From: "marcandre (Marc-Andre Lafortune)" <ruby-core@...>
Date: 2012-04-02T08:38:31+09:00
Subject: [ruby-core:44047] [ruby-trunk - Feature #6240] Enumerable#drop with negative argument


Issue #6240 has been updated by marcandre (Marc-Andre Lafortune).


Hi,

shugo (Shugo Maeda) wrote:
> How Enumerable#drop can know the total number of elements?
> The source of elements might be IO.  Besides that, the total number of elements might be infinite.

`drop` would have to consume the whole iteration, indeed, which is why I was talking about a buffer. The buffer holds elements until we know they are not in the last (-n) elements. Here's a Ruby implementation:

class Lazy
  def drop(n)
    return to_enum :drop, n unless block_given?
    if n < 0
      buffer = []
      each do |e|
        buffer << e
        yield buffer.shift if buffer.size > -n
      end
    else
      # ...
    end
  end
end

For infinite sequences, `drop` with negative argument would not be very useful, but it would still yield all elements.
----------------------------------------
Feature #6240: Enumerable#drop with negative argument
https://bugs.ruby-lang.org/issues/6240#change-25576

Author: marcandre (Marc-Andre Lafortune)
Status: Open
Priority: Normal
Assignee: 
Category: core
Target version: 2.0.0


Currently, Enumerable#drop works only for non-negative arguments.

It could be extended so that negative arguments means dropping from the end:

    [:hello, :world].drop(-1) # => [:hello]

This could especially be interesting for `Lazy#drop`, which would keep a circular buffer of elements before yielding them.

    (1..6).lazy.drop(-3).each{|x| puts x} # -> prints 1, 2 and 3

Thoughts?


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