day five (ft. merge sort)

dayfive.rkt

@@ -0,0 +1,44 @@
+#lang racket
+
+(define (square x)
+	(* x x))
+
+;; more obfuscated functions, please
+(define head car)
+(define tail cdr)
+
+; (define (merge-sort arr)
+
+
+(define (merge-helper list-a list-b new-list)
+	(if (null? list-a)
+		(if (null? list-b)
+			new-list ; both lists null, end everything
+			(merge-helper list-a (tail list-b) (cons (head list-b) new-list))) ; continue with elem from b
+		(if (null? list-b)
+			(merge-helper (tail list-a) list-b (cons (head list-a) new-list)) ; continue with elem from a
+			(if (< (head list-a) (head list-b)) ; is a[0] less than b[0]
+				(merge-helper (tail list-a) list-b (cons (head list-a) new-list)) ; if so, continue with elem from b
+				(merge-helper list-a (tail list-b) (cons (head list-b) new-list)))))) ; otherwise, continue with elem from b
+
+(define (merge list-a list-b) (reverse (merge-helper list-a list-b (list))))
+
+(merge (list 1 3 8) (list 2 4 5))
+
+(define (first-half-length lst) (exact-ceiling (/ (length lst) 2)))
+; (define (second-half-length lst) (- (length lst) (first-half-length lst)))
+
+(define (first-half lst) (take lst (first-half-length lst)))
+(define (second-half lst) (list-tail lst (first-half-length lst)))
+
+(first-half (list 1 2 3 4 5))
+(second-half (list 1 2 3 4 5))
+
+
+(define (merge-sort lst)
+	(if (> (length lst) 1)
+		(merge (merge-sort (first-half lst)) (merge-sort (second-half lst)))
+		lst)) ; return the list if only one elem
+
+(display "\nmerge sort:\n")
+(merge-sort (list 1 3 2 50 4 7)) ; ==> '(1 2 3 4 7 50)