clojure.core/conj

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Type: function Added: Clojure 1.0 Examples: 11 Runnable: 9

([] [coll] [coll x] [coll x & xs])

conj[oin]. Returns a new collection with the xs 'added'. (conj nil item) returns (item). (conj coll) returns coll. (conj) returns []. The 'addition' may happen at different 'places' depending on the concrete type.

Examples

by MrHus on 2010-07-18 ▶

;; notice that conjoining to a vector is done at the end (conj [1 2 3] 4) ;;=> [1 2 3 4] ;; notice conjoining to a list is done at the beginning (conj '(1 2 3) 4) ;;=> (4 1 2 3) (conj ["a" "b" "c"] "d") ;;=> ["a" "b" "c" "d"] ;; conjoining multiple items is done in order (conj [1 2] 3 4) ;;=> [1 2 3 4] (conj '(1 2) 3 4) ;;=> (4 3 1 2) (conj [[1 2] [3 4]] [5 6]) ;;=> [[1 2] [3 4] [5 6]] ;; conjoining to maps only take items as vectors of length exactly 2 (conj {1 2, 3 4} [5 6]) ;;=> {5 6, 1 2, 3 4} (conj {:firstname "John" :lastname "Doe"} {:age 25 :nationality "Chinese"}) ;;=> {:firstname "John", :lastname "Doe", :age 25, :nationality "Chinese"} ;; conj on a set (conj #{1 3 4} 2) ;;=> #{1 2 3 4}

by john.r.woodward on 2012-07-19 ▶

;; conjoin shows similar behaviour to cons ;; The main difference being that conj works on collections ;; but cons works with seqs. (conj ["a" "b" "c"] ["a" "b" "c"] ) ;;=> ["a" "b" "c" ["a" "b" "c"]]

by jamesqiu on 2012-10-08 ▶

;; conjoin nil with x or xs (conj nil 3) ;;=> (3) (conj nil 3 4) ;;=> (4 3)

by Alan Thompson on 2014-02-14 . May have evaluation errors. ▶

;; maps and sets are treated differently (conj {1 2} {3 4}) ;;=> {3 4, 1 2} ; the contents of {3 4} are added to {1 2} (conj #{1 2} #{3}) ;;=> #{1 2 #{3}} ; the whole set #{3} is added to #{1 2} (clojure.set/union #{1 2} #{3}) ;;=> #{1 2 3} ; must use (clojure.set/union) to merge sets, not conj

by fordsfords on 2014-12-02 . May have evaluation errors. ▶

;; When conjoining into a map, vector pairs may be provided: (conj {:a 1} [:b 2] [:c 3]) ;;=> {:c 3, :b 2, :a 1} ;; Or maps may be provided, with multiple pairings: (conj {:a 1} {:b 2 :c 3} {:d 4 :e 5 :f 6}) ;;=> {:f 6, :d 4, :e 5, :b 2, :c 3, :a 1} ;; But multiple pairings cannot appear in vectors: (conj {:a 1} [:b 2 :c 3]) ;;=> IllegalArgumentException Vector arg to map conj must be a pair... ;; And pairs may not be provided in lists: (conj {:a 1} '(:b 2)) ;;=> ClassCastException ...Keyword cannot be cast to ...Map$Entry...

by bsima on 2016-01-25 ▶

;; Useful snippet: "merge" two or more vectors with `(comp vec flatten conj)` (let [a [{:a "hi"} {:b "hey"}] b [{:c "yo"} {:d "hiya"}] c [{:e "bonjour"}]] ((comp vec flatten conj) a b c)) ;;=> [{:a "hi"} {:b "hey"} {:c "yo"} {:d "hiya"} {:e "bonjour"}]

by yinwuzhe on 2016-02-22 ▶

(conj nil 1) ;=>(1) (conj nil [1 2]) ;=>([1 2])

by edcaceres on 2016-11-22 ▶

;; Conj new-element into nested structures "conj-in" (def db {:users [{:name "Eduardo"}]}) (def new-element {:name "Eva"}) (assoc db :users (conj (:users db) new-element)) ;; => {:users [{:name "Eduardo"} {:name "Eva"}]}

by phreed on 2017-11-13 ▶

;; implement stack semantics with conj, peek and pop. ;; we start with a list (def stack '(2 1 0)) (peek stack) ;; => 2 (pop stack) ;; => (1 0) (type (pop stack)) ;; => cljs.core/List ;; push = conj (conj stack 3) ;; => (3 2 1 0) (type (conj stack 3)) ;; => cljs.core/List ;; now let us try a vector (def stack [0 1 2]) (peek stack) ;; => 2 (pop stack) ;; => [0 1] (type (pop stack)) ;; => clojure.lang.PersistentVector ;; push = conj (conj stack 3) ;; => [0 1 2 3] (type (conj stack 3)) ;; => clojure.lang.PersistentVector

by berczeck on 2020-08-24 ▶

;; Based on examples from the great book CLOJURE for the BRAVE and TRUE ;; https://www.braveclojure.com/core-functions-in-depth/ (defn add-first-vec [target addition] (apply conj (if (vector? addition) addition [addition]) target)) (add-first-vec [3 4 5 6] 1) ;; [1 3 4 5 6] (add-first-vec [3 4 5 6] [1 2]) ;; [1 2 3 4 5 6]

by tomdl89 on 2023-03-09 ▶

;; conj also has other arities: (conj) ;; => [] (conj [1 2 3]) ;; => [1 2 3] ;; Which are particularly useful when transducing, as the nullary form can be ;; used to produce init, and the unary form for completion

Note by me7

list prepend vector append

See also

• clojure.core/cons (4)
• clojure.core/into (9)
• clojure.core/peek (3)
• clojure.core/pop (2)
• clojure.core/concat (7)
• clojure.core/conj! (2)
• clojure.core/assoc (8)
• clojure.core/disj (2)

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