如何找到子字符串的索引？ [英] How to find index of a substring?
问题描述
正在寻找与Ruby等效的Elixir：
Looking for Elixir equivalent of Ruby's:
"john.snow@domain.com".index("@") # => 9
"john.snow@domain.com".index("domain") # => 10
推荐答案
TL; DR：String.index / 2是由于存在更智能的替代方案，因此故意丢失了。 String.split / 2 通常会解决潜在的问题-并且具有更好的性能。
TL;DR: String.index/2 is intentionally missing because smarter alternatives exist. Very often String.split/2 will solve the underlying problem - and with a way better performance.
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我假设我们在这里谈论的是UTF-8字符串，并希望能够干净地处理非ASCII字符。
I assume we are talking UTF-8 strings here and expect to cleanly deal with non-ASCII characters.
Elixir鼓励使用快速代码。事实证明，我们通常尝试使用String.index / 2解决的问题可以用一种更加智能的方式来解决，从而在不降低代码可读性的情况下极大地提高了性能。
Elixir encourages fast code. It turns out that problems we usually try solve with String.index/2 can be solved in a much smarter way, vastly improving performance without degrading code readability.
更聪明的解决方案是使用String.split / 2和/或其他类似的String模块功能。 String.split / 2在字节级别上运行，同时仍可以正确处理字素。不会出错，因为两个参数都是字符串！ String.index / 2必须在字素级别上工作，并缓慢地遍历整个String。
The smarter solution is to use String.split/2 and/or other similar String module functions. The String.split/2 works on a byte-level while still correctly handling graphemes. It can't go wrong because both arguments are Strings! The String.index/2 would have to work on a grapheme-level, slowly seeking throughout the String.
因此，String.index / 2为除非出现非常引人注目的用例，而这些用例不能被现有功能彻底解决。
For that reason the String.index/2 is unlikely be added to the language unless very compelling use cases come up that cannot be cleanly solved by existing functions.
另请参见关于elixir-lang-core的讨论，没关系：
https://groups.google。 com / forum /＃！topic / elixir-lang-core / S0yrDxlJCss
See also the elixir-lang-core discussion on that matter: https://groups.google.com/forum/#!topic/elixir-lang-core/S0yrDxlJCss
Elixir在其成熟的Unicode中非常独特支持。虽然大多数语言都在代码点级别上工作（俗称字符），但Elixir却使用了更高的字素概念。字素是用户认为的一个字符（更确切地说，它是对字符的理解）。字素可以包含多个代码点（依次可以包含多个字节）。
On a side note, Elixir is pretty unique in its mature Unicode support. While most languages work on a codepoint level (colloquially "characters"), Elixir works with a higher level concept of graphemes. Graphemes are what users perceive as one character (lets say its a more practical understanding of a "character"). Graphemes can contain more than one codepoint (which in turn can contain more than one byte).
最后，如果我们确实需要索引：
Finally, if we really need the index:
case String.split("john.snow@domain.com", "domain", parts: 2) do
[left, _] -> String.length(left)
[_] -> nil
end
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