Tk Label and instance variables

(Bart Masschelein) #1

Hello all,

Below is a small piece of test code for a label using Tk. I try to set a label using an instance variable, and one using a local variable. The label with the local variable shows correctly in the window, the one with the instance variable doesn't. I get a warning: instance variable @value not initialized. Although the puts in between proves differently. What is the magic between all of this?

I tried adding value to attr_reader, but that didn't solve anything. Not that I was expecting it to.

As you probably can see, I come from a C/C++ background, and am probably missing some Ruby fundamentals to solve this. My collection of example scripts is rather small for the moment, and hence is also my experience. So any directions are welcome.

gr.b.

class LabelTest
  def initialize(value)
    @value = value
  end
   def run
    root = TkRoot.new { title "Label test" }
            puts @value
            TkLabel.new(root) do
      text "#{@value}"
      pack { padx 15 ; pady 15; side 'left' }
    end
        value2 = 200
        TkLabel.new(root) do
      text "#{value2}"
      pack { padx 15 ; pady 15; side 'left' }
    end
        Tk.mainloop
  end
end

test = LabelTest.new(100)
test.run

(Bart Masschelein) #2

Oh, and of course I could get around with the following, but I would like to know why the instance variable appears to be not initialized inside the TkLabel initialization.

    value3 = @value
       TkLabel.new(root) do
      text "#{value3}"
      pack { padx 15 ; pady 15; side 'left' }
    end

Bart Masschelein wrote:

···

Hello all,

Below is a small piece of test code for a label using Tk. I try to set a label using an instance variable, and one using a local variable. The label with the local variable shows correctly in the window, the one with the instance variable doesn't. I get a warning: instance variable @value not initialized. Although the puts in between proves differently. What is the magic between all of this?

I tried adding value to attr_reader, but that didn't solve anything. Not that I was expecting it to.

As you probably can see, I come from a C/C++ background, and am probably missing some Ruby fundamentals to solve this. My collection of example scripts is rather small for the moment, and hence is also my experience. So any directions are welcome.

gr.b.

class LabelTest
def initialize(value)
   @value = value
end

def run
   root = TkRoot.new { title "Label test" }
          puts @value
          TkLabel.new(root) do
     text "#{@value}"
     pack { padx 15 ; pady 15; side 'left' }
   end
      value2 = 200
      TkLabel.new(root) do
     text "#{value2}"
     pack { padx 15 ; pady 15; side 'left' }
   end Tk.mainloop
end
end

test = LabelTest.new(100)
test.run

(Hidetoshi NAGAI) #3

Message-ID: <4314AAFD.3040900@skynet.be>

Oh, and of course I could get around with the following, but I would
like to know why the instance variable appears to be not initialized
inside the TkLabel initialization.

Maybe, that is a FAQ. :slight_smile:
The block given to 'new' method is evaluated by 'instance_eval'.
That is, in the block, self is the created widget object.
So, you can abbreviate the receiver widget object to configure
widget options or to call instance methods of the widget objcet.
In your test script, @value in the block is not an instance variable
of the LabelTest object but an uninitialized instance variable of
the widget object.

···

From: Bart Masschelein <bart.masschelein@skynet.be>
Subject: Re: Tk Label and instance variables
Date: Wed, 31 Aug 2005 03:53:02 +0900
--
Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)