clojure-koans/src/koans/05_maps.clj

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(meditations
"There are two ways to create maps"
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(= __ (hash-map))
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"Maps in clojure associate keys with values"
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(= __ (count (hash-map)))
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"A value must be supplied for each key"
(= {:a 1} (hash-map :a __))
"The size is the number of entries"
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(= __ (count {:a 1 :b 2}))
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"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"
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(= __ (get {:a 1 :b 2} :c))
"But you can provide your own default"
(= __ (get {:a 1 :b 2} :c :key-not-found))
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"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"))
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"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"}))))