Ruby array manipulation inside method

Question

In the following, input_1 changes:

def method_1(a)
  a << "new value"
end

input_1 = []
method_1(input_1)
input_1 #=> ["new value"]

In the following, input_2 does not change:

def method_2(a)
  a = ["new value"]
end

input_2 = []
method_2(input_2)
input_2 #=> []

Why does input_1 change while input_2 doesn't change?

Answer 1

It boils down to Ruby using "pass-reference-by-value".

The exact case that you encounter is described in this excellent blog post.

The gist:

In method_1 you are changing the value of an object that two different variables (input_1 and a) are both pointing to.

In method_2 you are reassigning a completely new object to one of the two variables (a).

Answer 2

With a bit of simplification we can say that a variable in Ruby is a reference to a value. In your case variable a holds a reference to an array.

a << (a.append) mutates the value stored in variable a. The reference is not changed, but the value did. It's the case of method_1

def method_1(a)
    a << "new value"
end

Assignment = changes the reference stored in a variable - it starts to point to a different value. References are copied when passed to a method. Because of that when you call

def method_2(a)
    a = ["new value"]
end
input = []
method_2(a)

You only change a reference stored in a that is local to the method, without any change to the reference stored in input nor to the value (and array of []) that is pointed by this reference.