adding about maps

koans/about_maps.clj

@@ -0,0 +1,48 @@
+(meditations
+  "Maps in clojure associate keys with values"
+  (= __ (.size (hash-map)))
+
+  "There are two ways to create maps"
+  (= {} (hash-map))
+
+  "A value must be supplied for each key"
+  (= {:a 1} (hash-map :a __))
+
+  "The size is the number of entries"
+  (= __ (.size {: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"
+  (= nil (get {:a 1 :b 2} :c))
+
+  "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 "Febuary"} 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"}))))
+

koans/path_to_enlightenment.clj

@@ -15,4 +15,5 @@
 (load "about_equalities")
 (load "about_vectors")
 (load "about_sets")
+(load "about_maps")
 (println "You have acheived clojure enlightenment. Namaste.")