CIS352 'S26

match.rkt

#lang racket
;; CIS352 -- Practice on Pattern Matching
;; 
;; If you want more practice, I have a video here: https://www.youtube.com/watch?v=RJFkmh9Wo8o

(define (filter f l)
  (if (empty? l)
      '()
      (cons (f (first l)) (filter f (rest l)))))

(define (filter-m f l)
  (match l
    ['() '()] ;; we know l = '()
    [`(,hd ,tl ...)
     (cons (f hd) (filter f tl))])) ;; we l = (cons hd tl)

;;(every-other '()) => '()
;;(every-other '(1)) => '(1)
;;(every-other '(1 2)) => '(1)
;;(every-other '(1 2 3)) => '(1 3)

(define (every-other l)
  (cond [(empty? l) '()]
        [(equal? (length l) 1) l]
        [else (cons (first l) (every-other (rest (rest l))))]))

(every-other-m '(1 2 3 4 5))

(define (every-other-m l)
  (match l
    ['() '()]
    [`(,hd) l]
    [`(,fst ,snd ,rst ...) ;; fst = 1, snd = 2 rest =  '(3 4 5)
     (cons fst (every-other-m rst))]))

;;(every-other-m '(1 2 3 4 5))
;;= (match '(1 2 3 4 5)
;;     ['() '()]
;;     [`(,hd) l]
;;     [`(,fst ,_ ,rst ...) ;; fst = 1, snd = 2 rest =  '(3 4 5)
;;      (cons fst (every-other-m rst))])
;; = (cons 1 (every-other-m '(3 4 5)))
;; = (cons 1 (cons 3 (every-other-m '(5))))
;; = (cons 1 (cons 3 '(5))) = '(1 3 5)

;; (every-other-m '(1 2 3 4 5 6))
;; = (cons 1 (every-other-m '(3 4 5 6)))
;; = (cons 1 (cons 3 (every-other-m '(5 6))))
;; = (cons 1 (cons 3 (cons 5 (every-other-m '()))))
;; = (cons 1 (cons 3 (cons 5 '())))

;; Translate the following to use match
(define (count-1s l)
  (cond [(empty? l) 0]
        [(equal? (first l) 1) (+ 1 (count-1s (rest l)))]
        [else (count-1s (rest l))]))

(define (count-1s-m l)
  (match l
    ['() 'todo]
    [`(1 ,tl ...) 'todo]
    [`(,x ,tl ...) 'todo]))

(define (map f l)
  (if (empty? l)
      '()
      (cons (f (first l)) (rest l))))

(define (map-m f l)
  (match l
    ['() 'todo]
    [`(,fst ,rst ...) 'todo]))

;; Translate from using match to using if / empty? / etc...
(define (foo l)
  (match l
    ['() 5]
    [`(1 ,rst ...) (+ 1 (foo rst))]
    [`(,x ,rst ...) (+ 2 (foo rst))]))

(define (foo-using-cond l)
  (cond [(equal? l '()) 5]
        [(equal? (first l) 1) (+ 1 (foo-using-cond (rest l)))]
        [else (+ 2 (foo-using-cond (rest l)))]))