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(meditations
"There are two ways to create maps"
(= {} (hash-map))
"Maps in clojure associate keys with values"
(= 0 (count (hash-map)))
"A value must be supplied for each key"
(= {:a 1} (hash-map :a 1))
"The size is the number of entries"
(= 2 (count {:a 1 :b 2}))
"You can look up the value for a given key"
(= 2 (get {:a 1 :b 2} :b))
"Maps can be used as lookup functions"
(= 1 ({:a 1 :b 2} :a))
"And so can keywords"
(= 1 (:a {:a 1 :b 2}))
"But map keys need not be keywords"
(= "Vancouver" ({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))
"But you can provide your own default"
(= :key-not-found (get {:a 1 :b 2} :c :key-not-found))
"You can find out if a key is present"
(= true (contains? {:a nil :b nil} :b))
"Or if it is missing"
(= false (contains? {:a nil :b nil} :c))
"Maps are immutable, but you can create a new, 'changed' version"
(= {1 "January" 2 "February"} (assoc {1 "January" } 2 "February"))
"You can also 'remove' an entry"
(= {1 "January"} (dissoc {1 "January" 2 "February"} 2))
"Often you will need to get the keys (which will be in hash order)"
(= (list 2006 2010 2014)
(sort (keys {2006 "Torino" 2010 "Vancouver" 2014 "Sochi"})))
"Or the values"
(= (list "Sochi" "Torino" "Vancouver")
(sort (vals {2006 "Torino" 2010 "Vancouver" 2014 "Sochi"}))))