Maps, Keywords, and Sets

Create a map literal:

{"title" "Parallels" "artist" "Fates Warning" "year" 1991}

Create a map using a function:

(hash-map "title" "Parallels" "artist" "Fates Warning" "year" 1991)

Retrieve a value from a map by its key:

(def album {"title" "Parallels" "artist" "Fates Warning" "year" 1991})
(get album "title") ; "Parallels"

Same, but using the map like a function:

(album "title") ; "Parallels"

If the key is not found, nil is returned:

(get album "duration") ; nil
(album "duration") ; nil

Use keywords instead of strings as map keys:

(def song {:title "Pale Fire"
           :artist "Fates Warning"
           :duration "4m17s"})
(get song :title) ; "Pale Fire"
(song :title) ; "Pale Fire"

Use the keyword like a function for map lookup:

(:title song) ; "Pale Fire"

Add a new entry (key-value pair) to a map:

(assoc song :album "Inside Out")
;; {:title "Pale Fire", :artist "Fates Warning",
;;  :duration "4m17s", :album "Inside Out"}

Add multiple new entries to a map:

(assoc song :album "Inside Out" :year 1994)
;; {:title "Pale Fire", :artist "Fates Warning",
;;  :duration "4m17s", :album "Inside Out", :year 1994}

Remove entries from a map:

(dissoc song :artist :duration) ; {:title "Pale Fire"}

Non-existant keys are ignored silently:

(dissoc song :artist :duration :genre :top-chart-position)
;; {:title "Pale Fire"}

Retrieve all the keys and values, respectively, from a map:

(keys song) ; (:title :artist :duration)
(vals song) ; ("Pale Fire" "Fates Warning" "4m17s")

The order of keys is not guaranteed, but matches the order of the values. Use sorted-map to create a map with keys kept in sorted order.

Check whether or not a map contains a key:

(contains? song :title) ; true
(contains? song :genre) ; false

Create a set literal:

#{"Monument" "One" "Pale Fire"}

Create a set using a function:

(hash-set "Monument" "One" "Pale Fire")

Check whether or not a set contains a value:

(def songs #{"Monument" "One" "Pale Fire"})
(contains? songs "One") ; true
(contains? songs "Two") ; false

Lookup of keywords like a function:

(def genres #{:metal :pop :electro})
(:metal genres) ; :metal
(:classic genres) ; nil

Extend a set:

(conj genres :classic) ; #{:metal :pop :electro :classic}

Remove a value from a set:

(disj genres :pop) ; #{:metal :electro}

Exercises

Change a Map

Given the following map:

(def server {:name "Persephone" :uptime "13d7h5m43s" :active false})

Write an expression to remove the :uptime key (with its value) from the map, to update the :active value to false in the map, and to add the key :decommissioned with the value 2025 to the map.

Hint: Use the assoc function to alter and add a key-value pair, and the disj function to remove a key-value pair.

Test: The map {:name "Persephone" :active false :decommissioned 2025} is returned.

(def server {:name "Persephone" :uptime "13d7h5m43s" :active false})
(assoc (assoc (dissoc server :uptime) :active false) :decommissioned 2025)

Change a Set

Given the following set:

(def members #{"Steve" "Dave" "Adrian" "Bruce" "Nicko"})

Write an expression to remove the value "Adrian" from the set, and to add the value "Janick" to the set.

Hint: Use the disj function to remove a value, and the conj function to add a value.

Test: The set {"Steve" "Dave" "Bruce" "Nicko" "Janick"} is returned.

(def members #{"Steve" "Dave" "Adrian" "Bruce" "Nicko"})
(conj (disj members "Adrian") "Janick")

Combine Maps and Sets

Given the following map and set:

(def guitarists #{"Dave" "Adrian" "Janick"})
(def song {:title "Paschendale" :duration "8m28s"})

Write an expression which first adds the guitarists set with the key :solos to the song map, and then retrieves the :solos set again from the map, thereby removing the value "Janick" from the set.

Hint: Use the assoc and disj functions.

Test: The set {"Dave" "Adrian"} is returned.

(def guitarists #{"Dave" "Adrian" "Janick"})
(def song {:title "Paschendale" :duration "8m28s"})
(disj (:solos (assoc song :solos guitarists)) "Janick")