In Ruby, why does Array.new(size, object) create an array consisting of multiple references to the same object?

As mentioned in this answer, Array.new(size, object) creates an array with size refer开发者_开发问答ences to the same object.

hash = Hash.new
a = Array.new(2, hash)
a[0]['cat'] = 'feline'
a # => [{"cat"=>"feline"},{"cat"=>"feline"}]
a[1]['cat'] = 'Felix'
a # => [{"cat"=>"Felix"},{"cat"=>"Felix"}]

Why does Ruby do this, rather than doing a dup or clone of object?

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Because that's what the documentation says it does. Note that Hash.new is only being called once, so of course there's only one Hash

If you want to create a new object for every element in the array, pass a block to the Array.new method, and that block will be called for each new element:

>> a = Array.new(2) { Hash.new }
=> [{}, {}]
>> a[0]['cat'] = 'feline'
=> "feline"
>> a
=> [{"cat"=>"feline"}, {}]
>> a[1]['cat'] = 'Felix'
=> "Felix"
>> a
=> [{"cat"=>"feline"}, {"cat"=>"Felix"}]

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For certain classes that can't be modified in-place (like Fixnums) the Array.new(X, object) form works as expected and is probably more efficient (it just calls memfill instead of rb_ary_store and yielding to the block):

For more complicated objects you always have the block form (e.g. Array.new(5) { Hash.new }).

*Edit:* Modified according to the comments. Sorry for the stupid example, I was tired when I wrote that.

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I came up with this answer, very short and simple solution

c_hash = Hash.new
["a","b","c","d","e","f"].each do |o|
  tmp = Hash.new
  [1,2,3].map {|r| tmp[r] = Array.new}
  c_hash[o] = tmp
end
c_hash['a'][1] << 10
p c_hash