [10 15 20 25] ---> [15 10 25 20] [:q :e :g :t :p] ---> [:e :q :t :g :p]

this is how i did it, but pretty sure there are better ways to do it!

(defn switch [s]
(loop [[a b & rest] s
       result []]
  (if (empty? rest)
    (cond
        ;;empty sequence
        (empty? s) result
        ;;odd sequence
        (nil? b) (conj result a)
        ;;even sequence
        :else (conj result b a)
    )
    (recur rest (conj result b a))))
    )

I'm voting to close this question as off-topic because: It appears to be a simple homework problem. — Alan Thompson
– Alan Thompson, Commented Nov 27, 2018 at 2:41
@AlanThompson That is not the official Stack Overflow policy on homework questions. We care about questions that are well asked, and that is independent of whether they are homework. This one looks fine to me, as it clearly defines the desired behavior and shows an attempt. It's even a working attempt, asking for ways to do it better. — amalloy
– amalloy, Commented Nov 27, 2018 at 19:36

akond · Accepted Answer · 2018-11-27 06:46:20Z

2

(let [A [:q :e :g :t :p]]
    (->> A
         (partition-all 2)
         (mapcat reverse)))

edited Nov 27, 2018 at 6:46

answered Nov 27, 2018 at 4:24

akond

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3 Comments

leetwinski Over a year ago

(map reverse) + flatten == (mapcat reverse)

akond Over a year ago

Indeed. It is more succinct your way.

amalloy Over a year ago

It's not just more succinct, but more correct. flatten is (almost) never the right answer. Imagine if A were instead defined as [[1 2] [3 4]]. We'd like the result to be [[3 4] [1 2]], but with the flatten version we would get [3 4 1 2].

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