day four!

dayfour.rkt

@@ -0,0 +1,64 @@
+#lang racket
+
+(define (square x)
+	(* x x))
+
+;; more obfuscated functions, please
+(define (head x) (car x))
+(define (tail x) (cdr x))
+
+;; foldl --> (foldl - '(1 2 3) 9)
+;; = (((9 - 1) - 2) - 3) = 3
+(define (foldl-helper op lst total)
+	(if (null? lst)
+		total
+		(foldl-helper op (tail lst) (op total (head lst)))))
+
+(display "should be 9 ==> ")
+(foldl-helper - '(1 2 3) 9)
+
+;; foldr --> (foldr - '(1 2 3) 9) = -7
+(define (foldr-helper op lst start)
+	(if (null? lst)
+		start
+		(op (head lst) (foldr-helper op (tail lst) start))))
+
+(display "should be -7 ==> ")
+(foldr-helper - '(1 2 3) 9)
+
+
+;; gets the length of a list
+(define (lst-length lst)
+	(foldl-helper (lambda (x y) (+ x 1)) lst 0))
+(display "should be 5 ==> ")
+(lst-length (list 1 3 2 4 3 8)) ; ==> 5
+
+;; gets the lesser of two numbers (or either if the same)
+(define (min-of-two x y)
+	(if (< x y)
+		x
+		y))
+;; gets the least number of a list of numbers
+(define (min-lst lst)
+	(foldl-helper min-of-two lst +inf.0))
+
+(display "should be 1 ==> ")
+(min-lst (list 7 2 8 3 1 4)) ; ==> 1
+
+;; gets the greater of two numbers (or either if the same)
+(define (max-of-two x y)
+	(if (> x y)
+		x
+		y))
+;; gets the greatest number of a list of numbers
+(define (max-lst lst)
+	(foldl-helper max-of-two lst -inf.0))
+
+(display "should be 8 ==> ")
+(max-lst (list 7 2 8 3 1 4)) ; ==> 8
+
+(define (id lst)
+	(foldl-helper (lambda (arr elem) (cons elem arr)) lst '()))
+(id (list 1 2 3))
+
+