ruby - Arrays misbehaving

Question

Here's the code:

# a = Array.new(3, Array.new(3))
a = [[nil,nil,nil],[nil,nil,nil]]
a[0][0] = 1
a.each {|line| p line}

With the output:

[1, nil, nil]
[nil, nil, nil]

but using the commented line:

[1, nil, nil]
[1, nil, nil]
[1, nil, nil]

So why is that?

Answer 1

The commented line is assigning three of the same reference to the array, so a change to one array will propagate across the other references to it.

As for the 2 arrays vs 3, that's simply a matter of the first line specifying 3 as its first parameter and only specifying 2 array literals in the second line.

To create the nested arrays without having any shared references:

a = Array.new(3) {Array.new(3)}

When passed a block ({...} or do ... end), Array.new will call the block to obtain the value of each element of the array.