Common Lisp：在列表中的连续对之间循环的最佳方法是什么？ [英] Common Lisp: what's the best way to loop through consecutive pairs in a list?

问题描述

有时我需要遍历列表中的连续对。我现在的操作方式是

Sometimes I need to loop through consecutive pairs in a list. The way I do it right now is

(loop for x on lst while (not (null (cdr x)))
       (do something on (car x) and (cadr x)))

我是

我有时需要这样的原因，例如，我想知道是否有更好的/内置的方法来做到这一点。一些添加连续对的函数

The reason I need this is sometimes I want, e.g. some function that add consecutive pairs

(1 2 3 4 5) ----> (3 5 7 9)

是否有任何内置函数（如reduce）可以让我得到？

Is there any built-in function like reduce which allow me to get this?

推荐答案

AFAIK，没有内置函数可以执行您想要的操作。您可以尝试将某些内容与 maplist 组合在一起，但是我的第一个直觉是也可以达到 loop 。

AFAIK, there isn't a built-in function to do what you want. You could try to put something together with maplist, but my first instinct would be to reach for loop too.

不过，您只需要注意几个便笺即可。首先，（not（null foo））等效于CL中的 foo ，因为非<$ c $布尔运算将c> NIL 的值视为 t 。其次， loop 可以分解其参数，这意味着您可以更优雅地编写为

Just a couple of notes on what you've got there though. First, (not (null foo)) is equivalent to foo in CL, since a non-NIL value is treated as t by boolean operations. Second, loop can destructure its arguments, meaning you can write this more elegantly as

(loop for (a b) on lst while b
      collect (+ a b))

地图列表版本看起来像

(maplist 
   (lambda (rest) 
     (when (cdr rest) 
        (+ (first rest) (second rest)))
   lst)

我认为它的可读性较差（这还将返回NIL作为结果的最后一个元素，而不是仅仅在此之前结束）。

which I consider less readable (this would also return NIL as the last element of its result, rather than just ending before that).

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