# Dynamically reference instance vars **URL:** \ **Category:** ruby-talk\ **Created:** [30 October 2010 02:14 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661 "2010-10-30T02:14:50Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Greg\_Willits](https://avatars.discourse-cdn.com/v4/letter/g/cab0a1/32.png) [@Greg\_Willits](https://rubytalk.org/u/Greg_Willits)\ **Post date:** [30 October 2010 02:14 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/1 "2010-10-30T02:14:50Z")
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/](http://www.ruby-forum.com/). ---
**Author:** ![Jeremy\_Bopp](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/jeremy_bopp/32/1937_2.png) [@Jeremy\_Bopp](https://rubytalk.org/u/Jeremy_Bopp)\ **Post date:** [30 October 2010 02:31 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/2 "2010-10-30T02:31:38Z")
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 ---
**Author:** ![Greg\_Willits](https://avatars.discourse-cdn.com/v4/letter/g/cab0a1/32.png) [@Greg\_Willits](https://rubytalk.org/u/Greg_Willits)\ **Post date:** [30 October 2010 19:27 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/3 "2010-10-30T19:27:42Z")
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/](http://www.ruby-forum.com/). ---
**Author:** ![John\_Mair](https://avatars.discourse-cdn.com/v4/letter/j/ac91a4/32.png) [@John\_Mair](https://rubytalk.org/u/John_Mair)\ **Post date:** [30 October 2010 20:06 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/4 "2010-10-30T20:06:44Z")
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/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Andrew\_Wagner](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@Andrew\_Wagner](https://rubytalk.org/u/Andrew_Wagner)\ **Post date:** [30 October 2010 12:28 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/5 "2010-10-30T12:28:46Z")
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 \ 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 ---
**Author:** ![Jesus\_Gabriel\_y\_Gala](https://avatars.discourse-cdn.com/v4/letter/j/96bed5/32.png) [@Jesus\_Gabriel\_y\_Gala](https://rubytalk.org/u/Jesus_Gabriel_y_Gala)\ **Post date:** [30 October 2010 19:33 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/6 "2010-10-30T19:33:49Z")
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 \ 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. ---
**Author:** ![Tony\_Arcieri4](https://avatars.discourse-cdn.com/v4/letter/t/c77e96/32.png) [@Tony\_Arcieri4](https://rubytalk.org/u/Tony_Arcieri4)\ **Post date:** [30 October 2010 19:49 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/7 "2010-10-30T19:49:20Z")
> 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 \ wrote: > > -- > Tony Arcieri > Medioh! A Kudelski Brand ---
**Author:** ![Ryan\_Davis1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/ryan_davis1/32/1848_2.png) [@Ryan\_Davis1](https://rubytalk.org/u/Ryan_Davis1)\ **Post date:** [30 October 2010 19:57 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/8 "2010-10-30T19:57:41Z")
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) ---
**Author:** ![Jeremy\_Bopp](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/jeremy_bopp/32/1937_2.png) [@Jeremy\_Bopp](https://rubytalk.org/u/Jeremy_Bopp)\ **Post date:** [30 October 2010 15:40 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/9 "2010-10-30T15:40:04Z")
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) ---
**Author:** ![Greg\_Willits](https://avatars.discourse-cdn.com/v4/letter/g/cab0a1/32.png) [@Greg\_Willits](https://rubytalk.org/u/Greg_Willits)\ **Post date:** [30 October 2010 20:11 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/10 "2010-10-30T20:11:32Z")
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! 🙂 Sorry, no prizes ☹ Thanks. -- gw > **···** > > > On Oct 30, 2010, at 12:27 , Greg Willits wrote: > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [31 October 2010 12:40 UTC](https://rubytalk.org/t/dynamically-reference-instance-vars/60661/11 "2010-10-31T12:40:13Z")
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\> ) =\> #\ 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\> ) =\> #\ 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! 🙂 > > Sorry, no prizes ☹ > > -- > remember.guy do |as, often| as.you\_can - without end > [http://blog.rubybestpractices.com/](http://blog.rubybestpractices.com/)