Clojure Koans Walkthrough

1. Introduction
2. Lists
3. Vectors
4. Sets
5. Maps
6. Functions
7. Conditionals
8. Higher Order Functions
9. Runtime Polymorphism
10. Lazy Sequences
11. Sequence Comprehensions
12. Creating Functions
13. Recursion
14. Destructuring
15. Refs
16. Atoms
17. Macros
18. Datatypes
19. Java Interop
20. Partition

Recommended Books

• ← Sets
• Functions →

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:

(meditations
  "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"}))))