Issue #17056 has been updated by matz (Yukihiro Matsumoto). I think it's good to add an offset: keyword argument to `Array#index` and `Array#rindex`, for consistency with `String#index` and `String#rindex`. As for `Array#find_index`, I think we should add `offset:` there as well, keeping it in sync with `Array#index`. We will leave `Enumerable#find_index` untouched, since adding an offset to a lazy enumerable feels unnatural. We have no plan to add an end index for now. Matz. ---------------------------------------- Feature #17056: Array#index: Allow specifying the position to start search as in String#index https://bugs.ruby-lang.org/issues/17056#change-116745 * Author: TylerRick (Tyler Rick) * Status: Open ---------------------------------------- I have a use case of finding the first matching line within a given section in a file. After finding the line number of the start of the section, I want to find the first match after that line. My workaround for now is to use `with_index`: ```ruby lines = pathname.read.lines section_start_line = lines.index {|line| line.start_with?(/#* #{section_name}/) } lines.index.with_index {|line, i| i > section_start_line && line.include?(sought) } ``` I'd like to do it in a more concise way using a feature of `Array#index` that I propose here, which is analogous to `String#index`. If the second parameter of `String#index` is present, it specifies the position in the string to begin the search: ```ruby 'abcabc'.index('a') # => 0 'abcabc'.index('a',2) # => 3 ``` I would expect to also be able to do: ```ruby 'abcabc'.chars.index('a') # => 0 'abcabc'.chars.index('a', 2) ``` Using such feature, I would be able to do: ```ruby lines.index(sought, section_start_line) ``` This would give Ruby better parity with other programming languages like Python: ```python

...

...

list('abcabc') ['a', 'b', 'c', 'a', 'b', 'c'] list('abcabc').index('a') 0 list('abcabc').index('a', 2) 3

## End index

We can further think of an optional parameter to specify the position to end the search. The following languages allow specifying both start and end indexes:

- [Python](https://docs.python.org/3/tutorial/datastructures.html)
- [C#](https://docs.microsoft.com/en-us/dotnet/api/system.array.indexof?view=netcore-3.1)

Ruby's `String#index` does not have one, so we could make a separate proposal to add `end` to both methods at the same time.



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