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