在方案中定义地图以接受许多列表 [英] defining map in scheme to take many lists

问题描述

我正在为方案中的方案创建一个评估程序，而我希望它具有的功能之一就是地图.但是，我发现的map的所有定义都不允许有多个列表. 例如:

I am creating an evaluator for scheme in scheme, and one of the features I want it to have is map. However, all the definitions for map I've found do not allow for multiple lists. For example:

(define (my-map proc lis)
   (cond ((null? lis)
          '())
         ((pair? lis)
          (cons (proc (car lis))
                (my-map proc (cdr lis))))))

此映射的定义是不完整的"，因为例如您不能像这样添加2个数字列表(它只允许一个列表作为参数):

This definition of map is 'incomplete' because, for example, you cannot add 2 lists of numbers with it like so (it only allows one list as an argument):

(my-map + '(1 2 3) '(4 5 6))

如何修改上面的地图定义以允许任意数量的列表?

How do I modify the above map definition to allow an arbitrary number of lists?

推荐答案

您可以定义一个multi-map过程，该过程可以处理多个列表，而one-map过程仅适用于一个列表(这就是您的程序正在执行，尽管第二个条件有点不正常.

You can define a multi-map procedure that works with multiple lists in terms of a one-map procedure that works for just one list (that's what your my-map procedure is doing, although the second condition is a bit unusual).

假设所有列表具有相同的长度，并且至少 个列表作为参数传递给multi-map:

Here's how, assuming that all the lists have the same length and that at least one list is passed as parameter to multi-map:

(define (one-map proc lst)
  (if (null? lst)
      '()
      (cons (proc (car lst))
            (one-map proc (cdr lst)))))

(define (multi-map proc . lst)
  (if (null? (car lst))
      '()
      (cons (apply proc (one-map car lst))
            (apply multi-map proc (one-map cdr lst)))))

例如:

(multi-map + '(1 2 3) '(4 5 6) '(7 8 9))
=> '(12 15 18)

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