clojure.core/drop-last

View this page on ClojureDocs

Type: function Added: Clojure 1.0 Examples: 2 Runnable: 1

([coll] [n coll])

Return a lazy sequence of all but the last n (default 1) items in coll

Examples

by zmila on 2010-07-28 ▶

(drop-last [1 2 3 4]) ;=> (1 2 3) (drop-last -1 [1 2 3 4]) ;=> (1 2 3 4) (drop-last 0 [1 2 3 4]) ;=> (1 2 3 4) (drop-last 5 [1 2 3 4]) ;=> () ;; works differently with any seq. ;; but with some the last items become ambiguous. (drop-last 2 (vector 1 2 3 4)) ;=> (1 2) (drop-last 2 (list 1 2 3 4 )) ;=> (1 2) (drop-last 2 {:a 1 :b 2 :c 3 :d 4}) ;=> ([:a 1] [:b 2])

by Crowbrammer on 2021-03-08 . May have evaluation errors. ▶

;; Using `drop-last` vs. using `butlast` (def v [1 2 3 4]) (butlast v) ;;=> (1 2 3) (drop-last v) ;;=> (1 2 3) (drop-last 2 v) ;;=> (1 2) butlast can't do this (butlast 2 v) ;;=> ArityException (drop-last 0 v) ;;=> (1 2 3 4) (drop-last -1 v) ;;=> (1 2 3 4) ;; Performance differences: ;; - `butlast`, linear time ;; - `drop-last`, lazy, efficient, more flexible, constant time(?) ;; - `pop`, constant but requires a vector (def r (range 10000)) (time (butlast r)) ;;=> "Elapsed time: 6.379 msecs" (time (drop-last r)) ;;=> "Elapsed time: 0.0475 msecs" (time (drop-last 2 r)) ;;=> "Elapsed time: 0.0569 msecs" (time (drop-last 0 r)) ;;=> "Elapsed time: 0.098 msecs" (time (drop-last -1 r)) ;;=> "Elapsed time: 0.0615 msecs" (def r-vec (vec r)) ;; to work like `butlast` pop needs a vector (time (pop r-vec)) ;;=> "Elapsed time: 0.062 msecs"

Note by huahaiy

drop-last is lazy and implemented with map, whereas butlast is not lazy and implemented with loop and recur.

See also

• clojure.core/drop (2)
• clojure.core/drop-while (2)
• clojure.core/butlast (4)
• clojure.core/pop (2)

---

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