Dynamically reference instance vars

ruby-talk
1

If I need to dynamically reference instance vars, is this the only way
to do it (var set example)?

  my_object.send(:instance_variable_set, "@#{iname}", ivalue)

I expected something more elegant, but this is the only way I can get it
to work. No biggie, just curious.

More complete example below.

-- gw

class Shape
  attr_accessor :size, :fill_color, :line_color, :line_width
  def initialize
    @size = ""
    @fill_color = ""
    @line_color = ""
    @line_width = ""
  end
end

my_shape = Shape.new

shape_details = {
  :size => 'small',
  :fill_color => 'red',
  :line_color => 'black',
  :line_width => '2'}

shape_details.each do |iname, ivalue|
  my_shape.send(:instance_variable_set, "@#{iname}", ivalue)
end

···

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

2

How about backing the accessors with a hash that also has an accessor?
That way you could merge in a hash of settings or set/get them neatly by
name:

class Shape
  attr_reader :details

  def initialize
    @details = {
      :size => "",
      :fill_color => "",
      :line_color => "",
      :line_width => ""
    }
  end

  def size
    @details[:size]
  end

  def size=(size)
    @details[:size] = size
  end

  def fill_color
    @details[:fill_color]
  end

  def fill_color=(fill_color)
    @details[:fill_color] = fill_color
  end

  def line_color
    @details[:line_color]
  end

  def line_color=(line_color)
    @details[:line_color] = line_color
  end

  def line_width
    @details[:line_width]
  end

  def line_width=(line_width)
    @details[:line_width] = line_width
  end
end

my_shape = Shape.new

shape_details = {
  :size => 'small',
  :fill_color => 'red',
  :line_color => 'black',
  :line_width => '2'
}

my_shape.details.merge!(shape_details)

-Jeremy

···

On 10/29/2010 09:14 PM, Greg Willits wrote:

If I need to dynamically reference instance vars, is this the only way
to do it (var set example)?

  my_object.send(:instance_variable_set, "@#{iname}", ivalue)

I expected something more elegant, but this is the only way I can get it
to work. No biggie, just curious.

More complete example below.

-- gw

class Shape
  attr_accessor :size, :fill_color, :line_color, :line_width
  def initialize
    @size = ""
    @fill_color = ""
    @line_color = ""
    @line_width = ""
  end
end

my_shape = Shape.new

shape_details = {
  :size => 'small',
  :fill_color => 'red',
  :line_color => 'black',
  :line_width => '2'}

shape_details.each do |iname, ivalue|
  my_shape.send(:instance_variable_set, "@#{iname}", ivalue)
end

3

Appreciate the effort, but the point has been lost. Don't focus on the
hash or mass assignment, that was just a device to create an example.

The question is whether there's a shorter way to do this:

  my_object.send(:instance_variable_set, "@#{iname}", ivalue)

In another language I used, I could simply do the equivalent of

  myobject.iname = ivalue

that's because the syntax was like this
  #myobject->#iname = ivalue

Where # denotes a local var, so it was obvious to the parser that #iname
was not a method name, whereas with Ruby it's not obvious.

I was hoping in Ruby that at least this was possible:

  myobject.send(iname) = ivalue
  --or--
  myobject.send(iname, ivalue)

but that doesn't work. I could maybe force it to work by manually
creating setter methods, but that's not an elegant solution either, I'd
rather have the occassional use of instance_variable_set than craft
setters.

Anyway, not a biggie, just a curiosity.

···

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

4

You realize that instance_variable_set isn't private right?

So you can simply go:

    my_object.instance_variable_set("@#{iname}", ivalue)

Greg Willits wrote in post #958159:

···

If I need to dynamically reference instance vars, is this the only way
to do it (var set example)?

  my_object.send(:instance_variable_set, "@#{iname}", ivalue)

I expected something more elegant, but this is the only way I can get it
to work. No biggie, just curious.

More complete example below.

