Question

I’m working my way through a Ruby tutorial (just for hoops and giggles) and while experimenting around one of the examples this behavior struck me as unexpected:

s = "hello"
s[1, 2]          # => "el"
s[1 .. 2]        # => "el"
s[-4 .. -3]      # => "el"
s[-4, -3]        # => nil ... but why?

I would have expected the last line to yield the same result as the one before it. After all it works that way with positive slice values. Where am I going wrong?

Answer 1

Because a negative length for the slice doesn’t make sense. The slice method takes an index, or a range of indexes, or a start and length.

s[-4 .. -3] # here you're passing the slice method a range of numbers
s[-4, -3]   # here you're passing the slice method: start = -4, length = -3

s[-4, 2] => "el"

Documentation for the slice method for the String class