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comp.lang.ruby

Wrong results using named arguments

Jason Vogel

11/29/2006 10:09:00 PM

Source:

[a.rb]
class A

attr_accessor :id
attr_accessor :name

def initialize(id = nil, name = nil)
@id = id
@name = name
end
end

[test.rb]
require 'a'

a1 = A.new(:name => "test")
a2 = A.new(:name => "test2")

p a1
p a2

Results:
#<A:0x28c2d80 @id={:name=>"test"}, @name=nil>
#<A:0x28c2678 @id={:name=>"test2"}, @name=nil>

I expected:

#<A:0x28c2d80 @id=nil, @name="test">
#<A:0x28c2678 @id=nil, @name="test2">

So what nuby mistake am I making?

Thanks,
Jason
10 Answers

Paul Lutus

11/29/2006 10:23:00 PM

0

Jason Vogel wrote:

> So what nuby mistake am I making?

Posting the same message six hours after receiving a raft of helpful replies
to the first copy of your message?

--
Paul Lutus
http://www.ara...

Jason Vogel

11/29/2006 10:28:00 PM

0

Sorry about the repost. Initially, I thought the original post went
successfully, but then when I went to check on it, it didn't show up...
suck.

I thought that the keyword arguments support was in Ruby 1.8. I'm used
to coding Oracle PL/SQL, so it seems really natural to me.

I had seen the hash stuff, but those were all posts about 1.6.

Thanks everyone for the replies, I'll keep going.
Jason


On Nov 29, 4:22 pm, Paul Lutus <nos...@nosite.zzz> wrote:
> Jason Vogel wrote:
> > So what nuby mistake am I making?Posting the same message six hours after receiving a raft of helpful replies
> to the first copy of your message?
>
> --
> Paul Lutushttp://www.ara...

Jason Vogel

11/29/2006 10:35:00 PM

0

Okay, I have a couple of questions. If I use the Hash approach,

- How do I handle argument default values?
- How do I handle required vs. optional arguments?
- Provide some sort of clue to the calling routines what the argument
"names" are?
- Is there a way to enumerate the valid choices for values (e.g. colors
= [red,white,blue])

Jason

On Nov 29, 4:22 pm, Paul Lutus <nos...@nosite.zzz> wrote:
> Jason Vogel wrote:
> > So what nuby mistake am I making?Posting the same message six hours after receiving a raft of helpful replies
> to the first copy of your message?
>
> --
> Paul Lutushttp://www.ara...

Paul Lutus

11/29/2006 10:52:00 PM

0

Jason Vogel wrote:

> Okay, I have a couple of questions. If I use the Hash approach,
>
> - How do I handle argument default values?

Initially:

@hash = { key => arg, key => arg, ... }

Then:

@hash[key] = arg # from initializer arguments

> - How do I handle required vs. optional arguments?

Please explain. Ordinarily if a required argument is not provided, the call
fails:

.... wrong number of arguments (0 for 1) (ArgumentError)

> - Provide some sort of clue to the calling routines what the argument
> "names" are?

Is this still a constructor for a class?

Perhaps it would be better, rather than asking a lot of hypothetical
questions, for you to say what you are trying to accomplish with your code.

> - Is there a way to enumerate the valid choices for values (e.g. colors
> = [red,white,blue])

Yes, but it is a bit involved.

--
Paul Lutus
http://www.ara...

Tim Hunter

11/29/2006 11:09:00 PM

0

Jason Vogel wrote:
> Okay, I have a couple of questions. If I use the Hash approach,
>
> - How do I handle argument default values?
> - How do I handle required vs. optional arguments?
> - Provide some sort of clue to the calling routines what the argument
> "names" are?
> - Is there a way to enumerate the valid choices for values (e.g. colors
> = [red,white,blue])
>
>
You might find the "Method Arguments" section on page 332 in the Pickaxe
(2nd ed.) helpful, as it describes all the ways you can pass arguments
to a method.

Ara.T.Howard

11/29/2006 11:10:00 PM

0

Joel VanderWerf

11/30/2006 7:17:00 AM

0

ara.t.howard@noaa.gov wrote:
> On Thu, 30 Nov 2006, Jason Vogel wrote:
>
>> Okay, I have a couple of questions. If I use the Hash approach,
>>
>> - How do I handle argument default values?
>
> def the_method opts = { :arg => 'default value', :another_arg =>
> 'default value too' }

Not really, at least in 1.8. That sets the default for _opts_, not for
each of the named args.

def the_method opts = { :arg => 'default value', :another_arg =>
'default value too' }
p opts
end

the_method :arg => "qwe" # ==> {:arg=>"qwe"}

Note that the hash doesn't retain the default value for :another_arg.

Has this changed in 1.9? Is there a way to set the defaults for keyword
args?

...
>> - Is there a way to enumerate the valid choices for values (e.g. colors
>> = [red,white,blue])
>
> def the_method opts = { :a => %w( a b c ).first, :b => %w( d e f
> ).first }

You know this gets evaluated every time you call the_method, right?

def m opts = { :foo => (puts "called m") }; end
m; m

output:

called m
called m

Anyway, this serves no purpose beyond documentation (it doesn't
validate). Why not just put this info into the rdoc for the method?

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Joel VanderWerf

11/30/2006 7:29:00 AM

0

Joel VanderWerf wrote:
> ara.t.howard@noaa.gov wrote:
>> On Thu, 30 Nov 2006, Jason Vogel wrote:
>>
>>> Okay, I have a couple of questions. If I use the Hash approach,
>>>
>>> - How do I handle argument default values?
>>
>> def the_method opts = { :arg => 'default value', :another_arg =>
>> 'default value too' }
>
> Not really, at least in 1.8. That sets the default for _opts_, not for
> each of the named args.

This is one way to handle defaults for keyword args:

DEFAULTS = {
:x => 1,
:y => 2
}

def m opts = {}
opts = DEFAULTS.merge(opts)
p opts
end

m :x => 5 # ==> {:x=>5, :y=>2}

Note that #merge is non-destructive (unlike #update, aka #merge!), so it
doesn't affect DEFAULTS.

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Ara.T.Howard

11/30/2006 3:06:00 PM

0

Joel VanderWerf

11/30/2006 5:07:00 PM

0

Ryan Williams wrote:
> On 11/29/06, Joel VanderWerf <vjoel@path.berkeley.edu> wrote:
>>
>> Joel VanderWerf wrote:
...
>> def m opts = {}
>> opts = DEFAULTS.merge(opts)
>> p opts
>> end
>>
>> m :x => 5 # ==> {:x=>5, :y=>2}
>>
>> Note that #merge is non-destructive (unlike #update, aka #merge!), so it
>> doesn't affect DEFAULTS.
>
>
> In most cases I think it is best if you define DEFAULTS inline, as in:
>
> def m opts = {}
> opts = {:x => 1,
> :y => 2}.merge(opts)
> p opts
> end
>
> That way it's closer to the call and thus more self-documenting, and you
> don't have to worry about naming a bunch of constants in your class. The
> downside is you're newing up a new defaults hash in every method call, and
> executing any contained statements.

If you define default option hashes as constants, you can comment them
and they will appear in RDOC output. RDoc is even smart enough to try to
put the constant value (i.e. the defaults hash) in the doc.

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407