如何在列表中搜索特定元素答案

【问题标题】：How to search for a specific element in list如何在列表中搜索特定元素
【发布时间】：2018-12-10 17:53:39
【问题描述】：

我想检索列表中第一个出现的"Apple" 的 id（在本例中为 1）。例如：

List = [["1","Apple"],["2","Orange"],["3","Apple"]].

【问题讨论】：

您的列表格式是预定义的吗？如果没有，您可能希望查看对单个项目使用元组并为 id 使用整数，例如[{1, "Apple"}, {2, "Orange"}, {3, "Apple"}, ...] 这将允许您使用 proplists (erlang.org/doc/man/proplists.html) 函数。
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标签： list erlang

【解决方案1】：

您可以为此使用lists:search/2：

List = [["1","Apple"],["2","Orange"],["3","Apple"]],
{value, [Id, "Apple"]} =
  lists:search(fun([Id, Name]) -> Name == "Apple" end, List),
Id.

【讨论】：

【解决方案2】：

一个简单的递归函数可能就是您在这里寻找的。p>

find_key([], _) -> error;
find_key([[Key, Value] | Rest], Search) when Value = Search -> Key;
find_key([_ | Rest], Search) -> find_key(Rest, Search).

【讨论】：

我认为在 C 中实现 BIF lists:keyfind/2 会更好。
@Hynek-Pichi-Vychodil 对于实际系统，我同意。我不知道他们是在为自己写这个作为练习，还是为了在现实世界中使用。我的回答是考虑到练习。

【解决方案3】：

在 Erlang 世界中，对于固定大小的数据类型，我们使用 tuples。你的 List 可能会长大，但我认为它的元素是固定大小的，所以我建议对其元素使用元组，你可以使用模块的 API 函数lists 和 proplists：

1> List = [{"1", "Apple"}, {"2", "Orange"}, {"3", "Apple"}].
[{"1","Apple"},{"2","Orange"},{"3","Apple"}]
%% Search in List for a tuple which its 2nd element is "Apple":
2> lists:keyfind("Apple", 2, List).
{"1","Apple"}
3> lists:keyfind("Unknown", 2, List).
false
%% Take first Tuple which its 2nd element is "Apple", Also yield Rest of List:
4> lists:keytake("Apple", 2, List).
{value,{"1","Apple"},[{"2","Orange"},{"3","Apple"}]}
%% Replace a tuple which its 1st element is "3" with {"3", "Banana"}
5> lists:keyreplace("3", 1, List, {"3", "Banana"}).
[{"1","Apple"},{"2","Orange"},{"3","Banana"}]

【讨论】：