-- gw

class Shape
  attr_accessor :size, :fill_color, :line_color, :line_width
  def initialize
    @size = ""
    @fill_color = ""
    @line_color = ""
    @line_width = ""
  end
end

my_shape = Shape.new

shape_details = {
  :size => 'small',
  :fill_color => 'red',
  :line_color => 'black',
  :line_width => '2'}

shape_details.each do |iname, ivalue|
  my_shape.send(:instance_variable_set, "@#{iname}", ivalue)
end

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

5

C'mon, this is ruby! Revel in it!

class Shape
  attr_reader :details
  def self.keys
    [:size, :fill_color, :line_color, :line_color]
  end

  def initialize
    @details = {}
    self.class.keys.each {|key| @details[key] = "" }
  end

  keys.each do |key|
    define_method key do
      @details[key]
    end

    define_method "#{key}=" do |input|
      @details[key] = input
    end
  end
end

my_shape = Shape.new

shape_details = {
:size => 'small',
:fill_color => 'red',
:line_color => 'black',
:line_width => '2'
}

my_shape.details.merge!(shape_details)

···

On Fri, Oct 29, 2010 at 10:31 PM, Jeremy Bopp <jeremy@bopp.net> wrote:

On 10/29/2010 09:14 PM, Greg Willits wrote:
> If I need to dynamically reference instance vars, is this the only way
> to do it (var set example)?
>
> my_object.send(:instance_variable_set, "@#{iname}", ivalue)
>
> I expected something more elegant, but this is the only way I can get it
> to work. No biggie, just curious.
>
> More complete example below.
>
> -- gw
>
>
> class Shape
> attr_accessor :size, :fill_color, :line_color, :line_width
> def initialize
> @size = ""
> @fill_color = ""
> @line_color = ""
> @line_width = ""
> end
> end
>
> my_shape = Shape.new
>
> shape_details = {
> :size => 'small',
> :fill_color => 'red',
> :line_color => 'black',
> :line_width => '2'}
>
> shape_details.each do |iname, ivalue|
> my_shape.send(:instance_variable_set, "@#{iname}", ivalue)
> end

How about backing the accessors with a hash that also has an accessor?
That way you could merge in a hash of settings or set/get them neatly by
name:

class Shape
attr_reader :details

def initialize
   @details = {
      :size => "",
     :fill_color => "",
     :line_color => "",
     :line_width => ""
   }
end

  def size
   @details[:size]
end

def size=(size)
   @details[:size] = size
end

def fill_color
   @details[:fill_color]
end

def fill_color=(fill_color)
   @details[:fill_color] = fill_color
end

def line_color
   @details[:line_color]
end

def line_color=(line_color)
   @details[:line_color] = line_color
end

def line_width
   @details[:line_width]
end

def line_width=(line_width)
   @details[:line_width] = line_width
  end
end

my_shape = Shape.new

shape_details = {
:size => 'small',
:fill_color => 'red',
:line_color => 'black',
:line_width => '2'
}

my_shape.details.merge!(shape_details)

-Jeremy

6

Crafting setters is pretty simple, though:

class A
attr_writer :a
end

a = A.new
a.a = 3

Jesus.

···

On Sat, Oct 30, 2010 at 9:27 PM, Greg Willits <lists@gregwillits.ws> wrote:

Appreciate the effort, but the point has been lost. Don't focus on the
hash or mass assignment, that was just a device to create an example.

The question is whether there's a shorter way to do this:

my_object.send(:instance_variable_set, "@#{iname}", ivalue)

In another language I used, I could simply do the equivalent of

myobject.iname = ivalue

that's because the syntax was like this
#myobject->#iname = ivalue

Where # denotes a local var, so it was obvious to the parser that #iname
was not a method name, whereas with Ruby it's not obvious.

I was hoping in Ruby that at least this was possible:

myobject.send(iname) = ivalue
--or--
myobject.send(iname, ivalue)

but that doesn't work. I could maybe force it to work by manually
creating setter methods, but that's not an elegant solution either, I'd
rather have the occassional use of instance_variable_set than craft
setters.

Anyway, not a biggie, just a curiosity.

7

The question is whether there's a shorter way to do this:

my_object.send(:instance_variable_set, "@#{iname}", ivalue)

In another language I used, I could simply do the equivalent of

myobject.iname = ivalue

This breaks the encapsulation that objects are supposed to provide for their
hidden state. I have to say I *like* the fact that
instance_variable_get/instance_variable_set are long and unwieldy. By using
them you hopefully remind yourself that you're doing something naughty by
touching an object's private parts.

I was hoping in Ruby that at least this was possible:

myobject.send(iname) = ivalue
--or--
myobject.send(iname, ivalue)

but that doesn't work. I could maybe force it to work by manually
creating setter methods, but that's not an elegant solution either, I'd
rather have the occassional use of instance_variable_set than craft
setters.

It's Ruby, a lot is possible! You could add some method_missing magic so:

  myobj.iv_foobar = 42

thunks to:

  instance_variable_set(:@foobar, 42)

I would strongly recommend against that sort of thing though.

