Forum: Ruby-core Inconsistent Array.slice()

Issue #4541 has been updated by alexeymuranov (Alexey Muranov).


stomar (Marcus Stollsteimer) wrote:
> Regarding the OP's criticism of poor API documentation, this has in the meantime 
been improved (with issue #6680).

I hope this does not mean that the specification is now based on the 
implementation :).

In my opinion,

  array = ['a', 'b', 'c']
  array[4..4] # => []

makes better sense than

  array = ['a', 'b', 'c']
  array[4..4] # => nil

I think that `nil` should only be returned if there is no way to return 
a meaningful value, but also no particular reason to raise an error. 
Here a meaningful value to return IMO is `[]`, the same as for 
`array[3..4]`.

Think of a line of small tiles with letters 'a', 'b', 'c' on them which 
you have enumerated in your head starting with 0 on the left.

If you are asked to form a new line with letters 1..5, you will get the 
line ['b', 'c'] by taking 'b', then 'c', then nothing, and then taking 
nothing 2 more times.
If you are asked to form a new line with letters 3..4, you will get the 
empty line by taking 2 times nothing.
If you are asked to form a new line with letters 4..4, you will get the 
empty line by taking 1 time nothing.
If you are asked to form a new line with letters 4..5, you will get the 
empty line by taking 2 times nothing.

You will always get a line, you will not get "nil".

----------------------------------------
Feature #4541: Inconsistent Array.slice()
https://bugs.ruby-lang.org/issues/4541#change-34678

Author: kbl (Marcin Pietraszek)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: next minor


=begin
Array slice/[] method is a bit inconsistent. Is it just poorly 
documented "feature" or a bug? In API doc I can't find this behaviour 
mentioned as a "special case".

 def test_array_slice
    array = ['a', 'b', 'c']
    assert_equal nil, array[3]
    assert_eaual nil, array[4]

    assert_eaual [], array[3, 0] #
    assert_equal nil, array[4, 0] # [] expected (or both nils in 
array[3, 0] and array[4, 0])

    assert_equal ['c'], array[2..2]
    assert_equal [], array[3..3] #
    assert_equal nil, array[4..4] # [] expected (or both nils in 
array[3..3] and array[4..4])
 end

Same behaviour can be reproduced on ruby 1.8.7 (2010-12-23 patchlevel 
330) [x86_64-linux].
=end