Class variable as parameter

Filipe · 23 November 2007 21:20

I have the following situation:

class MyClass
   def setVar(var,varValue)
      var=varValue
   end

   def initialize
       @myVar=0
       setVar(@myVar,1)
       puts(@myVar.to_s)
   end
end

test = MyClass.new

Unfortunately, this piece of code is returning 0 instead of the 1 I
would expect. Is there anything I could do?

Thanks in advance

Morton_Goldberg · 23 November 2007 22:00

Your call to setVar can not change the value of an variable. Only values of variables are passed in Ruby, not the variables themselves.

    def setVar(var,varValue)
       var=varValue # <-- this is the same as: return varValue
    end

    def initialize
        @myVar=0
        setVar(@myVar,1) # <-- at this point this is the same as: setVar(0,1)
        puts(@myVar.to_s)
    end

Ruby has a built-in method, Object#instance_variable_set that you can use for situations where you need to assign by name, so you don't need to define it for any derived class. Also, Ruby can define accessor methods for you:

class MyClass
    attr_accessor :myVar
    def initialize
        @myVar=0
        self.myVar = 1
        puts myVar # <-- to_s not needed; puts applies to_s to objects
    end
end
test = MyClass.new

This will output 1 and may be what you are looking for.

Regards, Morton

···

On Nov 23, 2007, at 4:20 PM, Filipe wrote:

I have the following situation:

class MyClass
   def setVar(var,varValue)
      var=varValue
   end

   def initialize
       @myVar=0
       setVar(@myVar,1)
       puts(@myVar.to_s)
   end
end

test = MyClass.new

Unfortunately, this piece of code is returning 0 instead of the 1 I
would expect. Is there anything I could do?

7stud · 23 November 2007 23:37

Filipe wrote:

I have the following situation:
def initialize
@myVar=0
setVar(@myVar,1)>

Unfortunately, this piece of code is returning 0 instead of the 1 I
would expect. Is there anything I could do?

Yes:

def initialize
   @myVar = 0
   @myVar = 1
   ...

···

--
Posted via http://www.ruby-forum.com/\.

Filipe · 23 November 2007 23:24

The key point is that the setVar method is supposed to do a couple
more things than just set the value of the variable. I removed them
from the example just for the sake of simplicity. Something like:

class MyClass
   def setVar(var,varValue)
      var=varValue
      #do something else...
   end

   def initialize
       @myVar1=0
       @myVar2=0
       @myVar3=0

       setVar(@myVar,1)
       setVar(@myVar,2)
       setVar(@myVar,3)

end
end

test = MyClass.new

Beeing able to "send" the variable to setVar and have its value
changed would save me many lines of code...

···

On 23 nov, 20:00, Morton Goldberg <m_goldb...@ameritech.net> wrote:

On Nov 23, 2007, at 4:20 PM, Filipe wrote:

> I have the following situation:

> class MyClass
> def setVar(var,varValue)
> var=varValue
> end

> def initialize
> @myVar=0
> setVar(@myVar,1)
> puts(@myVar.to_s)
> end
> end

> test = MyClass.new

> Unfortunately, this piece of code is returning 0 instead of the 1 I
> would expect. Is there anything I could do?

Your call to setVar can not change the value of an variable. Only
values of variables are passed in Ruby, not the variables themselves.

    def setVar(var,varValue)
       var=varValue # <-- this is the same as: return varValue
    end

    def initialize
        @myVar=0
        setVar(@myVar,1) # <-- at this point this is the same as:
setVar(0,1)
        puts(@myVar.to_s)
    end

Ruby has a built-in method, Object#instance_variable_set that you can
use for situations where you need to assign by name, so you don't
need to define it for any derived class. Also, Ruby can define
accessor methods for you:

class MyClass
    attr_accessor :myVar
    def initialize
        @myVar=0
        self.myVar = 1
        puts myVar # <-- to_s not needed; puts applies to_s to objects
    end
end
test = MyClass.new

This will output 1 and may be what you are looking for.

Regards, Morton

7stud · 23 November 2007 23:26

Morton Goldberg wrote:

Also, Ruby can define
accessor methods for you:

class MyClass
    attr_accessor :myVar
    def initialize
        @myVar=0
        self.myVar = 1
        puts myVar # <-- to_s not needed; puts applies to_s to objects
    end
end
test = MyClass.new

Here it is explicitly:

class MyClass

  def num=(val)
    @num = val
  end

  def num
      @num
  end

  def initialize(val)
    @num = val
  end

end

t = MyClass.new(1)
puts t.num

which is equivalent to:

class MyClass
attr_accessor :num

  def initialize(val)
    @num = val
  end

end

t = MyClass.new(1)
puts t.num

···

--
Posted via http://www.ruby-forum.com/\.

Morton_Goldberg · 24 November 2007 01:12

I have the following situation:

class MyClass
   def setVar(var,varValue)
      var=varValue
   end

   def initialize
       @myVar=0
       setVar(@myVar,1)
       puts(@myVar.to_s)
   end
end

test = MyClass.new

Unfortunately, this piece of code is returning 0 instead of the 1 I
would expect. Is there anything I could do?

