Issue #8811 has been updated by sawa (Tsuyoshi Sawada).


=begin
* Matz: As for the method name, phluid61's explanation is exactly what I had in mind. And, some use cases are as shown below.
* phluid61: Your implementation looks perfect to me.

==== Use case 1
I want to delete and get from an array `a` an element at position `i` if there is such element. That element can be possibly `nil`.

With `index?`, it works like this:

    a = ["a", nil, "b"]
    deleted = :none
    deleted = a.delete_at(3) if a.index?(3) # => Deletion did not happen.
    deleted # => :none # I can be sure that deletion did not happen.

    a = ["a", nil, "b"]
    deleted = :none
    deleted = a.delete_at(1) if a.index?(1) # => Deleted `nil`.
    deleted # => nil # I can be sure that an element `nil` was deleted.

Without checking the index, I cannot tell whether deletion happened:

    a = ["a", nil, "b"]
    deleted = :none
    deleted = a.delete_at(3) # => Deletion did not happen.
    deleted # => nil # I cannot tell whether an element `nil` was deleted or deletion did not happen.

    a = ["a", nil, "b"]
    deleted = :none
    deleted = a.delete_at(1) if a.index?(1) # => Deleted `nil`.
    deleted # => nil # I cannot tell whether an element `nil` was deleted or deletion did not happen.

=== Use case 2
I want to prepend with `:foo` an element at position `i` if there is such element. 

With `index?`, it works like this:

    a = ["a", "b"]
    a.insert(1, :foo) if a.index?(1)
    # => ["a", :foo, "b"]

    a = ["a", "b"]
    a.insert(3, :foo) if a.index?(3)
    # => ["a", "b"]

Without `index?`, unwanted `nil` will be inserted

    a = ["a", "b"]
    a.insert(1, :foo)
    # => ["a", :foo, "b"] # This is okay.

    a = ["a", "b"]
    a.insert(3, :foo)
    # => ["a", "b", nil, :foo] # Unwanted result

=end

----------------------------------------
Feature #8811: Counterpart to `Hash#key?` for `Array`
https://bugs.ruby-lang.org/issues/8811#change-41576

Author: sawa (Tsuyoshi Sawada)
Status: Feedback
Priority: Normal
Assignee: 
Category: 
Target version: 


=begin
`Hash` hash `key?` to tell if a key exists without checking the value. It would be convenient if there were a counterpart in `Array`. Suppose it is called `Array#index?`. Then it should behave as follows:

    [1, 2, 3].index?(2) # => true
    [1, 2, 3].index?(3) # => false
    [1, 2, 3].index?(-3) # => true
    [1, 2, 3].index?(-4) # => false

This is useful when we want to insert/move/delete elements to/from a certain position of an array. Without checking if a value exists, it can be messed up. Implementing a check is cumbersome now. With the proposed method, it would become easy.

=end



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