Exercise 06 - Maps

Creating a a Map

You can create a map in clojure by:

• {:a 1 :b 2}
• (hash-map :a 1 :b 2)

Working with Maps

Count will count the number of sets

(count {:a 1 :b 2})
  ;;=> 2

You lookup the value of assigned to a key

(get {:a 1 :b 2} :b)
  ;;=> 2

You can also use a map as a function to grab a value assigned to a key

({:a 1 :b 2} :a)
  ;;=> 1

You can also use a keywords and non-keywords as a function to grab a value from an assigned key

(:a {:a 1 :b 2})
  ;;=> 1

({2010 "Vancouver" 2014 "Sochi" 2018 "PyeongChang"} 2014)
  ;;=> "Sochi"
 

If a key can't be found, a nil is returned. However, you can create your won default value for a missing key

(get {:a 1 :b 2} :c)
  ;;=> nil
  
(get {:a 1 :b 2} :c :key-not-found)
  ;;=> :key-not-found

You can see if a key exists or doesn't exist using contains? (Boolean Returned)

(contains? {:a nil :b nil} :b)
  ;;=> true

(contains? {:a nil :b nil} :c)
  ;;=> false

Maps are Immutable AKA can't be changed. You can create cheap copies though!

You can easily add items by creating a new copy using assoc

(assoc {:monday "Pizza"} :tuesday "Burgers")
  ;;=> {:monday "Pizza" :tuesday "Burgers"}

And replace values in a a matching key

(assoc {:monday "Pizza :tuesday "Burgers"} :tuesday "Indian")
  ;;=> {:monday "Pizza" :tuesday "Indian"}

Or remove a key/value pair using dissoc

(dissoc {:a "alpha" :b "beta"} :b)
  ;;=> {:a "alpha"}

While we are at it, lets combine to maps using merge

(merge {:a "alpha" :b "beta"} {:c "charlie" :d "delta"})
  ;;=> {:a "alpha" :b "beta" :c "charlie" :d "delta"}

Note: If you are trying to add multiple versions of the same key, the right most wins.

(merge {:a 1} {:a 2} {:a 3})
  ;;=> {:a 3}

Now let's get fancy! You can use merge-with to decide how to handle matching keys

(merge-with + 
	{:a 99 :b 48 :c 63}
	{:a 1 :b 2 :c 7})

	;;=> {:a 100 :b 50 :c 70}

You can pull all keys or values from a map but the order is not reliable. You should pass it into a function that handles the order like sort

Note: Returned as a sequence

(sort(keys {:a 1 :c 2 :b 3}))
  ;;=> (:a :b :c)
  
(sort (vals {:a 10 :b 50 :c 40}))
  ;;=> (10 40 50)