instance variable passing and modification in a function

Question

How to pass an instance variable as an argument to a function ? Example:

@tmp = "blabla"
...
my_function(@tmp)

@tmp should be "hello"

def my_function(foo)
foo = "hello"
end

Note: I changed the wording of the title !

Duplicated http://stackoverflow.com/questions/7688391/why-doesnt-this-work-if-in-ruby-everything-is-an-object — Rafa Paez, Mar 09 '14 at 17:23

Joe · Accepted Answer · 2014-03-09T17:02:12.020

1

@temp should not be "hello" the way you are doing it. If what you want to update @tmp I would change the way my_function works a bit:

def my_function
    'hello'
end

@tmp = "blabla"
...
@tmp = my_function

Ruby is a "pass by value" language. This might help explain it a bit better than I can: https://www.ruby-forum.com/topic/41160

Basically your my_function is only assigning foo "hello" while in the function, it then returns foo, but does not reassign the passed in object.

edited Mar 09 '14 at 17:02

answered Mar 09 '14 at 16:57

Joe

2,987
15
24

my_function could be more complicate. It was jusr an example. – Bertaud Mar 09 '14 at 17:12
@Bertaud so show the complete code. You can only Assign the instance to what the function returns. – MrPizzaFace Mar 09 '14 at 17:13
@Bertaud, yes I understand that, but Ruby is still pass by value, meaning you will need to return the value and assign it to `@tmp` no matter how complicated `my_function` becomes. – Joe Mar 09 '14 at 17:14
shall i write: def my_function ... return value end and, after @tmp = my_function(@tmp) – Bertaud Mar 09 '14 at 17:24
You may want to read up on ruby methods: http://www.tutorialspoint.com/ruby/ruby_methods.htm but I think @sawa's answer is what you are looking for. – Joe Mar 09 '14 at 17:27
whatever the operations on @tmp ? – Bertaud Mar 09 '14 at 17:29

score 0 · Answer 2 · answered Mar 09 '14 at 17:14

Your question is somewhat under-specified, but it looks like you want to do some dynamic attribute assignment.

class Test
  attr_accessor :foo

  def initialize
    @foo = "asdf"
  end

  def my_function(var, set_to = "hello")
    self.instance_variable_set(var, set_to)
  end
end

usage:

t = Test.new # t.foo is "asdf"

t.my_function(:@foo) # t.foo is "hello"

t.my_function(:@foo, "bananas") # t.foo is "bananas"

Notably, you have to pass my_function a symbol representing your variable that you want to dynamically alter, not the variable itself.

instance variable passing and modification in a function

2 Answers2