clojure.core/group-by

View this page on ClojureDocs

Type: function Added: Clojure 1.2 Examples: 8 Runnable: 7

([f coll])

Returns a map of the elements of coll keyed by the result of f on each element. The value at each key will be a vector of the corresponding elements, in the order they appeared in coll.

Examples

by mvonrohr on 2010-07-14 ▶

;; group strings by their length (group-by count ["a" "as" "asd" "aa" "asdf" "qwer"]) ;;=> {1 ["a"], 2 ["as" "aa"], 3 ["asd"], 4 ["asdf" "qwer"]} ;; group integers by a predicate (group-by odd? (range 10)) ;;=> {false [0 2 4 6 8], true [1 3 5 7 9]}

by Brool on 2010-09-21 ▶

;; group by a primary key (group-by :user-id [{:user-id 1 :uri "/"} {:user-id 2 :uri "/foo"} {:user-id 1 :uri "/account"}]) ;;=> {1 [{:user-id 1, :uri "/"} ;; {:user-id 1, :uri "/account"}], ;; 2 [{:user-id 2, :uri "/foo"}]}

by Canna71 on 2016-08-21 ▶

;; group by multiple criteria (def words ["Air" "Bud" "Cup" "Awake" "Break" "Chunk" "Ant" "Big" "Check"]) (group-by (juxt first count) words) ;;{[\A 3] ["Air" "Ant"], ;;[\B 3] ["Bud" "Big"], ;;[\C 3] ["Cup"], ;;[\A 5] ["Awake"], ;;[\B 5] ["Break"], ;;[\C 5] ["Chunk" "Check"]}

by pavanred on 2017-02-07 ▶

(group-by :category [{:category "a" :id 1} {:category "a" :id 2} {:category "b" :id 3}]) ;;{"a" [{:category "a", :id 1} {:category "a", :id 2}], ;; "b" [{:category "b", :id 3}]} (group-by #(get % :category) [{:category "a" :id 1} {:category "a" :id 2} {:category "b" :id 3}]) ;;{"a" [{:category "a", :id 1} {:category "a", :id 2}], ;; "b" [{:category "b", :id 3}]} (defn my-category [item] (get item :category)) ;;#'user/my-category (group-by my-category [{:category "a" :id 1} {:category "a" :id 2} {:category "b" :id 3}]) ;;{"a" [{:category "a", :id 1} {:category "a", :id 2}], ;; "b" [{:category "b", :id 3}]}

by statcompute on 2018-06-03 . May have evaluation errors. ▶

(require '[ultra-csv.core :refer [read-csv]]) (def ds (read-csv "/home/liuwensui/Downloads/nycflights.csv")) (map (fn [x] {:year (first (key x)) :month (last (key x)) :flights (count (val x))}) (group-by (juxt :year :month) ds))

by MicahElliott on 2020-02-11 ▶

;; Find anagrams (def words ["meat" "mat" "team" "mate" "eat" "tea"]) (group-by set words) ;;=> {#{\a \e \m \t} ["meat" "team" "mate"], ;; #{\a \m \t} ["mat"], ;; #{\a \e \t} ["eat" "tea"]}

by jimka2001 on 2024-02-01 ▶

;; one way to iterate across return value of group-by (for [[k v] (group-by (fn [n] (mod n 3)) (range 20))] (list k v)) ;; -> ((0 [0 3 6 9 12 15 18]) ;; (1 [1 4 7 10 13 16 19]) ;; (2 [2 5 8 11 14 17]))

by pikariop on 2026-04-22 ▶

(let [items [{:id 1 :name "primary"} {:id 2 :name "secondary" :parent {:id 1}} {:id 3 :name "secondary" :parent {:id 1}} {:id 4 :name "also primary"}]] (group-by #(:id (or (:parent %) %)) items)) ;=> {1 [{:id 1, :name "primary"} ; {:id 2, :name "secondary", :parent {:id 1}} ; {:id 3, :name "secondary", :parent {:id 1}}], ; 4 [{:id 4, :name "also primary"}]}

See also

• clojure.core/partition-by (3)
• clojure.core/frequencies (2)

---

Content from the matching ClojureDocs page, with authors credited on each contribution.