In order to implement auto-vivification of Ruby hash, one can employ the following class
class AutoHash < Hash
def initialize(*args)
super()
@update, @update_index = args[0][:update], args[0][:update_key] unless
args.empty?
end
def [](k)
if self.has_key?k
super(k)
else
AutoHash.new(:update => self, :update_key => k)
end
end
def []=(k, v)
@update[@update_index] = self if @update and @update_index
super
end
def few(n=0)
Array.new(n) { AutoHash.new }
end
end
This class allows to do the following things
a = AutoHash.new
a[:a][:b] = 1
p a[:c] # => {} # key :c has not been created
p a # => {:a=>{:b=>1}} # note, that it does not have key :c
a,b,c = AutoHash.new.few 3
b[:d] = 1
p [a,b,c] # => [{}, {:d=>1}, {}] # hashes are independent
There is a bit more advanced definition of this class proposed by Joshua, which is a bit hard for me to understand.
Problem
There is one situation, where I think the new class can be improved. The following code fails with the error message NoMethodError: undefined method '+' for {}:AutoHash
a = AutoHash.new
5.times { a[:sum] += 10 }
What would you do to handle it? Can one define []+=
operator?
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