问题
I am going through some Clojure tutorials using Closure Box, and entered the following code:
user> (def stooges (vector "Moe" "Larry" "Curly"))
#'user/stooges
user> (contains? stooges "Moe")
false
Shouldn't this evaluate to TRUE ? Any help is appreciated.
回答1:
A vector is similar to an array. contains? returns true if the key exists in the collection. You should be looking for the "key/index" 0, 1 or 2
user=> (def stooges (vector "Moe" "Larry" "Curly"))
#'user/stooges
user=> (contains? stooges 1)
true
user=> (contains? stooges 5)
false
If you were using a hash...
user=> (def stooges {:moe "Moe" :larry "Larry" :curly "Curly"})
#'user/stooges
user=> (contains? stooges :moe)
true
user=> (contains? stooges :foo)
false
As mikera suggests, you probably want something like clojure.core/some
回答2:
This is a common trap! I remember falling into this one when I was getting started with Clojure :-)
contains? checks whether the index (0, 1, 2, etc.) exists in the collection.
You probably want something like:
(some #{"Moe"} stooges)
=> "Moe" <counts as boolean true>
(some #{"Fred"} stooges)
=> nil <counts as boolean false>
Or you can define your own version, something like:
(defn contains-value? [element coll]
(boolean (some #(= element %) coll)))
(contains-value? "Moe" stooges)
=> true
回答3:
contains? support Set, if you use clojure-1.4
user=> (contains? #{:a, :b} :a)
true
user=> (contains? (set stooges) "Moe")
true
来源：https://stackoverflow.com/questions/11963193/issue-with-clojure-contains