新手问题 数组取出子元素问题
为什么 3,4 时候返回时空数组，5 就返回了 nil？
array = [:peanut, :butter, :and, :jelly]
=> [:peanut, :butter, :and, :jelly]
2.1.2 :055 > array[3,0]
=> []
2.1.2 :056 > array[4,0]
=> []
2.1.2 :057 > array[5,0]
=> nil
2.1.2 :058 >
关于Array#slice
an empty array is returned when the starting index for an element range is at the end of the array.
Returns nil if the index (or starting index) are out of range.
来源 ruby-doc