ruby​​：使用散列作为另一个散列的默认值？

Question

我想使用散列作为较大散列的默认值。但我不知道如何通过外部散列设置内部散列的值。

h = Hash.new do 
    {:counter => 0} 
end 
h[:a][:counter] += 1 
=> 1 
h[:a][:counter] += 1 
=> 1 
h[:a][:counter] += 1 
=> 1 
h 
=> {} 

呃，什么是正确的方法？

Answer 1

如果散列initialize with a default value：

h = Hash.new({:counter => 5}) 

然后，您可以按照您的示例调用模式：

h[:a][:counter] += 1 
=> 6 
h[:a][:counter] += 1 
=> 7 
h[:a][:counter] += 1 
=> 8 

或者您可能希望与块这样的一个新的实例并初始化每次使用新密钥时都会创建:counter哈希值：

# Shared nested hash 
h = Hash.new({:counter => 5}) 
h[:a][:counter] += 1 
=> 6 
h[:boo][:counter] += 1 
=> 7 
h[:a][:counter] += 1 
=> 8 

# New hash for each default 
n_h = Hash.new { |hash, key| hash[key] = {:counter => 5} } 
n_h[:a][:counter] += 1 
=> 6 
n_h[:boo][:counter] += 1 
=> 6 
n_h[:a][:counter] += 1 
=> 7 

Answer 2

def_hash={:key=>"value"} 
another_hash=Hash.new(def_hash) 
another_hash[:foo] # => {:key=>"value"}