clojure.core/take

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Type: function Added: Clojure 1.0 Examples: 4 Runnable: 3

([n] [n coll])

Returns a lazy sequence of the first n items in coll, or all items if there are fewer than n. Returns a stateful transducer when no collection is provided.

Examples

by jandot on 2010-07-12 ▶

;; return a lazy seq of the first 3 items (take 3 '(1 2 3 4 5 6)) ;;=> (1 2 3) (take 3 [1 2 3 4 5 6]) ;;=> (1 2 3) ;; returns all items if there are fewer than n (take 3 [1 2]) ;;=> (1 2) (take 1 []) ;;=> () (take 1 nil) ;;=> () (take 0 [1]) ;;=> () (take -1 [1]) ;;=> ()

by citizen428 on 2010-07-30 ▶

;; similar to subvec but lazy and with seqs (take 3 (drop 5 (range 1 11))) ;;=> (6 7 8)

by bsvingen on 2015-07-02 ▶

;; Used without a collection, take will create a transducer: (def xf (take 5)) ;; We can now apply this transducer to a sequence: (transduce xf conj (range 5)) ;; => [0 1 2 3 4]

by cljlc on 2018-10-07 . May have evaluation errors. ▶

;; Note that usually more items are realized than needed. ;; In the example below we want a lazy sequence of 1 item ;; (the first item of the range), but actually the first 32 items are calculated. ;; This can be especially important when this extra realization requires a lot of ;; resources (CPU, time, memory, etc.) and at the end not needed at all. ;; Tip: try 32 and 33 instead of 1 in the take below. (let [x (map (fn [i] (println i) (Thread/sleep 100) i) (range 50))] (take 1 x)) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 (0) ;; Another interesting case (especially for debugging) when ;; an exception is thrown during this extra realization. (let [x (map (fn [i] (when (> i 40) (throw (Exception. (str i " is not accepted!"))))) (range 50))] (take 33 x)) ;; result: Exception 41 is not accepted! ;; When pmap is used instead of map, then not only one, but many exceptions might ;; be thrown during this extra realization. However, all of them are "swallowed" ;; by pmap: they are thrown inside, but never get out: (let [x (pmap (fn [i] (when (> i 40) (println "Exception is thrown...") (throw (Exception. (str i " is not accepted!"))))) (range 50))] (take 33 x)) (nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil Exception is thrown...Exception is thrown... Exception is thrown...Exception is thrown... Exception is thrown... Exception is thrown... nil nil nil nil nil nil nil nil nil nil nil nil) Exception is thrown... Exception is thrown...Exception is thrown... ;; Note: the REPL sees no exception! ;; "Normal" exceptions arrive as expected: (let [x (pmap (fn [i] (when (> i 40) (println "Exception is thrown..." i) (throw (Exception. (str i " is not accepted!"))))) (range 50))] (take 42 x)) (nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil Exception is thrown... Exception is thrown...Exception is thrown... 42 44Exception is thrown...Exception is thrown... 46 nilException is thrown... Exception is thrown...47 45 Exception is thrown... 49 43 48nil nil nil nil nil nil Exception is thrown...nil nil 41nil nil nil Exception 41 is not accepted! user/eval9941/fn--9942 (NO_SOURCE_FILE:337) nil nil nil nil nil nil ;; We can see above, that several exceptions are thrown, but only the one for i=41 ;; arrives to the REPL.

See also

• clojure.core/drop (2)
• clojure.core/take-while (3)
• clojure.core/take-last (1)
• clojure.core/take-nth (2)

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