clojure.core/reduce-kv

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Type: function Added: Clojure 1.4 Examples: 10 Runnable: 1

([f init coll])

Reduces an associative collection. f should be a function of 3 arguments. Returns the result of applying f to init, the first key and the first value in coll, then applying f to that result and the 2^nd key and value, etc. If coll contains no entries, returns init and f is not called. Note that reduce-kv is supported on vectors, where the keys will be the ordinals.

Examples

by viksit on 2014-10-14 . May have evaluation errors. ▶

Let's assume you want to apply a function to a vector of maps, Input: [{:a 1 :b 2} {:a 3 :b 4}] such that all vals are incremented by 1. Result: [{:a 2 :b 3} {:a 4 :b 5}] An easy way to do so is using reduce-kv, (def vector-of-maps [{:a 1 :b 2} {:a 3 :b 4}]) (defn update-map [m f] (reduce-kv (fn [m k v] (assoc m k (f v))) {} m)) (map #(update-map % inc) vector-of-maps) => ({:b 3, :a 2} {:b 5, :a 4})

by Dimagog on 2015-02-04 . May have evaluation errors. ▶

;; Swap keys and values in a map (reduce-kv #(assoc %1 %3 %2) {} {:a 1 :b 2 :c 3}) {1 :a, 2 :b, 3 :c}

by Luckvery on 2015-10-17 . May have evaluation errors. ▶

;; Swap keys with values, only if values are not empty, ;; while turning values into proper keys (def someMap { :foo "food", :bar "barista", :baz "bazaar"}) (defn swap [someMap] (reduce-kv (fn [m k v] (if (empty? v) m (assoc m (keyword v) (name k)))) {} someMap)) (swap someMap) => {:food "foo", :barista "bar", :bazaar "baz"}

by puppybits on 2015-11-07 . May have evaluation errors. ▶

;; Calculate total wins and winning streaks (def all-games [{:game 1 :won true} {:game 2 :won false} {:game 3 :won true} {:game 4 :won true}]) (reduce-kv (fn [result index game] (let [last-game (last result) wins (if (:won game) (inc (:total last-game 0)) (:total last-game)) streak (if (:won game) (inc (:streak last-game 0)) 0)] (println (assoc game :total wins :streak streak)) (conj result (assoc game :total wins :streak streak)))) [] all-games) ;; Output ;; {:game 1, :won true, :total 1, :streak 1} ;; {:game 2, :won false, :total 1, :streak 0} ;; {:game 3, :won true, :total 2, :streak 1} ;; {:game 4, :won true, :total 3, :streak 2} ;; [{:game 1, :won true, :total 1, :streak 1} {:game 2, :won false, :total 1, :streak 0} {:game 3, :won true, :total 2, :streak 1} {:game 4, :won true, :total 3, :streak 2}]

by jvanderhyde on 2016-09-08 . May have evaluation errors. ▶

;; You can define map-kv using reduce-kv, ;; to do something to every value in a map. (defn map-kv [f coll] (reduce-kv (fn [m k v] (assoc m k (f v))) (empty coll) coll)) (map-kv inc {:a 12, :b 19, :c 2}) ;;=> {:c 3, :b 20, :a 13} ;; It works on vectors, too. (map-kv inc [1 1 2 3 5]) ;;=> [2 2 3 4 6]

by overset on 2017-02-06 . May have evaluation errors. ▶

;; It works with indexes on vectors as well (reduce-kv (fn [res idx itm] (assoc res idx itm)) {} ["one" "two" "three"]) ;;=> {2 "three", 1 "two", 0 "one"}

by bsifou on 2017-02-11 ▶

(defn update-map-entries[m e] (reduce-kv (fn [r k v] (assoc r k v)) m e)) ;;user=> (update-map-entries {:a 1 :b 2 :c 3} {:a 5 :b 9}) ;;{:a 5, :b 9, :c 3} ;;user=> (update-map-entries {:a 1 :b 2 :c 3} {:a 5 :b 9 :d 8}) ;;{:a 5, :b 9, :c 3, :d 8}

