ruby - Why does array.each behavior depend on Array.new syntax?

Question

I'm using Ruby 1.9.2-p290 and found:

a = Array.new(2, []).each {|i| i.push("a")}    
=> [["a", "a"], ["a", "a"]]

Which is not what I would expect. But the following constructor style does do what I would expect:

b = Array.new(2) {Array.new}.each {|i| i.push("b")}
=> [["b"], ["b"]] 

Is the first example the expected behavior?

In ruby-doc it looks like my size=2 argument is the same kind of argument for both constructors. I think that if the each method is getting passed that argument that it would use it the same way for both constructors.

Answer 1

This is a common misunderstanding. In your first example you are creating an array with 2 elements. Both of those are a pointer to the same array. So, when you iterate through your outer array you add 2 elements to the inner array, which is then reflected in your output twice

Compare these:

> array = Array.new(5, [])
=> [[], [], [], [], []] 

# Note - 5 identical object IDs (memory locations)
> array.map { |o| o.object_id }
=> [70228709214620, 70228709214620, 70228709214620, 70228709214620, 70228709214620] 

> array = Array.new(5) { [] }
=> [[], [], [], [], []] 

# Note - 5 different object IDs (memory locations)
> array.map { |o| o.object_id }
=> [70228709185900, 70228709185880, 70228709185860, 70228709185840, 70228709185780]