Feature #17056

Updated by TylerRick (Tyler Rick) about 2 months ago

The docs for `String#index` say: 

 > If the second parameter is present, it specifies the position in the string to begin the search. 

 So you can do: 

 > 'abcabc'.index('a',2) 
 #=> 3 

 #=> 0 

 I would expect to also be able to do: 
 #=> 0 

 'abcabc'.chars.index('a', 2) 
 ArgumentError: wrong number of arguments (given 2, expected 0..1) 
 from (pry):19:in `index' 

 After using this feature on strings, it is surprising to find that it is not available on arrays. 

 ## Use case 

 One use case I have for this is scanning a file, trying to find the first matching line within a given section. So first I find the line number of the start of the section, and then I want to use that to find the first match _after_ that line. 

     lines = 
     section_start_line = lines.index {|line| line.start_with?(/#* #{section_name}/) } 
     lines.index(sought, section_start_line) 

 ## Feature parity 

 This would also give Ruby better parity with other programming languages that support this, like Python: 

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

 ## End index too? 

 Ideally, we would also add an optional end index arg as well, but `String#index` does not have one, so we could a separate proposal to add `end` to both methods at the same time. 

 Other languages that allow specifying both start and end indexes: 
 - [Python]( 
 - [C#]( 
 - ...