ruby​​：使用散列作爲另一個散列的默認值？

Question

我想使用散列作爲較大散列的默認值。但我不知道如何通過外部散列設置內部散列的值。

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

呃，什麼是正確的方法？

Answer 1

如果散列initialize with a default value：

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

然後，您可以按照您的示例調用模式：

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

或者您可能希望與塊這樣的一個新的實例並初始化每次使用新密鑰時都會創建: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"}