From: sin@...
Date: 2020-03-07T06:39:17+00:00
Subject: [ruby-core:97392] [Ruby master Feature#16665] Add an	Array#except_index method

Issue #16665 has been updated by prajjwal (Prajjwal Singh).


There needs to be more discussion on the behavior of this method.

Logically, `except_indices` should be the complement of `values_at`, so for the purpose of writing test cases I added this stopgap:

```ruby
def except_indices(*idx)
  self - self.values_at(*idx)
end
```

And ended up with following test cases:

```ruby
  def test_except_indices
    a = @cls['a', 'b', 'c', 'd', 'e']

    assert_equal(@cls['a', 'c'], a.except_indices(1, -2, -1))
    assert_equal(@cls['c', 'd', 'e'], a.except_indices(0, 1, 10))
    assert_equal(@cls['a', 'b', 'c', 'd', 'e'], a.except_indices(10, 20, 30))

    assert_equal(@cls[], a.except_indices(0..4))
    assert_equal(@cls['c', 'd', 'e'], a.except_indices(0..1))
    assert_equal(@cls['c'], a.except_indices(0..1, 3..4))
    assert_equal(@cls['a', 'b', 'c'], a.except_indices(3..15))
    # Logically except_indices is the complement of values_at, and values_at
    # returns an empty array when given a range starting with a negative number.
    # This could change in the future.
    # https://bugs.ruby-lang.org/issues/16678
    assert_equal(@cls['a', 'b', 'c', 'd', 'e'], a.except_indices(-1..2))
  end
```

However, I'm left wondering whether that last case is something that should be fixed (with `values_at` being changed to accomodate ranges starting with a negative), or if this is fine.

Any feedback welcome. Full commit is at - https://github.com/Prajjwal/ruby/commit/0bb2788fb249df4381982cea957c7feaadccd0ed.

----------------------------------------
Feature #16665: Add an Array#except_index method
https://bugs.ruby-lang.org/issues/16665#change-84522

* Author: alex_golubenko (Alex Golubenko)
* Status: Open
* Priority: Normal
----------------------------------------
The main idea is to implement a method that we can use to exclude elements from the array by their indices.

For example:

``` ruby
%w( a b c d e f).except_index(0, -1) 
=> ["b", "c", "d", "e"]

%w( a b c d e f g h ).except_index(0..1, 3, -2..-1)
=> ["c", "e", "f"]
```

I was meeting many questions on the StackOverflow about how to do such functionality also found many topics about it.
So I think it might a helpful addition.

I spent a few days finding the proper solution on Ruby that might be acceptable with integers and ranges(both positive and negative) and has good performance: 

```ruby
  def except_index(*indexes)
    indexes.each_with_object(dup) do |ind, obj|
      ind.is_a?(Range) ? ind.each { |i| obj[i] = false } : obj[ind] = false
    end.select(&:itself)
  end
```

As you can see it's have not the best readability so I think it's a good point to add a built-in method on C.



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