ruby-1.8.7-p174 > [0,1][2..3]
=> []
ruby-1.8.7-p174 > [0,1][3..4]
=> nil
In a 0-index setting where index 2, 3, and 4 are all in fact out of bounds of the 2-item array, why would these return different values?
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3Your question has less noise, but unfortunately: possible duplicate of Array slicing in Ruby: looking for explanation for illogical behaviour (taken from Rubykoans.com). In both cases the surprise is the same: nil vs [] at the end index.Ciro Santilli OurBigBook.com– Ciro Santilli OurBigBook.com2014-01-28 16:21:38 +00:00Commented Jan 28, 2014 at 16:21
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Wow, duplicate declined. Why?Ciro Santilli OurBigBook.com– Ciro Santilli OurBigBook.com2014-01-30 10:06:12 +00:00Commented Jan 30, 2014 at 10:06
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Good question. I raised a flag to ask to reevaluate or explain. I am curious to learn the reasoning of such.pjvleeuwen– pjvleeuwen2020-01-14 08:57:54 +00:00Commented Jan 14, 2020 at 8:57
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1 Answer
This is a known ugly odd corner. Take a look at the examples in rdoc for Array#slice.
This specific issue is listed as a "special case"
a = [ "a", "b", "c", "d", "e" ]
a[2] + a[0] + a[1] #=> "cab"
a[6] #=> nil
a[1, 2] #=> [ "b", "c" ]
a[1..3] #=> [ "b", "c", "d" ]
a[4..7] #=> [ "e" ]
a[6..10] #=> nil
a[-3, 3] #=> [ "c", "d", "e" ]
# special cases
a[5] #=> nil
a[5, 1] #=> []
a[5..10] #=> []
If the start is exactly one item beyond the end of the array, then it will return [], an empty array. If the start is beyond that, nil. It's documented, though I'm not sure of the reason for it.
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pjvleeuwen
See this answer and related comments where the difference is described in terms of 'fence-posting' and 'indexing'. I do agree with Ciro, this question is a duplicate. For those reading this Q&A, be aware that more details exist on the 'why' in that duplicate.
