user=> (def users [{:name "James" :age 26} {:name "John" :age 43}])
#'user/users
user=> (update-in users [1 :age] inc)
[{:name "James", :age 26} {:name "John", :age 44}]
user=> (def p {:name "James" :age 26})
#'user/p
user=> (update-in p [:age] inc)
{:name "James", :age 27}
;; remember, the value of p hasn't changed!
user=> (update-in p [:age] + 10)
{:name "James", :age 36}
(defn char-cnt [s]
"Counts occurence of each character in s"
(reduce
(fn [m k]
(update-in m [k] (fnil inc 0)))
{}
(seq s)))
;Note use of fnil above - returns 0 if nil is passed to inc (avoids null pointer exception)
Source
clojure/core.clj:5432
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(defn update-in
"'Updates' a value in a nested associative structure, where ks is a
sequence of keys and f is a function that will take the old value
and any supplied args and return the new value, and returns a new
nested structure. If any levels do not exist, hash-maps will be
created."
{:added "1.0"
:static true}
([m [k & ks] f & args]
(if ks
(assoc m k (apply update-in (get m k) ks f args))
(assoc m k (apply f (get m k) args)))))
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