ruby-on-rails - 想要翻译一段ruby的代码，有两句看不懂

Question

1. @item_array = Hash.new { |hash| hash = Hash.new(false) }
2. @item_array[i] = [] unless @item_array.member? sindex;

第一句觉得很奇怪啊，没理解错的话是创建了一个二维hash表吧。但是很不解的是为什么ruby的hash可以给定一个初始值？只有值吗？键呢？键在哪儿？

第二句那个unless很绕啊，真搞不懂为啥要有这样的关键字，那句话意思是说：
除非存在sindex这个键，我赋值为[]，否则。。。得，按我这样理解这个“否则”还不知道要干嘛！？

请大家帮我看看吧，先谢谢啦。

Answer 1

First question: http://ruby-doc.org/core-2.1.0/Hash.html#method-c-new
Let’s write a few more sentences. There are two ways to use it

xxx = Hash.new { |hash| Hash.new(false) }

or

xxx = Hash.new { |hash,key | hash[key] = Hash.new(false);nil }

Actually, the hash = Hash.new(false); written in the question is completely unnecessary. The meaning of this sentence is actually
Actually I just want

 Hash.new { Hash.new(false);}

is enough.

Second question: If item_array does not have a member with symbol sindex, then execute the previous @item_array[i] = []. This syntax is to post clauses such as if (or while unless). What it expresses is actually

xxx if condition

=>

if condition
  xxx
end

Essentially, there are a few lines missing. .