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comp.lang.ruby

When is a key not a key?

Dave Burt

11/10/2004 8:17:00 AM

Hi,

I hesitate to write because some of my code here is pretty evil, but this
problem is weirding me out a bit.

I have a hash (loan_shark) indexed by instances of a particular struct
(Player).

The hash seems to be failing to find its keys when I try to look them up.
Like this:

loan_shark.size #=> 4
loan_shark.has_key?(loan_shark.keys[0]) #=> false

Any ideas?

The background is something like this:

Player = Struct.new(:name, :bankroll, :hands)
....
loan_shark = {}
....
newb = Player.new("Fred", some_dollars)
....
loan_shark[newb] = some_dollars
....
players << newb

If you feel the need, you can get all the code (about 15k) here:
http://www.dave.burt.id.au/ruby/bl...
http://www.dave.burt.id.au/rub...
(The issue can be found near blackjack.rb:304, complete with futile
debugging code)

Cheers,
Dave


2 Answers

Robert Klemme

11/10/2004 8:32:00 AM

0


"Dave Burt" <dave@burt.id.au> schrieb im Newsbeitrag
news:D7kkd.31098$K7.23161@news-server.bigpond.net.au...
> Hi,
>
> I hesitate to write because some of my code here is pretty evil, but
this
> problem is weirding me out a bit.
>
> I have a hash (loan_shark) indexed by instances of a particular struct
> (Player).
>
> The hash seems to be failing to find its keys when I try to look them
up.
> Like this:
>
> loan_shark.size #=> 4
> loan_shark.has_key?(loan_shark.keys[0]) #=> false
>
> Any ideas?

Two possible reasons come to mind:

- keys are modified after they were inserted into the hash but you did
not rehash

- some of the instances that you put into the Player struct do not
implement #eql? and #hash appropriately. Did you verify that newb.eql?(
newb ) is true?

Solutions to both could be to implement #eql? and #hash on your own, for
example if only "name" is the key. From what I read I feel that :hands
does change over time and thus is maybe not suited to be included in the
key or the key's #hash and #eql? calculation.

Kind regards

robert



>
> The background is something like this:
>
> Player = Struct.new(:name, :bankroll, :hands)
> ...
> loan_shark = {}
> ...
> newb = Player.new("Fred", some_dollars)
> ...
> loan_shark[newb] = some_dollars
> ...
> players << newb
>
> If you feel the need, you can get all the code (about 15k) here:
> http://www.dave.burt.id.au/ruby/bl...
> http://www.dave.burt.id.au/rub...
> (The issue can be found near blackjack.rb:304, complete with futile
> debugging code)
>
> Cheers,
> Dave
>
>

Dave Burt

11/10/2004 9:11:00 AM

0

"Robert Klemme" <bob.news@gmx.net> wrote...
> - keys are modified after they were inserted into the hash but you did
> not rehash

Of course, that makes perfect sense.

I'll try and make my hash keys invariant to keep it simple.

Many thanks,
Dave

PS: Sorry about the bad links, they've been fixed now the problem's gone, so
you can see my evil but working code.