020 - Penultimate element - more sequence fun

How to get the second to last function when no function like it exists in
Clojure core.

Included solutions using loop/recur, second/reverse and comp

src/four_clojure/020_penultimate_element.clj

@@ -0,0 +1,117 @@
+(ns four-clojure.020-penultimate-element)
+
+;; #020 Penultimate Element
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Difficulty:	Easy
+;; Topics:	seqs
+
+;; Write a function which returns the second to last element from a sequence.
+
+;; (= (__ (list 1 2 3 4 5)) 4)
+;; (= (__ ["a" "b" "c"]) "b")
+;; (= (__ [[1 2] [3 4]]) [1 2])
+
+
+;; Deconstruct the problem
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; This exercise follows on nicely from #19 - last element.
+
+;; In this exercise we do not have a restricted function, however, the function that would solve this problem does not exist in Clojure core.
+
+;; The sequence functions by themselves do not give the answer we are looking for, but perhaps combining them will help.
+
+(first [1 2 3 4 5])
+;; => 1
+
+(second [1 2 3 4 5])
+;; => 2
+
+
+;; but there is not a function called `second-to-last`
+;; Like in #19, if we reverse the collection, then the `second` function will give us the correct answer.
+
+(second (reverse [1 2 3 4 5]))
+;; => 4
+
+
+;; We can put this into a function so the argument is passed to the reverse function
+
+(fn [arg]
+  (second (reverse arg)))
+
+;; or use the short form
+#(second (reverse %))
+
+
+;; Using the nth function
+;; The `nth` function could be used as it gives us a way to access a collection by an index (even if the data structure type doesnt have an index).
+
+;; In the 4Clojure exercise, the expression does not provide the size of the collection as an argument.
+;; so we have to write a function that gets the size of the collection,
+;; then we reduce that number by 2 so we get the second to last value using the `nth` function.
+;; `nth` uses an index that starts at zero, so we need to minus the count of the elements by 2 (one for the index starting at zero, and one for the second to last element)
+
+(nth [1 2 3 4 5] 3)
+;; => 4
+
+(fn [collection]
+  (nth collection (- (count collection) 2)))
+
+;; using the function definition short form
+#(nth % (- (count %) 2))
+
+
+
+;; Taking a functional composition approach we can instead use the `comp` function
+
+(comp second reverse)
+
+
+;; and we could even use the low level loop and recur approach, but this doent really give us any advantage.
+
+(loop [collection [1 2 3 4 5]]
+  (if (= 2 (count collection))
+    (first collection)
+    (recur (rest collection))))
+
+
+;; Answers summary
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Functional composition approach
+
+(comp second reverse)
+
+
+;; function definition
+(fn [arg]
+  (second (reverse arg)))
+
+;; or the short form for function definition
+#(second (reverse %))
+
+;; Using `nth` function to access the collection via an index
+
+(fn [collection]
+  (nth collection (- (count collection) 2)))
+
+;; using the function definition short form
+#(nth % (- (count %) 2))
+
+
+;; low level loop recur
+(loop [collection [1 2 3 4 5]]
+  (if (= 2 (count collection))
+    (first collection)
+    (recur (rest collection))))
+
+
+;; interesting 4Clojure answers
+
+;; `buttlast` is similar to `rest`, but drops the last element of the collection, rather than the first.
+;; If you wanted to iterate backwards through a collection using a loop recur or recursive function,
+;; then `buttlast` would be very useful.
+
+(comp last butlast)