From: Glenn <glenn_ritz@...>
Date: 2009-01-02T03:48:49+09:00
Subject: Re: How do I pass the name of an instance variable into an object?

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Excellent!  Thanks so much for your help.

I noticed that the class of the variable always seems to be String when I use add_instance_variable.  When I use instance_variable_set, though it seems to preserve the class of the object in thee 2nd parameter.

In this case, x is a Fixnum:

s = SomeClass.new
s.instance_variable_set('@x', 22)

In this case, though, x is a String:

s = SomeClass.new
s.add_instance_variable(:x, 22)

I think that this is because of the quotes around initial_value in this line:

self.instance_variable_set("@#{name}", "#{initial_value}")


When I rewrote the line as:

self.instance_variable_set("@#{name}", initial_value )


It maintains the original class of that 2nd parameter.  Do you think that there is any harm in leaving those quotes off?

Glenn



________________________________
From: Suman Gurung <sumangurung@gmail.com>
To: ruby-talk ML <ruby-talk@ruby-lang.org>
Sent: Thursday, January 1, 2009 12:46:51 PM
Subject: Re: How do I pass the name of an instance variable into an object?

i just tried putting everything together and it worked great.
I thought i will share the consolidated code, so here it is:

class SomeClass
  def initialize
    @first = 'first variable'
    @second = 'second variable'
  end

  def print_instance_variable(*args)
    args.each do |arg|
      begin
        raise unless instance_variable_defined?("@#{arg}")
        puts self.instance_variable_get("@#{arg}")
      rescue => ex
        puts  "#{ex.class}: #{ex.message}"
      end
    end
  end

  def add_instance_variable(name, initial_value)
    raise if instance_variable_defined?("@#{name}")
    self.instance_variable_set("@#{name}", "#{initial_value}")
  end
end


x = SomeClass.new
x.print_instance_variable(:first)
x.print_instance_variable(:second)

puts "\n****************** printing both values in one method call
*************"
x.print_instance_variable :first, :second

puts "\n*************** should throw an exception, because the
instance variable is undefined **************"
x.print_instance_variable :third


x.add_instance_variable(:third, "third value")

puts "\n*************** should print the third instance variable**************"
x.print_instance_variable :third
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