matrix = Array.new(2, Array.new(2, 0))
=> [[0, 0], [0, 0]]
matrix[0][0] = 5
=> 5
matrix
=> [[5, 0], [5, 0]]
Why does it happen? While modifing the assignment I get the following result:
matrix = [[0, 0], [0, 0]]
=> [[0, 0], [0, 0]]
matrix[0][0] = 5
=> 5
matrix
=> [[5, 0], [0, 0]]
In the first example, you are passing an array object [2, 0] in the second argument of Array.new. The main array will be filled with that same array in two positions. If you modify one of them, the same array in the other position will also be modified.
In the second example, each sub array is a different array. Modifying one of them does not modify the other.
matrix = Array.new(2) { Array.new(2, 0) }