Why does Ruby Array slicing sometimes returns nil and sometimes returns empty Array?

Question

I got this question when doing ruby koans. Given this array:

array = [1, 2, 3, 4]

array[4, 0] equals []. However, array[5, 0] equals nil.

Both 4 and 5 are out of index. Why do they return different things?

Answer 1

The first parameter of Array#slice(start,length) is the place between indices where slicing should begin :

array = [1, 2, 3, 4]
# elements            : [   1   2   3   4   ]
#                         ↑   ↑   ↑   ↑   ↑
# slice start indices :   0   1   2   3   4

slice(0,_) begins left of 1, slice(3,_) begins left of 4, and slice(4,_) begins at the last possible place : right of 4.

slice(4,0) is still inside array, it's the empty Array right of 4.

slice(5,0) isn't inside array anymore, it's nil.