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Possible Duplicate:
Array slicing in Ruby: looking for explanation for illogical behaviour (taken from Rubykoans.com)
Say you have an array
a = [1,2,3]
why the a.slice(3,6) returns [] while the a.slice(4,6) returns nil?
Because it makes assignment more general
The mechanism is designed this way so slices can work in a highly generalized way on the left-hand side of assignment operators.
It doesn’t really matter for
#sliceexactly because that result cannot be assigned but the same interpretation applies tox[3, 6]and those expressions can be assigned.It’s best to look at the array indices as identifying the spaces between elements, rather than the elements themselves.
This interpretation creates a consistent and useful interface … for example, code can be written that will handle replacing elements or appending to zero length or populated Arrays, and all without needing special-case tests.