From: lucasbuchala@...
Date: 2018-05-19T15:50:55+00:00
Subject: [ruby-core:87194] [Ruby trunk Feature#14777] Add Range#offset ?

Issue #14777 has been updated by lucasbuchala (Lucas Buchala).


Hello. I don't have any strong opinion about this feature, but I guess I would welcome such feature if it used standard operators. After seeing this issue, I remembered that I have tried this in the past, so I'm just sharing a snippet to give some ideas. I took this approach as a beginner. Hopefully someone more experienced will have a more idiomatic solution:

~~~ruby
class Range
  def _build(sym, *args)
    b = self.begin.__send__(sym, *args)
    e = self.end.__send__(sym, *args)
    self.class.new(b, e, exclude_end?)
  end

  def +(n); _build(:+, n) end
  def -(n); _build(:-, n) end
  def *(n); _build(:*, n) end
  def /(n); _build(:/, n) end
end
~~~

~~~ruby
p (0..4) + 5  #=> 5..9
p (5..9) - 5  #=> 0..4
p (1..4) * 2  #=> 2..8
p (2..8) / 2  #=> 1..4
                         
p (0...4) + 5  #=> 5...9
p (5...9) - 5  #=> 0...4
p (1...4) * 2  #=> 2...8
p (2...8) / 2  #=> 1...4
~~~

As a comparision, Perl 6 ranges implement the same behavior:

~~~
say (0..4) + 5;   #=> 5..9
say (5..9) - 5;   #=> 0..4
say (1..4) * 2;   #=> 2..8
say (2..8) / 2;   #=> 1.0..4.0

say (0..^4) + 5;  #=> 5..^9
say (5..^9) - 5;  #=> ^4
say (1..^4) * 2;  #=> 2..^8
say (2..^8) / 2;  #=> 1.0..^4.0
~~~

Also, in a superficial search, I found a similar already proposed feature: https://bugs.ruby-lang.org/issues/7580


----------------------------------------
Feature #14777: Add Range#offset ?
https://bugs.ruby-lang.org/issues/14777#change-72183

* Author: owst (Owen Stephens)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Hi,

As mentioned in https://bugs.ruby-lang.org/issues/14473#note-17 an addition to Range that we find useful is an `Range#offset(n)` method that adds (or subtracts) `n` to the range, for example:

~~~ruby
(1..10).offset(2)   # => (3..12)
(1...10).offset(1)  # => (2...11)
(1..10).offset(-10) # => (-9..0)
~~~

Similarly to `Range#step` we can support non-Numeric objects if they implement `succ`:
~~~ruby
('a'..'e').offset(2) # => ('c'..'g')
~~~

Alternative names could be `Range#shift` (i.e. shift the elements of the Range up or down) or perhaps `>>`/`<<`:
~~~ruby
(1..10).shift(2) # => (3..12)
(1..10) >> 2 # => (3..12)
(1...10) << 1 # => (0...9)
~~~
However, I don't think the operators are clear enough, so I prefer `offset` or `shift`.

An example pure Ruby implementation is:

~~~ruby
class Range
  def offset(n)
    add_n = ->(x) do
      if x.is_a?(Numeric)
        x + n
      elsif x.respond_to?(:succ)
        n.times { x = x.succ }
        x
      else
        raise ArgumentError, "Can't offset #{x.class}"
      end
    end

    if exclude_end?
      (add_n.call(first)...add_n.call(last))
    else
      (add_n.call(first)..add_n.call(last))
    end
  end
end
~~~

Please let me know your thoughts, I can then look to implement this properly in C.

Regards,
Owen.



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