clojure.core/butlast

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

([coll])

Return a seq of all but the last item in coll, in linear time

Examples

by dpritchett on 2010-07-14 ▶

(butlast [1 2 3]) (1 2) (butlast (butlast [1 2 3])) (1) (butlast (butlast (butlast [1 2 3]))) nil

by boxie on 2010-11-06 ▶

;really slow reverse ;put the last item of the list at the start of a new list, and recur over all but the last item of the list. ;butlast acts similar to next in that it returns null for a 1-item list. (defn my-reverse [xs] (when xs (cons (last xs) (my-reverse (butlast xs)))))

by reborg on 2018-03-16 ▶

;; A version of (into) that doesn't require (comp) ;; for multiple transducers. (defn into* [to & args] (into to (apply comp (butlast args)) (last args))) (into* [] (range 10)) ;; [0 1 2 3 4 5 6 7 8 9] (into* [] (map inc) (range 10)) ;; [1 2 3 4 5 6 7 8 9 10] (into* [] (map inc) (filter odd?) (range 10)) ;; [1 3 5 7 9]

by Crowbrammer on 2021-03-08 ▶

;; `butlast` (linear time) vs. `drop-last` (lazy, efficient, more flexible, ;; constant time(?)) vs. `pop` (fast 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 miner

When using a vector, pop is faster than butlast.

Note by huahaiy

pop will throw exception if the vector is empty, whereas butlast will return nil

Note by Crowbrammer

In my example, but-last is ~100 x slower than drop-last. butlast cannot remove multiple elements from the end, but drop-last can. butlast seems eager, and drop-last seems lazy. 'butlast' will return nil when empty, and drop-last will return '() when empty.

See also

• clojure.core/first (2)
• clojure.core/rest (6)
• clojure.core/last (2)
• clojure.core/next (2)
• clojure.core/drop-last (1)
• clojure.core/take-last (1)
• clojure.core/pop (2)

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