方案/球拍:使用map和lambda转换列表 [英] Scheme / Racket : transposing lists with map and lambda

问题描述

我当前正在尝试编写一个程序，该程序获取一个列表列表并返回一个列表列表，其中包含输入列表的每个第一项.例如，我将为您提供输入和输出.

I'm currently trying to code a procedure which gets a list of lists and returns a list of lists, which contains each first item of the input lists. For example I'll provide you with a Input and Output.

输入:

(list (list 1 5) (list 2 6) (list 3 7) (list 4 8)))

输出:

(list (list 1 2 3 4) (list 5 6 7 8))

我的想法是使用lambda构建函数来构建某种程度的映射过程.我目前正在努力将任何函数映射到组合列表元素上，因为 map 只处理列表中的一个项目(据我所知).

My idea was to build a somewhat map procedure using lambda for function. I'm currently struggling on mapping any function on combined list elements, since map only processes a single item on the list (to my understanding).

谁能为我提供有用的见解?

Could anyone provide me with helpful insights?

非常感谢您.

推荐答案

这可以在Scheme/Racket中使用

This can be done in Scheme / Racket with

(apply map list (list (list 1 5) (list 2 6) (list 3 7) (list 4 8)))

在DrRacket中返回的

which in DrRacket returns

'((1 2 3 4) (5 6 7 8))

基本上，调用(应用地图列表 [abc ... n] )(在 pseudocode 中)与调用

Basically, calling (apply map list [a b c ... n]) (in pseudocode), is the same as calling

(map list a b c ... n)

可以这么说，参数列表是开放的" .

The argument list is "opened up", so to speak.

• cf.方案中的矩阵乘法，列表列表

(lambda x x)代替 list :

(apply map (lambda x x) (list ....... ))

这是因为 map 可以与多个列表一起用作其输入，而不仅仅是一个.在这种情况下，lambda函数的参数数量必须与输入列表的数量匹配.

This is because map can be used with several lists as its inputs, not just one. In such a case the number of arguments to the lambda function must match the number of input lists.

特殊情况是(lambda x ...)，其中 x 不在括号内.这意味着这样的lambda函数可以应用于任意个参数，这些参数将全部收集并作为列表传递给.因此，(lambda x x)的行为与内置的 list 相同，并且可以视为其实现.

A special case is (lambda x ...) where x is not enclosed in parentheses. This means such a lambda function can be applied to any number of arguments, which will all be collected and passed in as a list. So (lambda x x) will act just the same as the built-in list does, and can be seen as its implementation.

顺便说一下，这不是合并"，而是换位".

And by the way, this isn't "merging", it is "transposition".

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