466 Eric Normand Newsletter

00 custom sort order.md

Custom sort order

The sort function sorts a collection based on its "natural ordering." sort-by allows you to sort a collection based on the natural ordering of the result of some function applied to the elements. Your task is to write a function sort-with which takes a sequence of elements that define the sort order.

Example

           ; define ordering               ; collection to sort
(sort-with [:breakfast :lunch :dinner]     #{:lunch :breakfast :dinner}) ;=> (:breakfast :lunch :dinner)
(sort-with [2 3 4 :jack :queen :king :ace] [4 2 4 :king 2]) ;=> [2 2 4 4 :king]

Thanks to this site for the problem idea, where it is rated Expert in PHP. The problem has been modified.

Please submit your solutions as comments on this gist.

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Here's a more conventional version of the same idea without transducers.

(defn sort-with [ordering coll]
  (sort-by (reduce (fn [res x] (assoc! res x (count res))) (transient {}) ordering) coll))

The original problem asks for the unmatched elements to be appended to the result, in natural sort order. Just for fun, I came up with a solution for this variant:

(defn sort-with-original [order xs]
  (let [m (zipmap order (range))]
    (sort-by #(if-let [i (m %)]
                [i]
                [nil %])
             xs)))

(apply str (sort-with-original "edc" "abcdefzyx"))   ; => "edcabfxyz"

(defn sort-with
  "Sort `xs` according to the order defined by `template`.
  See https://edabit.com/challenge/5cefoardyvgEb52JB for full instructions.

   Note: the original exercise suggests that if the character isn't in the template,
  then sort it alphabetically - Eric doesn't mention this because he tries to make it work on more generic items.
  Thus we don't implement this requirement either."
  [template xs]
  ;; I'd like to use sort-by and just found proper 'key-fn'
  ;; So I think it could be something that returns an index of the element in the template?
  (sort-by (fn [item] (.indexOf (vec template) item))
           xs))

(defn sort-with [order xs]
  (sort-by (->> order (into {} (map-indexed (fn [i x] [x i]))))
           xs))