Access variables programmatically by name in Ruby

Question

I'm not entirely sure if this is possible in Ruby, but hopefully there's an easy way to do this. I want to declare a variable and later find out the name of the variable. That is, for this simple snippet:

foo = ["goo", "baz"]

How can I get the name of the array (here, "foo") back? If it is indeed possible, does this work on any variable (e.g., scalars, hashes, etc.)?

Edit: Here's what I'm basically trying to do. I'm writing a SOAP server that wraps around a class with three important variables, and the validation code is essentially this:

  [foo, goo, bar].each { |param|
      if param.class != Array
        puts "param_name wasn't an Array. It was a/an #{param.class}"
        return "Error: param_name wasn't an Array"
      end
      }

My question is then: Can I replace the instances of 'param_name' with foo, goo, or bar? These objects are all Arrays, so the answers I've received so far don't seem to work (with the exception of re-engineering the whole thing ala dbr's answer)

Answer 1

You need to re-architect your solution. Even if you could do it (you can't), the question simply doesn't have a reasonable answer.

Imagine a get_name method.

a = 1
get_name(a)

Everyone could probably agree this should return 'a'

b = a
get_name(b)

Should it return 'b', or 'a', or an array containing both?

[b,a].each do |arg|
  get_name(arg)
end

Should it return 'arg', 'b', or 'a' ?

def do_stuff( arg )
  get_name(arg)
do
do_stuff(b)

Should it return 'arg', 'b', or 'a', or maybe the array of all of them? Even if it did return an array, what would the order be and how would I know how to interpret the results?

The answer to all of the questions above is "It depends on the particular thing I want at the time." I'm not sure how you could solve that problem for Ruby.

Answer 2

You can't, you need to go back to the drawing board and re-engineer your solution.

Answer 3

There's Kernel::local_variables, but I'm not sure that this will work for a method's local vars, and I don't know that you can manipulate it in such a way as to do what you wish to acheive.

Answer 4

I do not know of any way to get a local variable name. But, you can use the instance_variables method, this will return an array of all the instance variable names in the object.

Simple call:

object.instance_variables

or

self.instance_variables

to get an array of all instance variable names.

Answer 5

It seems you are trying to solve a problem that has a far easier solution..

Why not just store the data in a hash? If you do..

data_container = {'foo' => ['goo', 'baz']}

..it is then utterly trivial to get the 'foo' name.

That said, you've not given any context to the problem, so there may be a reason you can't do this..

[edit] After clarification, I see the issue, but I don't think this is the problem.. With [foo, bar, bla], it's equivalent like saying ['content 1', 'content 2', 'etc']. The actual variables name is (or rather, should be) utterly irrelevant. If the name of the variable is important, that is exactly why hashes exist.

The problem isn't with iterating over [foo, bar] etc, it's the fundamental problem with how the SOAP server is returing the data, and/or how you're trying to use it.

The solution, I would say, is to either make the SOAP server return hashes, or, since you know there is always going to be three elements, can you not do something like..

{"foo" => foo, "goo" => goo, "bar"=>bar}.each do |param_name, param|
      if param.class != Array
        puts "#{param_name} wasn't an Array. It was a/an #{param.class}"
        puts "Error: #{param_name} wasn't an Array"
      end
end

Answer 6

What if you turn your problem around? Instead of trying to get names from variables, get the variables from the names:

["foo", "goo", "bar"].each { |param_name|
  param = eval(param_name)
  if param.class != Array
    puts "#{param_name} wasn't an Array. It was a/an #{param.class}"
    return "Error: #{param_name} wasn't an Array"
  end
  }

If there were a chance of one the variables not being defined at all (as opposed to not being an array), you would want to add "rescue nil" to the end of the "param = ..." line to keep the eval from throwing an exception...