CODEWITHSUNDEEP
clojure
QuestionProgram
QuestionProgram

Reputation: 45

Remove n instances of matched elements from collection

What is the best way to remove n instances of matched elements of collection-2 from collection-1?

(let [coll-1 [8 2]
      coll-2 [8 8 8 2]

Here's what I first came up with to solve original problem:

   ...
;; (remove (set coll-1) coll-2))
;;     --> ()

But realised I must achieve:

   ...
;; (some-magic coll-1 coll-2))
;;     --> (8 8)

Clarification:

 (some-magic {8 2} [8 8 8 2]) ;;Removes 1x8 and 1x2 from vector.
 (some-magic {8 8 2} [8 8 8 2]) ;;Removes 2x8 and 1x2 from vector.

Edit:

Preserving the order is desired.

Upvotes: 3

Views: 95

Answers (2)

Mike Fikes
Mike Fikes

Reputation: 3527

Here is a lazy solution, written in the style of distinct:

(defn some-magic [count-map coll]
  (let [step (fn step [xs count-map]
               (lazy-seq
                 ((fn [[f :as xs] count-map]
                    (when-let [s (seq xs)]
                      (if (pos? (get count-map f 0))
                        (recur (rest s) (update-in count-map [f] dec))
                        (cons f (step (rest s) count-map)))))
                   xs count-map)))]
    (step coll count-map)))

The first argument needs to be a map indicating how many of each value to remove:

(some-magic {8 1, 2 1} [8 8 8 2]) ;; Removes 1x8 and 1x2 
;=> (8 8)

(some-magic {8 2, 2 1} [8 8 8 2]) ;; Removes 2x8 and 1x2
;=> (8)

Here is an example dealing with falsey values and infinite input:

(take 10 (some-magic {3 4, 2 2, nil 1} (concat [3 nil 3 false nil 3 2] (range))))
;=> (false nil 0 1 4 5 6 7 8 9)

Upvotes: 3

Arthur Ulfeldt
Arthur Ulfeldt

Reputation: 91544

I don't see any of the built in sequence manipulation functions quite solving this, though a straitforward loop can build the result nicely:

user> (loop [coll-1 (set coll-1) coll-2 coll-2 result []]
        (if-let [[f & r] coll-2]
          (if (coll-1 f)
            (recur (disj coll-1 f) r result)
            (recur coll-1 r (conj result f)))
          result))
[8 8]

Upvotes: 2

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