by themustafabasit on 2020-08-05 . May have evaluation errors. ▶

;; increment map values (def abc-map {:a 1 :b 2 :c 3}) (seq abc-map) ;; => ([:a 1] [:b 2] [:c 3]) ;; via reduce (reduce (fn [new-map [k v]] (assoc new-map k (inc v))) {} abc-map) ;; => {:a 2, :b 3, :c 4} ;; via reduce-kv (reduce-kv (fn [new-map k v] (assoc new-map k (inc v))) {} abc-map) ;; => {:a 2, :b 3, :c 4}

by cdol on 2020-10-12 . May have evaluation errors. ▶

;; filter a coll of maps with arbitrary # of preds ;; setup coll (def people '( {:gender "male", :age-group "child", :origin "Germany", :prename "Hans"} {:gender "male", :age-group "adult", :origin "France", :prename "Jacques"} {:gender "male", :age-group "senior", :origin "Estonia", :prename "Rasmus"} {:gender "male", :age-group "adult", :origin "Poland", :prename "Jakub"} {:gender "male", :age-group "senior", :origin "Germany", :prename "Uwe"} {:gender "female", :age-group "adult", :origin "France", :prename "Amélie"} {:gender "female", :age-group "child", :origin "Estonia", :prename "Sofia"} {:gender "female", :age-group "child", :origin "Germany", :prename "Emma"} {:gender "female", :age-group "child", :origin "Estonia", :prename "Alisa"} {:gender "female", :age-group "senior", :origin "Poland", :prename "Anna"})) ;; select elements using reduce-kv & filter (defn multi-pred [coll m] (reduce-kv (fn [erg k v] (filter #(= v (k %)) erg)) coll m) ) ;; two "preds" (multi-pred people {:gender "male", :age-group "senior"}) ;; => ({:gender "male", :age-group "senior", :origin "Estonia", :prename "Rasmus"} {:gender "male", :age-group "senior", :origin "Germany", :prename "Uwe"}) ;; three "preds" (multi-pred people {:gender "female", :age-group "child", :origin "Estonia"}) ;; => ({:gender "female", :age-group "child", :origin "Estonia", :prename "Sofia"} {:gender "female", :age-group "child", :origin "Estonia", :prename "Alisa"})

by cloudbuck3t on 2021-12-13 . May have evaluation errors. ▶

;; an example to show how to add a new key to nested maps. ;; consider a map with submaps {:a {:b 1} :c {:d 2} :e {:f 3}} ;; reduce-kv keeps an accumulator that ;; starts with the value provided in init, ;; in this case an empty map. ;; you can return the original collection via reduce-kv this way (reduce-kv (fn [acc k v] (assoc acc k v)) {} {:a {:b 1} :c {:d 2} :e {:f 3}})) ;; the output is identical to the input. => {:a {:b 1} :c {:d 2} :e {:f 3}} ;; you can insert a new key-value pair into each submap ;; by using this instead, generalized as a function ;; that takes a map and a new-key and its new-key-value as inputs: (defn add-submap [m new-key new-keys-value] (reduce-kv (fn [acc k v] (assoc acc k (assoc v new-key new-keys-value))) {} m)) ;; notice how the value "v" of the key "k" is actually the submap on each key, ;; when we visit :a we are looking at the first k-v pair, ;; the key is :a, ;; and the value is {:b 1} ;; ;; so we assoc a new key into that submap (the value) ;; for each key-and-submap pairing. ;; so when you run this function => (add-submap {:a {:b 1} :c {:d 2} :e {:f 3}} :email-verified nil) ;; the result is the original data with the new key and its val in each submap. => {:a {:b 1, :email-verified nil}, :c {:d 2, :email-verified nil}, :e {:f 3, :email-verified nil}}

Note by KingMob

Currently broken for subvectors: CLJ-2065

Note by diegonayalazo

Seems to be fixed for subvectors as of release 1.11. https://clojure.atlassian.net/browse/CLJ-2065

See also

• clojure.core/reduce (22)
• clojure.core/reduced (3)

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