有没有更好的方法来获取列表中的第n个项目? [英] Is there a better way to get the nth item in a list?

问题描述

以下两个表达式是等效的:

The following two expressions are equivalent:

(third (list 1 2 3 4))

(first (nthcdr 2 (list 1 2 3 4)))

但是，使用第三"，第四"，第五"等并不总是可行的，并且(first (nthcdr n list))似乎有些冗长.有没有办法说出类似(item 2 (list 1 2 3 4))这样的内容来获取列表中的第n个项目?

However, using "third," "fourth," "fifth," etc. isn't always practical and (first (nthcdr n list)) seems a little verbose. Is there a way to say something like (item 2 (list 1 2 3 4)) to get the nth item in a list?

推荐答案

(nth 3 (list 1 2 3 4))

返回第4个项目(从零开始！)

returns 4th item (zero based!)

根据 HyperSpec :

访问者 NTH

说明:

Accessor NTH

Description:

nth 定位 list 的 n 个元素， list 是零"元素.具体来说，

nth locates the nth element of list, where the car of the list is the “zeroth” element. Specifically,

(nth n list) ==  (car (nthcdr n list))

示例:

(nth 0 '(foo bar baz)) =>  FOO
(nth 1 '(foo bar baz)) =>  BAR
(nth 3 '(foo bar baz)) =>  NIL
(setq 0-to-3 (list 0 1 2 3)) =>  (0 1 2 3)
(setf (nth 2 0-to-3) "two") =>  "two"
0-to-3 =>  (0 1 "two" 3)

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