ruby variable as same object (pointers?)

Question

>> a = 5
=> 5
>> b = a
=> 5
>> b = 4
=> 4
>> a
=> 5

how can I set 'b' to actually be 'a' so that in the example, the variable a will become four as well. thanks.

Answer 1

class Ref
  def initialize val
    @val = val
  end

  attr_accessor :val

  def to_s
    @val.to_s
  end
end

a = Ref.new(4)
b = a

puts a   #=> 4
puts b   #=> 4

a.val = 5

puts a   #=> 5
puts b   #=> 5

When you do b = a, b points to the same object as a (they have the same object_id).

When you do a = some_other_thing, a will point to another object, while b remains unchanged.

For Fixnum, nil, true and false, you cannot change the value without changing the object_id. However, you can change other objects (strings, arrays, hashes, etc.) without changing object_id, since you don't use the assignment (=).

Example with strings:

a = 'abcd'
b = a

puts a  #=> abcd
puts b  #=> abcd

a.upcase!          # changing a

puts a  #=> ABCD
puts b  #=> ABCD

a = a.downcase     # assigning a

puts a  #=> abcd
puts b  #=> ABCD

Example with arrays:

a = [1]
b = a

p a  #=> [1]
p b  #=> [1]

a << 2            # changing a

p a  #=> [1, 2]
p b  #=> [1, 2]

a += [3]          # assigning a

p a  #=> [1, 2, 3]
p b  #=> [1, 2]

Answer 2

You can't. Variables hold references to values, not references to other variables.

Here's what your example code is doing:

a = 5 # Assign the value 5 to the variable named "a".
b = a # Assign the value in the variable "a" (5) to the variable "b".
b = 4 # Assign the value 4 to the variable named "b".
a # Retrieve the value stored in the variable named "a" (5).

See this article for a more in-depth discussion of the topic: pass by reference or pass by value.