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36 lines
1.1 KiB
Clojure
36 lines
1.1 KiB
Clojure
(ns koans.09-higher-order-functions
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(:require [koan-engine.core :refer :all]))
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(meditations
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"The map function relates a sequence to another"
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(= [4 8 12] (map (fn [x] (* 4 x)) [1 2 3]))
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"You may create that mapping"
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(= [1 4 9 16 25] (map (fn [x] (* x x)) [1 2 3 4 5]))
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"Or use the names of existing functions"
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(= [false false true false false] (map nil? [:a :b nil :c :d]))
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"A filter can be strong"
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(= '() (filter (fn [x] false) '(:anything :goes :here)))
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"Or very weak"
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(= '(:anything :goes :here) (filter (fn [x] true) '(:anything :goes :here)))
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"Or somewhere in between"
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(= [10 20 30] (filter (fn [x] (< x 40)) [10 20 30 40 50 60 70 80]))
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"Maps and filters may be combined"
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(= [10 20 30] (map (fn [x] (* x 10)) (filter (fn [x] (< x 4)) [1 2 3 4 5 6 7 8])))
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"Reducing can increase the result"
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(= 24 (reduce (fn [a b] (* a b)) [1 2 3 4]))
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"You can start somewhere else"
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(= 2400 (reduce (fn [a b] (* a b)) 100 [1 2 3 4]))
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"Numbers are not the only things one can reduce"
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(= "longest" (reduce (fn [a b]
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(if (< (count a) (count b)) b a))
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["which" "word" "is" "longest"])))
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