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39 lines
1.4 KiB
Clojure
39 lines
1.4 KiB
Clojure
(ns koans.22-group-by
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(:require [koan-engine.core :refer :all]))
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(defn get-odds-and-evens [coll]
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(let [{odds true evens false} (group-by odd? coll)]
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[odds evens]))
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(meditations
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"To categorize a collection by some function, use group-by."
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(= {3 '("foo" "bar") 5 '("hello" "world")} (group-by count ["hello" "world" "foo" "bar"]))
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"You can simulate filter + remove in one pass"
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(= (get-odds-and-evens [1 2 3 4 5])
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((juxt filter remove) odd? [1 2 3 4 5])
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[[1 3 5] [2 4]])
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"You can also group by a primary key"
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(= {1 '({:id 1 :name "Bob"}
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{:id 1 :last-name "Smith"})
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2 '({:id 2 :name "Jennifer"})}
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(group-by :id [{:id 1 :name "Bob"}
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{:id 2 :name "Jennifer"}
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{:id 1 :last-name "Smith"} ]))
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"But be careful when you group by a non-required key"
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(= {"Bob" [{:name "Bob" :id 1}]
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"Jennifer" [{:name "Jennifer" :id 2}]
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nil [{:last-name "Smith" :id 1}]}
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(group-by :name [{:id 1 :name "Bob"}
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{:id 2 :name "Jennifer"}
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{:id 1 :last-name "Smith"}]))
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"The true power of group-by comes with custom functions"
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(= {:naughty-list '({:name "Jimmy" :bad true} {:name "Joe" :bad true}) :nice-list '({:name "Jane" :bad false})}
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(group-by #(if (:bad %) :naughty-list :nice-list)
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[{:name "Jimmy" :bad true}
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{:name "Jane" :bad false}
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{:name "Joe" :bad true}])))
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