Question

In Ruby, I have this class:

class Position
  attr_reader :x, :y
  def initialize(x, y)
      @x, @y = x, y
  end
end

What I want to do is to access x and y variables using the symbol, something like this:

axis = :x
pos = Position.new(5,6)
 #one way:
pos.axis # 5 (pos.x)
 #other way:
pos.get(axis) # 5 (pos.x)

Thanks to this question I've found with this code, I can achieve the second behavior.

#...
class Position
  def get(var)
    instance_variable_get(("@#{var}").intern)
  end
end

But it seems ugly and inefficient (especially converting symbol to string and back to symbol). Is there a better way?

Answer 1

Here are ways to do both techniques. Assuming we already have your class definition,

position = Position.new(1, 2)
axis = :x
position.send axis #=> 1
axis = :y
position.send axis #=> 2

The Object#send method accepts at least a symbol representing the name of the method to call, and call it. You can also pass arguments to the method after the name and a block, too.

The second way to do this (using your Position#get method) is

class Position
  def get(axis)
    send axis
  end
end

position = Position.new(1, 2)
axis = :x
position.get axis #=> 1
axis = :y
position.get axis #=> 2

I recommend this way because it encapsulates the technique for getting the values. Should you need to change it later, you don’t need to change all the code that uses Position.