json - 这个 nil 键是如何进入哈希的？

Question

我试图实现一个 json 解析器。输入是

"{ \"key\": { \"foo\": \"bar\", \"hello\" : 100 } }"

我格式化成

"{key:{foo:bar,hello:100}}"

然后我将其标记化以获得这样的数组

["{", "key", ":", "{", "foo", ":", "bar", ",", "hello", ":", "100", "}", "}"]

我可能也忽略了 ,。无论如何，当我尝试从该数组构建 ruby​​ 哈希时，我得到了这个

{nil=>{"key"=>{"foo"=>"bar", "hello"=>"100"}}}

我不知道这个 nil 是如何被用作键的。这是我用来构建哈希的方法

def build_hash(arr)
  h = {}
  keys = []
  while arr.size > 0
    current_token = arr.shift
    if alpha?(current_token)
      if keys.empty? # that means a new key is to be expected
        keys << current_token
      else
        h[keys.pop] = current_token
      end
    elsif current_token == '}'
      # these shouldn't be any key left in the keys stack. if there is one, raise error
      # otherwise close the hash and return from method
      if not keys.empty?
        raise_invalid_format_error
      else
        return h
      end
    elsif current_token == ','
      # continue reading. new key will appear. There shouldn't be any key in keys[]
      raise_invalid_format_error unless keys.empty?
    elsif current_token == '{'
      # this means new hash is starting, nested. Should be a value to a key
      # recursive call, assign the result to the existing key
      h[keys.pop] = build_hash(arr)
    end
  end
  h
end

如果字符是字母或数字，方法 alpha? 返回 true

def alpha?(s)
  s.match(/\A[[:alnum:]]+\z/)
end

这是怎么回事？

Answer 1

数组中的第一个标记将是 { ，这将落入您的最终 elsif案例:

elsif current_token == '{'
  # this means new hash is starting, nested. Should be a value to a key
  # recursive call, assign the result to the existing key
  h[keys.pop] = ....

keys数组为空，所以 pop返回 nil ，您将其用作哈希的键。