Your call to setVar can not change the value of an variable. Only
values of variables are passed in Ruby, not the variables themselves.

    def setVar(var,varValue)
       var=varValue # <-- this is the same as: return varValue
    end

    def initialize
        @myVar=0
        setVar(@myVar,1) # <-- at this point this is the same as:
setVar(0,1)
        puts(@myVar.to_s)
    end

Ruby has a built-in method, Object#instance_variable_set that you can
use for situations where you need to assign by name, so you don't
need to define it for any derived class. Also, Ruby can define
accessor methods for you:

class MyClass
    attr_accessor :myVar
    def initialize
        @myVar=0
        self.myVar = 1
        puts myVar # <-- to_s not needed; puts applies to_s to objects
    end
end
test = MyClass.new

This will output 1 and may be what you are looking for.

Regards, Morton

The key point is that the setVar method is supposed to do a couple
more things than just set the value of the variable. I removed them
from the example just for the sake of simplicity.

That was a bad idea, since it made it impossible for anyone to understand your real problem and give you the help you were looking for.

Something like:

class MyClass
   def setVar(var,varValue)
      var=varValue
      #do something else...
   end

   def initialize
       @myVar1=0
       @myVar2=0
       @myVar3=0

       setVar(@myVar,1)
       setVar(@myVar,2)
       setVar(@myVar,3)

   end
end

test = MyClass.new

Beeing able to "send" the variable to setVar and have its value
changed would save me many lines of code...

You can use Object#instance_variable_set to solve your problem by sending the variable's name to setVar.

<code>
class MyClass
    def setVar(name, val)
       instance_variable_set(name, val)
       #do something else...
    end

    def initialize
       @myVar1=0
       @myVar2=0
       @myVar3=0
       setVar(:@myVar1, 1)
       setVar(:@myVar2, 2)
       setVar(:@myVar3, 3)
   end
end

MyClass.new.inspect # => "#<MyClass:0x24b58 @myVar3=3, @myVar2=2, @myVar1=1>"
</code>

Regards, Morton

···

On Nov 23, 2007, at 6:24 PM, Filipe wrote:

On 23 nov, 20:00, Morton Goldberg <m_goldb...@ameritech.net> wrote:

On Nov 23, 2007, at 4:20 PM, Filipe wrote:

Matt_Todd · 25 November 2007 13:50

The most stylish, Ruby way to do what you are trying to do is as follows:

class MyClass

  def initialize
    @myVar1 = 0 # direct value manipulation
    self.myVar1 = 1 # sent through your wrapper
    puts @myVar1
  end

  def myVar1=(value)
    @myVar1 = value
    # process extra steps here
  end

end

(Ehm, I should admit, there may be a better way to actually get the
accessor method to fire inside of initialize than self.myVar1, but
that's what I found to work.)

···

>> On Nov 23, 2007, at 4:20 PM, Filipe wrote:
>>> I have the following situation:
>>> class MyClass
>>> def setVar(var,varValue)
>>> var=varValue
>>> end
>>> def initialize
>>> @myVar=0
>>> setVar(@myVar,1)
>>> puts(@myVar.to_s)
>>> end
>>> end
>>> test = MyClass.new
>>> Unfortunately, this piece of code is returning 0 instead of the 1 I
>>> would expect. Is there anything I could do?

Filipe · 25 November 2007 22:40

I would still have to writer a myVar=(value) method for every single
variable. That's what I'm trying to get rid of.

···

On Nov 25, 11:50 am, Matt Todd <chiol...@gmail.com> wrote:

> >> On Nov 23, 2007, at 4:20 PM,Filipewrote:
> >>> I have the following situation:
> >>> class MyClass
> >>> def setVar(var,varValue)
> >>> var=varValue
> >>> end
> >>> def initialize
> >>> @myVar=0
> >>> setVar(@myVar,1)
> >>> puts(@myVar.to_s)
> >>> end
> >>> end
> >>> test = MyClass.new
> >>> Unfortunately, this piece of code is returning 0 instead of the 1 I
> >>> would expect. Is there anything I could do?

The most stylish, Ruby way to do what you are trying to do is as follows:

class MyClass

  def initialize
    @myVar1 = 0 # direct value manipulation
    self.myVar1 = 1 # sent through your wrapper
    puts @myVar1
  end

  def myVar1=(value)
    @myVar1 = value
    # process extra steps here
  end

end

(Ehm, I should admit, there may be a better way to actually get the
accessor method to fire inside of initialize than self.myVar1, but
that's what I found to work.)

Bernardo_Monteiro_Ru · 26 November 2007 01:50

You can do something like

···

--
class Foo

alias_method :method_missing2, :method_missing;

  def method_missing(method, *args, &block)
    if method.to_s =~ /=$/
      set_var($`, args.first);
    else
      method_missing2(method, *args, &block);
    end
  end

  private
  def set_var(name, value)
    instance_variable_set("@#{name}", value);
    #do stuff here...
  end

end

foo = Foo.new;
foo.car = "test";
p foo #=> #<Foo:0x2fb77e4 @car="test">
--

--
Bernardo Rufino

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