Confusion about ruby Array [] method results when starting index greater than (array.size -1)

Question

Given

array = [:a,:b,:c,:d] # with 4 elements:

array[3] => :d  # zero-based indices, I get that
array[4] => nil
array[5] => nil

array[3,0] => [] # OK since I asked for a slice with zero elements

Doco for array[start,length] says that it "Returns nil if the index (or starting index) are out of range."

array[5,0] => nil # OK
array[4,0] => []  # Hunh??

How come array[4,0] returns an array as opposed to nil ?

[edit] Looks like this has already come up: see Array slicing in Ruby: looking for explanation for illogical behaviour (taken from Rubykoans.com)

To me the explanation looks a bit like hand waving, but I'll settle for it and just accept that ruby violates PLS here.

Answer 1

I believe that the documentation does not word the behavior correctly. But intuitively, the bahavior makes sense. Just like it is possible to define a 0-element "sliver" at the beginning of the array, before all elements by indexing array[0,0] , it should also be possible, symmetrically, to get a 0-element "sliver" at the end of the array, after all the elements, by indexing array[array.size,0] . Another example would be that it should intuitively always be possible to get a copy of an array by indexing over its whole range, ie array[0,array.size] ; but this would not be possible for empty arrays if we returned nil when the starting index is equal to the array size.