Syntax error warning: useless use of a variable in void context

ruby-talk
1

class Tl
    def initialize(a,b)
        @a = a
        @b = b
    end
        def to_s
        "#{@a} #{@b}"
    end
        def <=>(tlr)
        if @a > tlr.@a
            1
        elsif
            @a == tlr.@a
                0
        else
            -1
        end
    end
    end

aa = Tl.new("tom","reilly")
ab = Tl.new("jack","charity")

r = aa <=> ab
p r

···

--------------------------
>ruby -cw tmp.rb
tmp.rb:12: syntax error
tmp.rb:12: warning: useless use of a variable in void context
tmp.rb:14: syntax error
tmp.rb:15: syntax error
tmp.rb:15: warning: useless use of a variable in void context
tmp.rb:19: warning: else without rescue is useless
tmp.rb:22: syntax error
>Exit code: 1

Considering I've been using Ruby for a couple of years, I've been totally frustrated
making this bit of code work.

Help!
Thanks

2

Tom Reilly wrote:

class Tl
   def initialize(a,b)
       @a = a
       @b = b
   end
      def to_s
       "#{@a} #{@b}"
   end
      def <=>(tlr)
       if @a > tlr.@a
           1
       elsif
           @a == tlr.@a
               0
       else
           -1
       end
   end

aa = Tl.new("tom","reilly")
ab = Tl.new("jack","charity")

r = aa <=> ab
p r

--------------------------
>ruby -cw tmp.rb
tmp.rb:12: syntax error
tmp.rb:12: warning: useless use of a variable in void context
tmp.rb:14: syntax error
tmp.rb:15: syntax error
tmp.rb:15: warning: useless use of a variable in void context
tmp.rb:19: warning: else without rescue is useless
tmp.rb:22: syntax error
>Exit code: 1

Considering I've been using Ruby for a couple of years, I've been totally frustrated
making this bit of code work.

Help!
Thanks

You seem to be having difficulty accessing the variables in the tla object.
This seems to work:

#!/usr/bin/env ruby

class Tl
   attr_reader :a, :b

   def initialize(a,b)
       @a = a
       @b = b
   end

   def to_s
       "#{@a} #{@b}"
   end

   def <=>(tlr)
       if @a > tlr.a
           1
       elsif
           @a == tlr.a
               0
       else
           -1
       end
   end
end

aa = Tl.new("tom","reilly")
ab = Tl.new("jack","charity")

r = aa <=> ab
p r

3

Selon Tom Reilly :

class Tl
   def initialize(a,b)
       @a = a
       @b = b
   end
      def to_s
       "#{@a} #{@b}"
   end
      def <=>(tlr)
       if @a > tlr.@a

                  ^^^^^^

           1
       elsif
           @a == tlr.@a

                    ^^^^^^

               0
       else
           -1
       end
   end

aa = Tl.new("tom","reilly")
ab = Tl.new("jack","charity")

r = aa <=> ab
p r

--------------------------
>ruby -cw tmp.rb
tmp.rb:12: syntax error
tmp.rb:12: warning: useless use of a variable in void context
tmp.rb:14: syntax error
tmp.rb:15: syntax error
tmp.rb:15: warning: useless use of a variable in void context
tmp.rb:19: warning: else without rescue is useless
tmp.rb:22: syntax error
>Exit code: 1

Considering I've been using Ruby for a couple of years, I've been totally frustrated
making this bit of code work.

Instance variables are completely private by default in Ruby, and you can only make them accessible by adding accessor methods (what the attr_* methods do for you). You can't access the private instance variable of a different object, even if they have the same class, so your "tlr.@a" simply won't work.

The only ways to make this work is:
- to make the instance variable @a readable with "attr_reader :a" and replace "tlr.@a" with "tlr.a"
- to bypass encapsulation by replacing "tlr.@a" with "tlr.instance_variable_get(:@a)".

···

--
Christophe Grandsire.

http://rainbow.conlang.free.fr

You need a straight mind to invent a twisted conlang.

4

^
                     ^

                   you can't do this.

probably you want

   attr "a"
   ...

   tlr.a

hth.

-a

···

On Thu, 24 Nov 2005, Tom Reilly wrote:

class Tl
  def initialize(a,b)
      @a = a
      @b = b
  end
     def to_s
      "#{@a} #{@b}"
  end
     def <=>(tlr)
      if @a > tlr.@a

--

ara [dot] t [dot] howard [at] noaa [dot] gov
all happiness comes from the desire for others to be happy. all misery
comes from the desire for oneself to be happy.
-- bodhicaryavatara

===============================================================================

5

Thanks everybody.
I had thought that an @_whatever could be seen anywhere the class was used.
TR

6

To be pedantic:
a) attr_reader doesn't make an instance variable readable, it's a convenient way to add a method that returns the value of the instance variable.

b) There are other ways (variations on your latter) such as tlr.instance_eval{ @a }

···

On Nov 23, 2005, at 3:57 PM, Christophe Grandsire wrote:

The only ways to make this work is:
- to make the instance variable @a readable with "attr_reader :a" and replace "tlr.@a" with "tlr.a"
- to bypass encapsulation by replacing "tlr.@a" with "tlr.instance_variable_get(:@a)".