5. Maps - Koans Walkthrough

This video covers the map (hashmap) data structure. Maps store keys and values, and Clojure provides many innovations making them a pleasure to use.

Here is the 05_maps.clj source code:

  "There are two ways to create maps"
  (= __ (hash-map))

  "Maps in clojure associate keys with values"
  (= __ (count (hash-map)))

  "A value must be supplied for each key"
  (= {:a 1} (hash-map :a __))

  "The size is the number of entries"
  (= __ (count {:a 1 :b 2}))

  "You can look up the value for a given key"
  (= __ (get {:a 1 :b 2} :b))

  "Maps can be used as lookup functions"
  (= __ ({:a 1 :b 2} :a))

  "And so can keywords"
  (= __ (:a {:a 1 :b 2}))

  "But map keys need not be keywords"
  (= __ ({2006 "Torino" 2010 "Vancouver" 2014 "Sochi"} 2010))

  "You may not be able to find an entry for a key"
  (= __ (get {:a 1 :b 2} :c))

  "But you can provide your own default"
  (= __ (get {:a 1 :b 2} :c :key-not-found))

  "You can find out if a key is present"
  (= __ (contains? {:a nil :b nil} :b))

  "Or if it is missing"
  (= __ (contains? {:a nil :b nil} :c))

  "Maps are immutable, but you can create a new, 'changed' version"
  (= {1 "January" 2 __} (assoc {1 "January" } 2 "February"))

  "You can also 'remove' an entry"
  (= {__ __} (dissoc {1 "January" 2 "February"} 2))

  "Often you will need to get the keys (which will be in hash order)"
  (= (list __ __ __)
     (sort (keys {2006 "Torino" 2010 "Vancouver" 2014 "Sochi"})))

  "Or the values"
  (= (list "Sochi" "Torino" __)
     (sort (vals {2006 "Torino" 2010 "Vancouver" 2014 "Sochi"}))))

Clojure Koans Walkthrough in Light Table IDE

This screencast tutorial helps you learn the Clojure programming language. Experience the joy of Clojure in the Light Table IDE as we tour through the Clojure Koans, taking you all the way from Beginner to Intermediate to Advanced.

Clojure is a Lisp created by Rich Hickey that runs on the JVM, as an alternative to Java. ClojureScript can target the web browser environment, and node.js, by compiling down to JavaScript, using the Google Closure compiler. Clojure features immutability, functional programming, and being a Lisp, macros.