codingexam0-practice.rkt

#lang racket

; takes two lists and builds a list of pairs of each element
; return empty list for unused elements as in '(0 1) '(2 3 4)
; (zip '(0 1) '(2 3)) --> '((0 . 2) (1 . 3))
(define (zip lst0 lst1)
  (match (cons lst0 lst1)
    [(cons `(,hd0 . ,tl0) `(,hd1 . ,tl1)) (cons (cons hd0 hd1) (zip tl0 tl1))]
    [(cons '() '()) '()]
    [(cons _ _) '()]))

(define zip-test-list '((0 . 4) (1 . 5) (2 . 6)))

; takes a list and unzips it, assuming each element is a pair,
; return a pair of lists
(define (unzip lst)
  (match lst
    ['() (cons '() '())]
    [`((,fst-car . ,fst-cdr) . ,tl)
     ; this is a cons cell whose car contains the cars of tl
     ; and whose cdr contains the cdrs of tl
     (define unzipped-tl (unzip tl))
     (cons (cons fst-car (car unzipped-tl)) (cons fst-cdr (cdr unzipped-tl)))]))

; map a function over a list
(define (map f lst)
  (define (loop f lst acc)
    (match lst
      ['() (reverse acc)]
      [`(,hd . ,tl) (loop f tl (cons (f hd) acc))]))
  (loop f lst '()))

; append two lists
; (append '() l) = l
; (append (cons e rst) l) = (cons e (append rst l))
(define (append lst0 lst1)
  (match lst0
    ['() lst1]
    [`(,hd . ,tl) (cons hd (append tl lst1))]))