Anonymous functions in Clojure

Passes unnamed functions to map and filter over a week of rainfall readings.

(def rainfall-mm [4 0 12 7 0 21])

;; fn makes a function with no name, used right where it is written
(println (map (fn [mm] (/ mm 10.0)) rainfall-mm))

;; #(...) is reader shorthand for the same thing, and % is the argument
(println (filter #(pos? %) rainfall-mm))

;; %1 and %2 name the arguments when there is more than one
(def notes (map #(str %1 " mm on day " %2) rainfall-mm (range 1 7)))
(println (first notes) "/" (last notes))

;; give the shorthand a name once the body starts earning one
(def wet-day? #(>= % 10))
(println "wet days:" (count (filter wet-day? rainfall-mm)))

How it works

1. (fn [mm] (/ mm 10.0)) converts each reading to centimeters inside map.
2. #(pos? %) is the reader shorthand for the same idea, with % standing in for the argument.
3. #(str %1 " mm on day " %2) names two arguments, and wet-day? shows the shorthand once it earns a name.

Keywords and builtins used here

• count
• def
• filter
• first
• fn
• last
• map
• println
• range
• str

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Step 2 of 3 in Functions, step 8 of 27 in Language basics.

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Anonymous functions in other languages

• Elixir