Forum: Ruby Attribute writers

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Michael J. (Guest)
on 2006-01-06 11:16
I'm working on a survey system in Ruby and am having a style crisis with
a simple bit of code that has left me paralyzed and unable to work.

Respondent data in research surveys is traditionally stored as big
strings of text like this: "123145992348241110111 11 11  1 1111 3331 1"

This was a made up example, in the real world however, there's sometimes
hundreds of thousands of characters in each respondent's data string.
In the stats and analysis section of my program, I'm working out how to
access subsets of these strings in an object-oriented way.

Something like this appeals to me and is very readable:

if currentRespondent.ascii(0..2) == 123
  currentRespondent.ascii(0..2) = 777
end

How would one actually implement this?

class Respondent
  def ascii(range)
    return @ascii[range]
  end

  # doesn't work
  def ascii(range)=(value)
    @ascii[range] = value
  end
end

currentRespondent = Respondent.new

This is broken code, as Ruby doesn't like me trying to do "def
a(b)=(c)".  There's other ways to accomplish the same purpose (e.g. def
setter(range,value)) but I'm obsessed with the attribute writer form --
which is just so dang cool.

Anyone have some light they could spare?

Argh.  I've been struggling with this for a week!
Hal F. (Guest)
on 2006-01-06 11:59
(Received via mailing list)
Michael J. wrote:
>
> Something like this appeals to me and is very readable:
>
> if currentRespondent.ascii(0..2) == 123
>   currentRespondent.ascii(0..2) = 777
> end
>
> How would one actually implement this?

I'd suggest doing a little metaprogramming and making the final code
a little easier.

Here is a very rough outline -- if you like it I can help flesh out
the details:


class Respondent

   Fields = { :this => 0..2,
              :that => 3..6,
              :etc  => 7..10 }

   Fields.each_pair do |field,range|
     # add a reader basically doing @resp[range]
     # add a writer basically doing @resp[range]=val
   end

   def initialize(str)
     @resp = str
   end

end


resp = Respondent.new(whatever)
if resp.this == Something
   resp.this = SomethingElse
end
do_whatever if resp.that != resp.this


Make sense?

Hal
Ross B. (Guest)
on 2006-01-06 12:05
(Received via mailing list)
On Fri, 06 Jan 2006 09:16:50 -0000, Michael J.
<removed_email_address@domain.invalid> wrote:

>
> Something like this appeals to me and is very readable:
>
> if currentRespondent.ascii(0..2) == 123
>   currentRespondent.ascii(0..2) = 777
> end
>
> How would one actually implement this?
>

Is this any help?

	irb(main):006:0> s = "123145992348241110111 11 11  1 1111 3331 1"
	  => "123145992348241110111 11 11  1 1111 3331 1"
	irb(main):007:0> s[0..2]
	  => "123"
	irb(main):008:0> s[0..2] = '777'
	  => "777"
	irb(main):009:0> s
	  => "777145992348241110111 11 11  1 1111 3331 1"

>
>
As far as I'm aware you can't do indexed attribute writers. You can of
course do:

	def []=(idx, val)
	  puts "at #{idx} = #{val}"
	end

But Ruby won't have it if you prefix the [] with a name.

What I think I'd do is just remove the writer (for this purpose) and
have
ascii return a String. The you can do:

	if currentRespondent[0..2] == '777'
	  currentRespondent.ascii[0..2] = "123"
	end

(since String's [] is pretty flexible as I showed above)

But I have a feeling I may have missed something in your question. Is
that
helpful?
Hal F. (Guest)
on 2006-01-06 12:11
(Received via mailing list)
Michael J. wrote:
> I'm working on a survey system in Ruby and am having a style crisis with
> a simple bit of code that has left me paralyzed and unable to work.

Here is the rest of my solution.

Does this help?


Hal


class Respondent

   Fields = { :this => 0..2,
              :that => 3..6,
              :etc  => 7..10 }

   Fields.each_pair do |field,range|
     # add a reader basically doing @resp[range]
     define_method(field) { @resp[range] }
     # add a writer basically doing @resp[range]=val
     define_method((field.to_s+"=").to_sym) do |val|
       raise "Wrong field length" if val.size != range.to_a.size
       @resp[range] = val
     end
   end

   def initialize(str)
     @resp = str
   end

end


resp = Respondent.new("abcdefghij")

puts resp.this
puts resp.that
puts resp.etc

resp.that = "foob"

puts resp.this
puts resp.that
puts resp.etc

resp.that = "foo"  # error!
Robert K. (Guest)
on 2006-01-06 12:35
(Received via mailing list)
Michael J. wrote:
> data string. In the stats and analysis section of my program, I'm
>
>
> currentRespondent = Respondent.new
>
> This is broken code, as Ruby doesn't like me trying to do "def
> a(b)=(c)".  There's other ways to accomplish the same purpose (e.g.
> def setter(range,value)) but I'm obsessed with the attribute writer
> form -- which is just so dang cool.
>
> Anyone have some light they could spare?
>
> Argh.  I've been struggling with this for a week!

Hm, I wouldn't use a string internally at all - at least not if you are
trying to do it OO.  This is what I'd do:

class Results
  include Enumerable

  def initialize(from_string = "")
    @fields = from_string.split(/\s+/).map {|s| s.to_i}
  end

  def field() @fields end

  def each(&b)
    @fields.each(&b)
    self
  end

  def size() @fields.size end
  def empty?() @fields.empty? end

  def to_str
    @fields.map {|d| d.to_s}.join ' '
  end

  alias :to_s :to_str

  def to_a() @fields end
end

You could also simply subclass Array and create appropriate constructors
and to_s, to_str methods:

class Results < Array
  def initialize(from_string = "")
    super( from_string.split(/\s+/).map! {|s| s.to_i} )
  end

  def to_str
    map {|d| d.to_s}.join ' '
  end

  alias :to_s :to_str
end

HTH

Kind regards

    robert
Michael J. (Guest)
on 2006-01-08 23:52
I want to say thanks for everyone's responses.  It took me a couple days
to go through them all and it helped me learn quite a bit.

  * Robert's example with Enumerable was neat.
  * Hal's use of symbols was cool and interesting.
  * Ross's use of a simple string probably made the most sense for my
situation

In the end I decided that simplest was best and subclassed String for
each respondent: adding save, load, and changed? methods.

respondents.each do |respondent|
        respondent.open
  	respondent[0..2] = 213
        respondent.close
end


I've been struggling with object oriented programming for years with
Perl, and only now (with Ruby) do I feel like I'm starting to really
"get" it.  It's all about creating an interface that makes sense to use
from a larger perspective.  The object hides the gritty details.

Like with the above interface, I could load the respondent from memory,
from database, or disk and it wouldn't matter at all to the rest of the
program.  That's sexy and makes my life easier.

Thanks again for everyone's help.  I certainly became a better
programmer after playing with your examples.

Regards,

Mike Judge
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