clojure.core/pop

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Type: function Added: Clojure 1.0 Examples: 4 Runnable: 2

([coll])

For a list or queue, returns a new list/queue without the first item, for a vector, returns a new vector without the last item. If the collection is empty, throws an exception. Note - not the same as next/butlast.

Examples

by pkolloch on 2010-08-20 ▶

(peek [1 2 3]) 3 (pop [1 2 3]) [1 2] (peek '(1 2 3)) 1 (pop '(1 2 3)) (2 3)

by HomoEfficio on 2017-02-03 . May have evaluation errors. ▶

(peek ()) nil (pop ()) IllegalStateException Can't pop empty list (peek []) nil (pop []) IllegalStateException Can't pop empty vector (peek (clojure.lang.PersistentQueue/EMPTY)) nil (into [] (pop (clojure.lang.PersistentQueue/EMPTY))) [] ;; Can pop empty Queue

by reborg on 2018-02-09 . May have evaluation errors. ▶

;; Use a vector as a LIFO stack to check for balanced brackets (require '[clojure.set :refer [map-invert]]) (defn balance [form] (let [brackets {\[ \] \( \) \{ \}} scan (fn [q x] (cond (brackets x) (conj q x) ((map-invert brackets) x) (if (= (brackets (peek q)) x) (pop q) (throw (ex-info (str "Unmatched delimiter " x) {}))) :else q))] (reduce scan [] form))) (balance "(let [a (inc 1]) (+ a 2))") ;; ExceptionInfo Unmatched delimiter ] (balance "(let [a (inc 1)] (+ a 2))") ;; []

by ibercode on 2018-02-15 ▶

;;basic example on vector and list ;;pop on vector returns a new vector removing the last element (pop [1 2 3]) ;; [1 2] ;;pop on list returns a new list removing the first element (pop '(1 2 3)) ;; (2 3)

Note by tomby42

Small reminder:

Do not work for arbitrary seq but just for persistent types implementing clojure.lang.IPersistentStack (like clojure.lang.Persistent*).

Example:

user> (pop (cons 1 '()))
; Evaluation aborted.

do not work because type is clojure.lang.Cons but

user> (pop (conj '() 1))
()

works because type is clojure.lang.PersistentList.

See also

• clojure.core/peek (3)
• clojure.core/rest (6)
• clojure.core/conj (9)
• clojure.core/butlast (4)
• clojure.core/pop!

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