[ruby-core:100134] [Ruby master Feature#17056] Array#index: Allow specifying the position to start search as in String#index

Issue #17056 has been updated by Eregon (Benoit Daloze).


What if a block is given, and one want to use a start index? (for efficiency and not run the block for the first `start` elements).

`ary.index(start) { |i| ... }` seems confusing.

Probably keyword arguments are better:
`ary.index(from: start) { |i| ... }` or `ary.index(start: start) { |i| ... }`

Although personally I'm not convinced we need these complications.
One can do `'abcabc'.chars[2..].index('a') + 2` instead of `'abcabc'.chars.index('a', 2)`.
And the `[2..]` is quite cheap considering that arrays use copy-on-write.

It can also be done with `ary = 'abcabc'.chars; (2...ary.size).find { |i| ary[i] == 'a' }`.
That's a little bit more complicated, but it's also usable in many more situations than just `index`.
I would expect it's fairly rare to need a start offset, so I think there is no need for a shortcut.

----------------------------------------
Feature #17056: Array#index: Allow specifying the position to start search as in String#index
https://bugs.ruby-lang.org/issues/17056#change-87705

* Author: TylerRick (Tyler Rick)
* Status: Open
* Priority: Normal
----------------------------------------
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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