Attribute writers


#1

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)=©”. 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!


#2

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


#3

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?


#4

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!


#5

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


#6

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)=©”. 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