SICP Section 2.2.1

Exercise 2.17

#lang racket(define (last-pair l)  (if (null? (cdr l))       l      (last-pair (cdr l))))

Exercise 2.18

#lang racket; (define (reverse list);   (define (mid-reverse list midlist);     (if (null? list);         midlist;         (mid-reverse (cdr list) (cons (car list) midlist))));   (mid-reverse list '()));'() can't be replaced as (list) why?;use append(define (reverse items)  (if (null? items)      (list)      (append        (reverse (cdr items))                (list (car items)))))      

Exercise 2.19

#lang racket(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? null?)(define except-first-denomination cdr)(define first-denomination car)(define us-coins (list 50 25 10 5 1))(cc 100 us-coins)

Exercise 2.20

#lang racket;the two number is same even-odd parity?(define (same-kind? first second)  (= (remainder (- first second) 2) 0));compare the first to the rest recursely(define (same-parity first . dulp)  (let ((dulpp dulp))    (define (parity first dulpp)        (if (null? dulpp)            (list first)            (let ((comparison (car dulpp))                  (rest (cdr dulpp)))              (if (same-kind? first comparison)                    (cons first (parity comparison rest))                  (parity first rest)))))     (parity first dulpp)));output(same-parity 2 4 6 8 9 10)            

Summarize

      闭包很强大，它允许我们轻松的创造层级结构，换言之，它能够创造很多基本的或复杂的数据结构，比如序列和树。任何强有力的编程语言都会提供这种能力。