问散列中不存在键的自定义消息

EN

> hash = Hash.new{|hash,key| hash[key] = "No element against #{key}"}
 => {}
> hash['a']
 => "No element against a" 
> hash['a'] = 123
 => 123 
> hash['a'] 
 => 123 
> hash['b'] = nil
 => nil 
> hash['b']
 => nil 

irb(main):001:0> h = Hash.new('No element present')
=> {}
irb(main):002:0> h[1]
=> "No element present"
irb(main):003:0> h[1] = nil
=> nil
irb(main):004:0> h[1]
=> nil
irb(main):005:0> h[2]
=> "No element present"

val = h.has_key?('key') ? h['key'] : "No element present"