`clojure.set/subset?`¶

Type: function Added: Clojure 1.2 Examples: 3 Runnable: 0

([set1 set2])

Is set1 a subset of set2?

Examples¶

by dakrone on 2010-12-30 . May have evaluation errors.

by jafingerhut on 2018-07-27 . May have evaluation errors.

;; Advice: Do not call subset? with non-set arguments.  If you are
;; concerned that you may be unintentionally doing so, and want an
;; exception to be thrown if you do, consider using the library
;; funjible (https://github.com/jafingerhut/funjible) which provides
;; its own definition of subset? that is identical to Clojure's,
;; except it checks the types of its arguments.

;; subset? might or might not return what you expect if you give it
;; values that are not sets.  The implementation of subset? _does not
;; check_ whether you are actually giving it values that are sets.  It
;; _assumes_ so.

;; If the second argument were a set with the same elements, you would
;; expect the return value false here.
(subset? #{1 3} [2 4 6 8])
true

;; Here, if the second argument were a set with the same elements as
;; the vector, you would expect the return value true.
(subset? #{1 3 5} [1 3 5])
false

;; And similarly here:
(subset? #{"1" "3"} ["1" "3" "5"])
false

;; Switching to considering cases where the first argument is not a set, this
;; appears to do what one would hope:
(subset? [1 2 3 4] #{1 2 3 4 5})
true

;; ... but this does not (at least up until Clojure 1.10 this behavior is
;; because of how subset? is implemented, first comparing the count of the
;; two collections to make it possible to quickly return false when the first
;; collection is larger):
(subset? [1 2 3 4 1 2 3 4] #{1 2 3 4 5})
false

;; Why not change the definition of subset? so it always throws
;; an exception if you give it a non-set argument?  I would guess that
;; the primary reason is that the extra run-time type checks would
;; slow subset? down by an amount that the Clojure core team does
;; not want everyone to have to pay on every such call.

;; Related Clojure tickets: 
;; https://dev.clojure.org/jira/browse/CLJ-1953
;; https://dev.clojure.org/jira/browse/CLJ-2287

by wxnn08 on 2024-11-26 . May have evaluation errors.

Note by arkdrag¶

Do note that this will only work as expected for sets. subset? uses contains? in it's implementation. The functionality in contains? for vectors can be slightly unintuitive at first, and causes functions like this to perform in completely unexpected ways when using vectors instead of sets. For example,

user> (clojure.set/subset?  [0 1] [2 3])
true
user> (clojure.set/subset?  [2 3] [2 3])
false

It is slightly more intuitive with maps, as it checks whether every item in the first collection is a key in the map passed in as the 2^nd parameter, but still is probably a bit confusing to someone reading your code.

user> (clojure.set/subset? '(7 8)  {8 1 7 3})
true
user> (clojure.set/subset? '(9 10)  {8 1 7 3})
false

Long story short, if you're using functions from the clojure.set namespace, make sure you're passing in sets.

clojure.set/subset?¶

Examples¶

Note by arkdrag¶

See also¶

`clojure.set/subset?`¶