$ ruby --version
ruby 1.6.8 (2002-12-24) [i386-cygwin]

irb(main):001:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):002:0> a[0,5]
=> [1, 2, 3]
irb(main):003:0> a[1,5]
=> [2, 3]
irb(main):004:0> a[2,5]
=> [3]
irb(main):005:0> a[3,5]
=> []
irb(main):006:0> a[4,5]
=> nil

要素無しのArray?