What is the best way to test whether a list contains a given value in Clojure?
In particular, the behaviour of contains? is currently confusing me:
(contains? '(100 101 102) 101) => false
I could obviously write a simple function to traverse the list and test for equality, but there must surely be a standard way to do this?
If you have a vector or list and want to check whether a value is contained in it, you will find that contains? does not work.
Michał has already explained why.
; does not work as you might expect
(contains? [:a :b :c] :b) ; = false
There are four things you can try in this case:
Consider whether you really need a vector or list. If you use a set instead, contains? will work.
(contains? #{:a :b :c} :b) ; = true
Use some, wrapping the target in a set, as follows:
(some #{:b} [:a :b :c]) ; = :b, which is truthy
The set-as-function shortcut will not work if you are searching for a falsy value (false or nil).
; will not work
(some #{false} [true false true]) ; = nil
In these cases, you should use the built-in predicate function for that value, false? or nil?:
(some false? [true false true]) ; = true
If you will need to do this kind of search a lot, write a function for it:
(defn seq-contains? [coll target] (some #(= target %) coll))
(seq-contains? [true false true] false) ; = true
Also, see Michał’s answer for ways to check whether any of multiple targets are contained in a sequence.
Another option:
((set '(100 101 102)) 101)
Use java.util.Collection#contains():
(.contains '(100 101 102) 101)