···

On Sat, Oct 30, 2010 at 1:27 PM, Greg Willits <lists@gregwillits.ws> wrote:

--
Tony Arcieri
Medioh! A Kudelski Brand

8

You're confusing methods and variables. They're not the same, but you're on the right track with the code above, but you'd be calling the getter, not the setter. Check it:

class X
  attr_accessor :x # creates x and x= methods
end

o = X.new
o.send("x=", 42)
p x

···

On Oct 30, 2010, at 12:27 , Greg Willits wrote:

I was hoping in Ruby that at least this was possible:

myobject.send(iname) = ivalue
--or--
myobject.send(iname, ivalue)

9

Excellent point! Thanks!

-Jeremy

···

On 10/30/2010 07:28 AM, Andrew Wagner wrote:

C'mon, this is ruby! Revel in it!

class Shape
  attr_reader :details
  def self.keys
    [:size, :fill_color, :line_color, :line_color]
  end

  def initialize
    @details = {}
    self.class.keys.each {|key| @details[key] = "" }
  end

  keys.each do |key|
    define_method key do
      @details[key]
    end

    define_method "#{key}=" do |input|
      @details[key] = input
    end
  end
end

my_shape = Shape.new

shape_details = {
:size => 'small',
:fill_color => 'red',
:line_color => 'black',
:line_width => '2'
}

my_shape.details.merge!(shape_details)

10

Ryan Davis wrote in post #958245:

I was hoping in Ruby that at least this was possible:

myobject.send(iname) = ivalue
--or--
myobject.send(iname, ivalue)

You're confusing methods and variables. They're not the same, but you're
on the right track with the code above, but you'd be calling the getter,
not the setter. Check it:

class X
  attr_accessor :x # creates x and x= methods
end

o = X.new
o.send("x=", 42)
p x

I wasn't confusing them, I was hoping Ruby's send would be flexible
enough to work with both of them (in conjunction with the accessors
being defined). What I didn't think of in this context was Ruby's x=
being considered the setter method and not just x, which I knew, and
should have recognized -- so yep, that was the ticket. You win! :slight_smile:
Sorry, no prizes :frowning:

Thanks.

-- gw

···

On Oct 30, 2010, at 12:27 , Greg Willits wrote:

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

11

Here are two other approaches using widely underused class Struct and that differ in the way they deal with values not present in the Hash:

Shape = Struct.new :size, :fill_color, :line_color, :line_width do
   def self.from_hash_1(h)
     sh = new
     h.each {|k,v| sh[k] = v}
     sh
   end

   def self.from_hash_2(h)
     sh = new
     members.each {|m| sh[m] = h[m]}
     sh
   end
end

irb(main):015:0* s1 = Shape.from_hash_1(
irb(main):016:1* :size => 'small',
irb(main):017:1* :fill_color => 'red',
irb(main):018:1* :line_color => 'black',
irb(main):019:1* :line_width => '2'
irb(main):020:1> )
=> #<struct Shape size="small", fill_color="red", line_color="black", line_width="2">
irb(main):021:0> s2 = Shape.from_hash_2(
irb(main):022:1* :size => 'small',
irb(main):023:1* :fill_color => 'red',
irb(main):024:1* :line_color => 'black',
irb(main):025:1* :line_width => '2'
irb(main):026:1> )
=> #<struct Shape size="small", fill_color="red", line_color="black", line_width="2">

If you like that better you can as well define

def Shape(h)
   sh = Shape.new
   # logic from above
   sh
end

Then you can do

s1 = Shape(
   :size => 'small',
   :fill_color => 'red',
   :line_color => 'black',
   :line_width => '2'
)

Kind regards

  robert

···

On 30.10.2010 22:11, Greg Willits wrote:

Ryan Davis wrote in post #958245:

On Oct 30, 2010, at 12:27 , Greg Willits wrote:

I was hoping in Ruby that at least this was possible:

  myobject.send(iname) = ivalue
  --or--
  myobject.send(iname, ivalue)

You're confusing methods and variables. They're not the same, but you're
on the right track with the code above, but you'd be calling the getter,
not the setter. Check it:

class X
   attr_accessor :x # creates x and x= methods
end

o = X.new
o.send("x=", 42)
p x

I wasn't confusing them, I was hoping Ruby's send would be flexible
enough to work with both of them (in conjunction with the accessors
being defined). What I didn't think of in this context was Ruby's x=
being considered the setter method and not just x, which I knew, and
should have recognized -- so yep, that was the ticket. You win! :slight_smile:
Sorry, no prizes :frowning:

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/