如果不使用Racket中的任何其他高级功能,“map”功能的定义是什么?
我需要一个堆栈递归版本。
答案 0 :(得分:5)
地图功能的简单定义可以是:
(define (map f l)
(if (null? l)
'()
(cons (f (car l)) (map f (cdr l)))))
(map (lambda (n) (* n n)) '(1 2 3 4)) ;; => (1 4 9 16)
答案 1 :(得分:1)
通常您会找到使用fold
制作的地图,但我更喜欢使用pair-for-each
(CL中为maplist
)执行所有操作。这定义pair-for-each
,map
,filter-map
,filter
,zip
和unzip
与SRFI-1 List library中的相同过程兼容。
#!racket/base
(define-values (pair-for-each map filter-map filter zip unzip)
(let ((%MAP-PASS (list 'MAP-PASS))
(%MAP-END (list 'MAP-END)))
;; pair-for-each-1 applies proc to every cons
;; in order until proc returns %MAP-END
;; when proc evaluates to %MAP-PASS the result is skipped
(define (pair-for-each-1 proc lst (next cdr))
(let loop ((lst lst))
(let ((res (proc lst)))
(cond ((eq? res %MAP-END) '())
((eq? res %MAP-PASS) (loop (next lst)))
(else (cons res
(loop (next lst))))))))
;; Transform a typical map procedure to include
;; a %MAP-END when the list argument is eq? a certain value
(define (stop-at value proc)
(lambda (lst)
(if (eq? value lst)
%MAP-END
(proc lst))))
;; Takes a lists of lists and returns a
;; new list with the cdrs
(define (cdrs lsts)
(pair-for-each-1 (stop-at '() cdar) lsts))
;; Takes a list of lists and returns a
;; new list with the cars except if one of
;; the sublists are nil in which the result is also nil
(define (cars lsts)
(call/cc (lambda (exit)
(pair-for-each-1 (stop-at '()
(lambda (x)
(let ((x (car x)))
(if (null? x)
(exit '())
(car x)))))
lsts))))
;; Takes a list of lists and returns #t if any are null
(define (any-null? lsts)
(if (null? lsts)
#f
(or (null? (car lsts))
(any-null? (cdr lsts)))))
;; Return value definitions starts here
;; pair-for-each is called maplist in CL
(define (pair-for-each proc lst . lsts)
(if (null? lsts)
(pair-for-each-1 (stop-at '() (lambda (x) (proc x))) lst)
(pair-for-each-1 (lambda (args)
(if (any-null? args)
%MAP-END
(apply proc args)))
(cons lst lsts)
cdrs)))
;; Multi arity map
(define (map f lst . lsts)
(if (null? lsts)
(pair-for-each-1 (stop-at '() (lambda (x) (f (car x)))) lst)
(pair-for-each-1 (lambda (x)
(let ((args (cars x)))
(if (null? args)
%MAP-END
(apply f args))))
(cons lst lsts)
cdrs)))
;; filter-map is like map except it skips false values
(define (filter-map proc . lsts)
(apply map (lambda x
(or (apply proc x) %MAP-PASS)))
lsts)
;; filter only takes one list and instead of the result it
;; takes the original argument as value (which may be #f)
(define (filter predicate? lst)
(pair-for-each-1 (stop-at '()
(lambda (x)
(let ((x (car x)))
(if (predicate? x)
x
%MAP-PASS))))
lst))
;; zip (zip '(1 2 3) '(a b c)) ; ==> ((1 a) (2 b) (3 c))
(define (zip lst . lsts)
(apply map list (cons lst lsts)))
;; unzip does the same except it takes a list of lists as argument
(define (unzip lsts)
(apply map list lsts))
;; return procedures
(values pair-for-each map filter-map filter zip unzip)))
答案 2 :(得分:1)
我不清楚OP所要求的实施方式,所以这是map
的另一种变体。
; map : function list -> list
; (map f '()) = '()
; (map f (cons x xs)) = (cons (f x) (map f xs))
(define (my-map f xs)
; loop : list list -> list
; (loop (list x1 ... xn) (list y1 ... ym)) = (list (f x1) ... (f xn) ym ... y1)
(define (loop xs ys)
(match xs
['() (reverse ys)]
[(cons x xs) (loop xs (cons (f x) ys))]))
(loop xs '()))
示例:
(my-map sqrt'(1 4 9 16)) '(1 2 3 4)