Map Update

A value in a map can either be replaced by the assoc function or updated using the update or update-in functions.

Associating new values with assoc

The assoc function can be used by assigning a new value to an existing key.

(def hello {:count 1 :words "hello"})

(assoc hello :count 0)
;=> {:count 0, :words "hello"}

(assoc hello :words "Hello Clojure World")
;=> {:count 1, :words "Hello Clojure World"}

Updating existing values with update & update-in

The update function applies a function to the existing value to create a new value for a specific key

The update function is passed:

  • the map to be updated
  • the key that points to the value to be updated
  • the function that determines how the value will be updated

The value of specified key will be the first argument of the given function.

(def hello {:count 1 :words "hello"})

(update hello :count inc)
;=> {:count 2, :words "hello"}
(update hello :words str ", world")
;=> {:count 1, :words "hello, world"}

The update-in function works like update, but takes a vector of keys to update at a path to a nested map.

(def mine {:pet {:age 5 :name "able"}})

(update-in mine [:pet :age] - 3)
;=> {:pet {:age 2, :name "able"}}



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