After passing a reference to an method, any mods using that reference are not visible outside the method - Ruby?

Question

I am passing the reference of name to *mod_name*, I modify the referenced object from within the method but the change is not visible outside of the method, if I am referring to the same object from all locations how come the value is different depending on where I reference it?

name = "Jason"

puts name.object_id      #19827274

def mod_name(name)
  puts name.object_id    #19827274
  name = "JasonB"
end

puts name.object_id      #19827274

puts name                #Jason

String might be a bad example, but I get the same result even if I use a Fixnum.

Answer 1

In this case, the name = "JasonB" line is creating a new local variable named name and assigning "JasonB" to it, rather than modifying the string which was passed in.

Answer 2

As Greg mentions, in your example you're creating a new local variable called name that is shadowing your parameter. This is due to behavior called copy-on-write. If you wanted the function to affect the object the parameter references, you could use replace instead of doing an assignment, like this:

def mod_name(name)
  name.replace('JasonB')
end

Answer 3

mutating the content of a string :

def mod_name(name)
  print "%i %s\n" % [name.object_id, name]
  name[0..-1] = "what"
  print "%i %s\n" % [name.object_id, name]
  name << "ever"
  print "%i %s\n" % [name.object_id, name]
end

Also :

• in your sample code you never call the mod_name function.

• You seemed to believe that writing 'var = x' would change var's content. It can't. It can only change what object gets pointed to by var. Asking for its object_id before and after affectation would have showed that it was indeed another string.

• There's no way to make this function work with Fixnums because Fixnums are immutable in ruby. You can't make any modification to a Fixnum. Worse, they're not even passed by reference and in some corner cases won't behave like proper objects.

Answer 4

I think you have gotten answers to the question you asked. Maybe you need an answer to what would have caused you to ask this question.

name = "jason"

def mod_name(name)
  local_name = "jasonb"
end

puts name = mod_name(name)
puts name

Depending upon where this method lives (if it were in a class for example) maybe you would utilize an instance variable @name and depending upon the scope wouldn't have to be passed.

@name = "jason"

def mod_name()
  @name = "jasonb"
end

puts @name