(在Lisp中定义(平均....)) [英] (define (average ....)) in Lisp

问题描述

我只是在研究scheme/lisp，并在考虑如何修正自己对average的定义.我不确定如何做一些我认为必需的事情.

I'm just playing around with scheme/lisp and was thinking about how I would right my own definition of average. I'm not sure how to do some things that I think are required though.

• 定义一个接受任意数量参数的过程
• 计算这些参数
• 将参数列表传递给(+)以将它们加在一起

有人提供定义average的示例吗?我对LISP的了解似乎不足，无法形成Web搜索来获取我想要的结果.

Does someone have an example of defining average? I don't seem to know enough about LISP to form a web search that gets back the results I'm looking for.

推荐答案

该定义将是一个非常简单的单行代码，但是在不破坏它的情况下，您应该研究一下:

The definition would be a very simple one-liner, but without spoiling it, you should look into:

• 一个"rest"参数-该(define (foo . xs) ...xs...)将foo定义为一个接受任意数量参数的函数，它们可作为列表使用，该列表将是xs的值.

• a "rest" argument -- this (define (foo . xs) ...xs...) defines foo as a function that takes any number of arguments and they're available as a list which will be the value of xs.

length返回列表的长度.

apply接受一个函数和一个值列表，并将该函数应用于这些值.

apply takes a function and a list of values and applies the function to these values.

当您了解到这一点时，可以去做更多:

When you get that, you can go for more:

• 参见foldl函数，以避免将列表应用于可能非常大的列表(在某些实现中，参数列表的长度受到限制，这可能很重要，但是在Racket中不会有太大的区别).

• see the foldl function to avoid applying a list on a potentially very big list (this can matter in some implementations where the length of the argument list is limited, but it wouldn't make much difference in Racket).

请注意，Racket具有确切的原理，您可以使用exact->inexact制作更有效的浮点版本.

note that Racket has exact rationals, and you can use exact->inexact to make a more efficient floating-point version.

破坏者是:

• (define (average . ns) (/ (apply + ns) (length ns)))

使其需要一个参数:(define (average n . ns) (/ (apply + n ns) (add1 (length ns))))

使用foldl:(define (average n . ns) (/ (foldl + 0 (cons n ns)) (add1 (length ns))))

使其使用浮点:(define (average n . ns) (/ (foldl + 0.0 (cons n ns)) (add1 (length ns))))

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