clojure.core/keep-indexed

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Type: function Added: Clojure 1.2 Examples: 7 Runnable: 5

([f] [f coll])

Returns a lazy sequence of the non-nil results of (f index item). Note, this means false return values will be included. f must be free of side-effects. Returns a stateful transducer when no collection is provided.

Examples

by pkolloch on 2010-08-20 ▶

(keep-indexed #(when (odd? %1) %2) [:a :b :c :d :e]) (:b :d)

by kumarshantanu on 2010-12-12 ▶

(keep-indexed #(if (pos? %2) %1) [-9 0 29 -7 45 3 -8]) (2 4 5) ;; f takes 2 args: 'index' and 'value' where index is 0-based ;; when f returns nil the index is not included in final result (keep-indexed (fn [idx v] (if (pos? v) idx)) [-9 0 29 -7 45 3 -8]) (2 4 5)

by yasuto on 2011-05-25 . May have evaluation errors. ▶

(defn position [x coll & {:keys [from-end all] :or {from-end false all false}}] (let [all-idxs (keep-indexed (fn [idx val] (when (= val x) idx)) coll)] (cond (true? from-end) (last all-idxs) (true? all) all-idxs :else (first all-idxs)))) (position [1 1] [[1 0][1 1][2 3][1 1]]) 1 (position [1 1] [[1 0][1 1][2 3][1 1]] :from-end true) 3 (position [1 1] [[1 0][1 1][2 3][1 1]] :all true) (1 3) (def foo (shuffle (range 10))) #'user/foo foo (5 8 9 1 2 7 0 6 3 4) (position 5 foo) 0 (position 0 foo) 6

by PetrGlad on 2015-10-09 ▶

;; Simpler version of "position" above ;; Get position of first element that satisfies the predicate (let [predicate #(= 1 %) sequence [3 2 4 1 5 6 7]] (first (keep-indexed (fn [i x] (when (predicate x) i)) sequence))) ;; The same logic but implemented with map-indexed (let [predicate #(= 1 %) sequence [3 2 4 1 5 6 7]] (some identity (map-indexed (fn [i x] (when (predicate x) i)) sequence)))

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

(require '[clojure.pprint :as p] '[clojure.java.jdbc :as j]) (def db {:classname "org.sqlite.JDBC" :subprotocol "sqlite" :subname "/home/liuwensui/Downloads/chinook.db"}) (def orders (j/query db "select billingcountry as country, count(*) as orders from invoices group by billingcountry;")) (def clist '("USA" "India" "Canada" "France") (def country (map #(get % :country) orders)) (p/print-table (map #(nth orders %) (flatten (pmap (fn [c] (keep-indexed (fn [i v] (if (= v c) i)) country)) clist)))) ;| :country | :orders | ;|----------+---------| ;| USA | 91 | ;| India | 13 | ;| Canada | 56 | ;| France | 35 |

by gluer on 2018-11-23 ▶

;; a simple example (map-indexed #(when (< % 2) (str % %2)) [:a :b :c]) ;;=> ("0:a" "1:b" nil) (keep-indexed #(when (< % 2) (str % %2)) [:a :b :c]) ;;=> ("0:a" "1:b")

by kevinmungai on 2020-07-11 ▶

;; returns a new list by removing the elements at odd positions (defn filter-list-by-position [lst] (into [] (keep-indexed #(when (odd? %1) %2)) ;; transducer lst)) ;; 0 1 2 3 4 5 6 8 (def nums [2 5 3 4 6 7 9 8]) (filter-list-by-position nums) ;;=> [5 4 7 8]

See also

• clojure.core/map-indexed (4)
• clojure.core/keep (8)

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