Returns a map of the elements of coll keyed by the result of
f on each element. The value at each key will be a vector of the
corresponding elements, in the order they appeared in coll.
user=> (group-by count ["a" "as" "asd" "aa" "asdf" "qwer"])
{1 ["a"], 2 ["as" "aa"], 3 ["asd"], 4 ["asdf" "qwer"]}
user=> (group-by odd? (range 10))
{false [0 2 4 6 8], true [1 3 5 7 9]}
;; group by a primary key
user=> (group-by :user-id [{:user-id 1 :uri "/"}
{:user-id 2 :uri "/foo"}
{:user-id 1 :uri "/account"}])
{1 [{:user-id 1, :uri "/"} {:user-id 1, :uri "/account"}],
2 [{:user-id 2, :uri "/foo"}]}
Source
clojure/core.clj:5541
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(defn group-by
"Returns a map of the elements of coll keyed by the result of
f on each element. The value at each key will be a vector of the
corresponding elements, in the order they appeared in coll."
{:added "1.2"}
[f coll]
(persistent!
(reduce
(fn [ret x]
(let [k (f x)]
(assoc! ret k (conj (get ret k []) x))))
(transient {}) coll)))
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