017 - Sequences - map

Mapping functions over sequences.

src/four_clojure/017_sequences_map.clj

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+(ns four-clojure.017-sequences-map)
+
+;; #17 Sequences: map
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+
+;; Difficulty:	Elementary
+;; Topics:
+
+;; The map function takes two arguments: a function (f) and a sequence (s). Map returns a new sequence consisting of the result of applying f to each item of s. Do not confuse the map function with the map data structure.
+
+;; (= __ (map #(+ % 5) '(1 2 3)))
+
+
+;; Deconstruct the problem
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+
+;; Discovering how to use the map function to process all the elements of a collection, using a given function.
+
+;; REPL experiments
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+
+;; map will work on a single collection, assuming that the function we use with `map` works with a single argument.
+;; each element of the collection will be passed to the function in turn and the result put into a new collection.
+;; once all the elements of the collection have been passed to the function, the resulting colleciton is returned.
+;; The `map` function is responsible for building up the new collection.
+
+(map inc [1 2 3])
+;; => (2 3 4)
+
+;; The map function can work over multiple collections, assuming the function can take multiple arguments
+(map + [1 2 3] [3 4 5])
+;; => (4 6 8)
+
+(map + [1 2 3] [3 4 5] [6 7 8])
+;; => (10 13 16)
+
+;; The `+` function can take multiple arguments, so it can be mapped over multiple collections
+(map + [1 2 3] [3 4 5] [6 7 8] [9 10 11])
+;; => (19 23 27)
+
+
+;; As well as using a function from Clojure core, or any other library,
+;; we can write our own lambda function to be used by the `map` function.
+
+;; our function simply needs to add five to the argument given
+
+(fn [arg]
+  (+ arg 5))
+
+;; In the test this is written in the short form
+
+#(+ % 5)
+
+;; So when we map this function over the collection
+;; the map function returns a new collection with each element increased by 5
+(map #(+ % 5) '(1 2 3))
+;; => (6 7 8)
+
+
+
+;; Answers summary
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(6 7 8)