#100DaysOfClojure (Day [9] Maps)

Today I will look at Maps, one of the most important collection types in Clojure. Maps are collections of key-value pairs. Keys can be of any type in Clojure including functions, lists, other maps etc. The are 3 types of maps in Clojure: Hash Maps, Sorted Maps and Array Maps. Sorted maps keep the key value pairs in a certain order which implies that a lookup time may not be constant and it also allows you to traverse the values in the order of the keys. Sorted maps have a constraint in that the keys have to be comparable. 

You can create them as follows

; Hash maps

{"number" 1 2 2.25 :pi 22/7}

=> {"number" 1, 2 2.25, :pi 22/7}

or alternatively

(hash-map "number" 1 2 2.25 :pi 22/7)

=> {:pi 22/7, "number" 1, 2 2.25}

;Sorted maps

(sorted-map "number" 1 "2" 2.25 "pi" 22/7)

=> {"2" 2.25, "number" 1, "pi" 22/7}  ; note the order

; you can also sort using a custom comparator

(sorted-map-by #(< (count %1) (count %2))

               "number" 1 "2" 2.25 "pi" 22/7)

=> {"2" 2.25, "pi" 22/7, "number" 1}

You can lookup values as follows

(get {"number" 1 2 2.25 :pi 22/7} :pi)

=> 22/7

or alternatively use the key as a function

(:pi {"number" 1 2 2.25 :pi 22/7})

=> 22/7

if you don't find the key

(get {"number" 1 2 2.25 :pi 22/7} :missing)

=> nil

 You can add new key value pairs as follows

(assoc {"number" 1 2 2.25 :pi 22/7} :association {:a 2.00 :b 5.78})

=> {"number" 1, 2 2.25, :pi 22/7, :association {:a 2.0, :b 5.78}}

You can remove key value pairs as follows

(dissoc {"number" 1, 2 2.25, :pi 22/7, :association {:a 2.0, :b 5.78}} :pi )

=> {"number" 1, 2 2.25, :association {:a 2.0, :b 5.78}}

You can get a list of all the keys as follows

(keys {"number" 1, 2 2.25, :pi 22/7, :association {:a 2.0, :b 5.78}})

=> ("number" 2 :pi :association)

You can also get all the values as follows

(vals {"number" 1, 2 2.25, :pi 22/7, :association {:a 2.0, :b 5.78}})

=> (1 2.25 22/7 {:a 2.0, :b 5.78})

You can also combine maps as follows

(merge {"2" 2.25, "number" 1, "pi" 22/7} {"number" 1, 2 2.25, :pi 22/7, :association {:a 2.0, :b 5.78}})

=> {"2" 2.25, "number" 1, "pi" 22/7, 2 2.25, :pi 22/7, :association {:a 2.0, :b 5.78}}

That's all for today, tomorrow I will move onto Sets

(println "Bye 4 Now!!")