018 - Sequences - filter

Filtering a sequence so it only returns the elements that return true when
applying the given function.  I nice continuation from the sequence map exercise
- number 17 in 4Clojure.

src/four_clojure/018_sequences_filter.clj

@@ -0,0 +1,40 @@
+(ns four-clojure.018-sequences-filter)
+
+;; #18 - Sequences: filter
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Difficulty:	Elementary
+;; Topics:
+
+;; The filter function takes two arguments: a predicate function (f) and a sequence (s). Filter returns a new sequence consisting of all the items of s for which (f item) returns true.
+
+;; (= __ (filter #(> % 5) '(3 4 5 6 7)))
+
+
+;; Deconstruct the problem
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; filter works in a similar way to the `map` function in that it uses a function over the elements of a collection.
+
+;; The `filter` function only returns the values in the collection that return true from the given function.
+
+(filter odd? [1 2 3 4 5])
+;; => (1 3 5)
+
+;; The function used in the 4Clojure test checks to see if a value is greater than 5
+;; in its full form, the function is
+(fn [arg]
+  (> arg 5))
+
+;; in the 4Clojure test, it uses the short form
+#(> % 5)
+
+;; So when we filter the collection with this function only values greater than 5 are returned
+(filter #(> % 5) '(3 4 5 6 7))
+;; => (6 7)
+
+
+;; Answers summary
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(6 7)