78c5704f34
Works fine either way - this is just since underscores are required in filenames for real projects, so it's something good for folks to get used to.
50 lines
1.4 KiB
Clojure
50 lines
1.4 KiB
Clojure
(meditations
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"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"
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(= {:a 1} (hash-map :a __))
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"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"
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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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(= __ (get {:a 1 :b 2} :c))
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"But you can provide your own default"
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(= __ (get {:a 1 :b 2} :c :key-not-found))
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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 "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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