;; CSE 413 21sp
;; Lecture 4 more list functions, scope,  function definitions and examples

#lang racket

(define magic 42)
(define xvii 17)
(define pi 3.1415926535)

;; some sample lists

(define nums '(5 17 3 12))
(define ab '(a b))
(define xyz '(x y z))
(define stuff '(1 banana (2 3) 4 (5)))

;; Functions on lists

;; done previously: list functions: cons, car, cdr; list, quoting
;; also done previously: predicates: null?, pair?, list? and functions like length

;; sum of numbers in a list
(define (sum lst)
  (if (null? lst)
      0
      (+ (car lst) (sum (cdr lst)))))

;; sum numbers in list ignoring others
(define (numsum lst)
  (cond [(null? lst) 0]
        [(number? (car lst)) (+ (car lst) (numsum (cdr lst)))]
        [else (numsum (cdr lst))]))

;; = length of lst
(define (len lst)
  (if (null? lst)
      0
      (+ 1 (len (cdr lst)))))

;; First attempt to append lists - buggy
(define (badapp lst1 lst2)
  (cons lst1 lst2))


;; Fixed append
(define (app lst1 lst2)
  (if (null? lst1)
      lst2
      (cons (car lst1) (app (cdr lst1) lst2))))

;; new - attempt to reverse a list

;; first attempt
(define (rev1 lst)
  (if (null? lst)
      '()
      (cons (rev1 (cdr lst)) (car lst))))

;; second attempt
(define (rev2 lst)
  (if (null? lst)
      '()
      (append (rev2 (cdr lst)) (car lst))))

;; fixed!  (?)
(define (rev3 lst)
  (if (null? lst)
      '()
      (append (rev3 (cdr lst)) (list (car lst)))))


;; return 2 copies of the reversed list
(define (revrev lst)
  (append (rev3 lst) (rev3 lst)))

;; version 2 - local name for reversed list
(define (r2 lst)
  (let ([r (rev3 lst)])
    (append r r)))

;; append list to its reverse
(define (apprev lst)
  (let ([r (rev3 lst)])
    (append lst r)))

(define (apprev2 lst)
  (let ([r (rev3 lst)])
    (append r xyz)))