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1.3.0 permalink

1.2.0
1.3.0

contains?

Returns true if key is present in the given collection, otherwise
returns false. Note that for numerically indexed collections like
vectors and Java arrays, this tests if the numeric key is within the
range of indexes. 'contains?' operates constant or logarithmic time;
it will not perform a linear search for a value. See also 'some'.

2 Examples top

link | changes
```
;; `contains?` is straightforward for maps:
user=> (contains? {:a 1} :a)
true
user=> (contains? {:a nil} :a)
true
user=> (contains? {:a 1} :b)
false


;; It's likely to surprise you for other sequences because it's 
;; about *indices*, not *contents*:

user=> (contains? [:a :b :c] :b)
false

user=> (contains? [:a :b :c] 2)
true

user=> (contains? "f" 0)
true

user=> (contains? "f" 1)
false


;; It can be applied to non-sequences:

user=> (contains? 5 3)
false


;; Although lists are sequences, `contains?` seems to always return
;; `false` for them. (Clojure 1.1)

user=> (contains? '(1 2 3) 1) => false
```
;; `contains?` is straightforward for maps: user=> (contains? {:a 1} :a) true user=> (contains? {:a nil} :a) true user=> (contains? {:a 1} :b) false ;; It's likely to surprise you for other sequences because it's ;; about *indices*, not *contents*: user=> (contains? [:a :b :c] :b) false user=> (contains? [:a :b :c] 2) true user=> (contains? "f" 0) true user=> (contains? "f" 1) false ;; It can be applied to non-sequences: user=> (contains? 5 3) false ;; Although lists are sequences, `contains?` seems to always return ;; `false` for them. (Clojure 1.1) user=> (contains? '(1 2 3) 1) => false
link | changes
```
;; Can be used to test set membership
user=> (def s #{"a" "b" "c"})

user=> (contains? s "a")
true

user=> (contains? s "z")
false
```
;; Can be used to test set membership user=> (def s #{"a" "b" "c"}) user=> (contains? s "a") true user=> (contains? s "z") false

Source clojure/core.clj:1377 top

(defn contains?
  "Returns true if key is present in the given collection, otherwise
  returns false.  Note that for numerically indexed collections like
  vectors and Java arrays, this tests if the numeric key is within the
  range of indexes. 'contains?' operates constant or logarithmic time;
  it will not perform a linear search for a value.  See also 'some'."
  {:added "1.0"
   :static true}
  [coll key] (. clojure.lang.RT (contains coll key)))

Vars in clojure.core/contains?:

Used in 0 other vars

Comments top

1 comment(s) for contains?.

uvtc

link

As Rich points out on the ML:

contains? tells you whether or not get will succeed. It is not a "rummager".

contains? and get abstract over fast lookup.

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