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] 

Answer 2

In the first case you're using a single instance of an Array as a default for the elements of the main Array:

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

The second argument is simply recycled, so the push is applied to the same instance twice. You've only created one instance here, the one being supplied as an argument, so it gets used over and over.

The second method is the correct way to do this:

b = Array.new(2) {Array.new}.each {|i| i.push("b")

This deliberately creates a new instance of an Array for each position in the main Array. The important difference here is the use of the block { ... } which executes once for each position in the new Array. A short-form version of this would be:

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