Scheme List Operation

list-operation.ss

;Scheme中关于list的操作.
; author: cyanlong
; date: 2016-11-02

;一:构造list
;通过Scheme提供的序对操作.
;cons :construct
(cons 1
      (cons 2
            (cons 3 nil)))
;output: (1 . #<promise:temp2>)

;list 过程便捷构造
(list 1 2 3 4)

;二:访问元素
;car 选取表的第一个元素, cdr选取表中除去第一项之后剩下的所有项形成的子表
(define list1 (list 1 2 3 4))
(car list1) ;1
(cdr list1) ;(2 3 4)

;2.依次遍历输出list
;(define (print items)
;  (if (null?items)
;      "."
;      (car items)))

;三:常规的表操作
;1.(list-ref items n) 返回这个表中的第n个项.
;方法1:
(define (my-list-ref items n)
  (define (list-iter its count)
    (if (>= count n)
        (car its)
        (list-iter (cdr its) (+ count 1))))
  (list-iter items 0))

;方法2:
(define (my-list-ref2 items n)
  (if (= n 0)
      (car items)
      (my-list-ref2 (cdr items) (- n 1))))

;test
(my-list-ref (list 1 2 3 4) 3)
(my-list-ref2 (list 1 2 3 4) 3)


;2.(length items) 返回一个列表的长度
;迭代.
(define (my-length items)
  (define (length-iter items count)
    (if (null? items)
        count
        (length-iter (cdr items) (+ count 1))))
  (length-iter items 0))
;递归.
(define (my-length-recu items)
  (if (null? items)
      0
      (+ 1 (my-length-recu (cdr items)))))

;test
(my-length (list 1 2 3 4))
(my-length-recu (list 1 2 3 4))

;3.(append items item) 添加item(基本类型值)到items列表末尾
(define (my-append items item)
  (if (null? items)
      (cons item (nil))
      (cons (car items) 
            (my-append (cdr items) item))))

;test
(define app-list (my-append (list 1 2 3) 4))
(list-ref app-list 3)
 

;(append list1 list2) 两个list连接.
(define (my-append list1 list2)
  (if (null? list1)
      list2
      (cons (car list1) 
            (append (cdr list1) list2))))

;test
(define con-list (my-append (list 1 2 3) (list 4 5 6)))
(list-ref con-list 4)

;4: (last-pari items) 返回只包含给定表里最后一个元素的表.
(define (my-last-pari items)
  (if (null?(cdr items))
      items
      (my-last-pari (cdr items))))
;test
(my-last-pari (list 1 2 3 4))

;5.(reverse items) 返回与参数表元素相反的表.
;(define (my-reverse items)
;  (if (null? items)
;      
;      (cons )))

;四:高阶操作.
;如果没有map等高阶抽象,我们需要将注意力集中到对表的依次遍历上.
(define (scale-list items factor)
  (if (null? items)
      nil
     (cons (* (car items) factor)
           (scale-list (cdr items) factor))))

(scale-list (list 1 2 3) 10)

;map: 它由一个过程参数和一个表参数,返回将这一过程应用于表中各个元素得到的结果形成的表.
(define (my-map proc items)
  (if (null? items)
      nil
      (cons (proc (car items))
            (my-map proc (cdr items)))))
(define abs-list (my-map abs (list -1 2 -3)))

;test
(list-ref abs-list 0)
(my-list-ref abs-list 2)

;2.for-each
;特殊形式begin可以确保多语句按顺序求值.
;它可以将多条表达式当作一条表达式来运行.
(define (my-for-each proc items)
  (if (not (null? items))
      (begin
        (proc (car items))
        (for-each proc (cdr items)))))
"for-each"
(my-for-each (lambda (x) (newline)(display x)) (list 1 2 3))