sicp_2.17-2.20

#lang racket

;;;;;;;;;;;;;;;;;;;;;;;;2.17
(define (last-pair L)
  (if (null? (cdr L))
      L
      (last-pair (cdr L))))

;;;;;;;;;;;;;;;;;;;;;;;2.18
(define (reverse L)
  (if (null? (cdr L))
      L
      (cons (reverse (cdr L)) (car L))))

;;;;;;;;;;;;;;;;;;;;;;2.19
(define us-coins (list 50 25 10 5 1))
(define us-coins2 (list 1 5 10 25 50))
(define uk-coins (list 100 50 20 10 5 2 1))

(define (cc amount coin-values)
  (cond ((= amount 0) 1)
        ((or (< amount 0) (no-more? coin-values)) 0)
        (else
         (+ (cc amount
                (except-first-denomination coin-values))
            (cc (- amount
                   (first-denomination coin-values))
                coin-values)))))

(define (no-more? coins)
  (null? coins))

(define (except-first-denomination coin-values)
  (cdr coin-values))

(define (first-denomination coins)
  (car coins))

;;;;;;;;;;;;;;;;;;;;2.20
(define (same-parity sample . others)
  (filter (if (even? sample)
              even?
              odd?)
          (cons sample others)))

;;;;;;;;;;;;;;;;;;;;;;;test
;(last-pair (list 23 37 149 34))
;(reverse (list 1 4 9 16 25))
;(cc 100 us-coins)
;(cc 100 us-coins2)
;(same-parity 2 3 4 5 6 7)