frequencies & group-by in Clojure

Word counts with frequencies, then subway stops bucketed by group-by.

(require '[clojure.string :as str])

(def notice "the tide is out the boats are out the gulls are loud")

;; frequencies counts occurrences and returns a map of item to count
(def word-counts (frequencies (str/split notice #"\s+")))
(println (get word-counts "the") (get word-counts "out"))

(def stops [{:stop "Wharf" :line "A"} {:stop "Mill" :line "B"}
            {:stop "Quay" :line "A"} {:stop "Ash" :line "C"}])

;; group-by builds a map from key fn result to a vector of the matches
(def by-line (group-by :line stops))
(println (map :stop (get by-line "A")))

;; group sizes are group-by plus a walk over the values
(println (reduce-kv (fn [m k v] (assoc m k (count v))) {} by-line))

How it works

1. (frequencies (str/split notice #"\s+")) counts each word into a map of item to count.
2. (group-by :line stops) returns a map from line name to a vector of the matching stops.
3. reduce-kv walks by-line and swaps each vector for its count.

Keywords and builtins used here

• assoc
• count
• def
• fn
• frequencies
• get
• map
• println
• require

The run, in numbers

Lines

17

Characters to type

679

Tokens

120

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 103 seconds.

Type this snippet

Step 1 of 3 in Seq toolkit, step 16 of 27 in Language basics.

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