[ruby-core:35643] [Ruby 1.9 - Bug #4541] Inconsistent Array.slice()

Issue #4541 has been updated by Marc-Andre Lafortune.


Hi,

On Tue, Apr 5, 2011 at 3:04 AM, Yui NARUSE <[e-mail]> wrote:
>
> Issue #4541 has been updated by Yui NARUSE.
>
> Status changed from Open to Assigned
> Assignee set to Yukihiro Matsumoto
>
> The fix may be following but it can be a spec...
>
> diff --git a/array.c b/array.c
> index bdeb768..4721387 100644
> --- a/array.c
> +++ b/array.c
> @@ -948,7 +948,7 @@ rb_ary_subseq(VALUE ary, long beg, long len)
>  {
>     VALUE klass;
>
> -    if (beg > RARRAY_LEN(ary)) return Qnil;
> +    if (beg >= RARRAY_LEN(ary)) return Qnil;
>     if (beg < 0 || len < 0) return Qnil;
>
>     if (RARRAY_LEN(ary) < len || RARRAY_LEN(ary) < beg + len) {

With all due respect, I am strongly opposed to that change. There is no way that the following results should change:

    array[0...3] # => ["a", "b", "c"]
    array[1...3] # => ["b", "c"]
    array[2...3] # => ["c"]
    array[3...3] # => [], as it *must* be.

The only possible change would be that array[42..42] returns [] instead of nil. For consistency's sake, `String#slice` would also have to be changed. I am not in favor of that change either, for compatibility reason and because I feel the current API makes sense. See my comment on issue #4245 [ruby-core:34197]. This issue is a duplicate of #4245.

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Bug #4541: Inconsistent Array.slice()
http://redmine.ruby-lang.org/issues/4541

Author: Marcin Pietraszek
Status: Assigned
Priority: Normal
Assignee: Yukihiro Matsumoto
Category: 
Target version: 1.9.2
ruby -v: ruby 1.9.2p136 (2010-12-25 revision 30365) [x86_64-linux]


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].


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