Hash-tables containing references to hash-tables

Hash-tables containing references to hash-tables

[Nordlöw]

Is it possible for a hash-table to contain a *reference* to another
hash-table, similar to how we can build arbitrary graphs using conses/
lists and setf?
If so how? If not, is there reason for being so or is it just an emacs
todo?

If we have
  (puthash parent-key (make-hash-table : size CHILD-SIZE) parent-hash)
then I want
  (gethash parent-key parent-hash)
to return a *reference* to hash-table contained in parent-hash.

I already have a working code for this but I am uncertain how this
will perform when CHILD-SIZE is really large.

/Nordlöw

Re: Hash-tables containing references to hash-tables

[tomas]

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On Mon, Nov 23, 2009 at 03:12:22AM -0800, Nordlöw wrote:
> Is it possible for a hash-table to contain a *reference* to another
> hash-table, similar to how we can build arbitrary graphs using conses/
> lists and setf?

Yes, it is -- in a way.

> If so how? If not, is there reason for being so or is it just an emacs
> todo?
> 
> If we have
>   (puthash parent-key (make-hash-table : size CHILD-SIZE) parent-hash)
> then I want
>   (gethash parent-key parent-hash)
> to return a *reference* to hash-table contained in parent-hash.

It already does. In Lisp, you _always_ get a reference for anything
beyond simple data types (as integers and friends).

> I already have a working code for this but I am uncertain how this
> will perform when CHILD-SIZE is really large.

Ah, it's performance you are worried about. No -- gethash isn't
returning a copy of the child table, but "the child table itself".
Change it and you'll see...

Regards
- -- tomás
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Re: Hash-tables containing references to hash-tables

[Pascal J. Bourguignon]

Nordlöw <per.nordlow@gmail.com> writes:

> Is it possible for a hash-table to contain a *reference* to another
> hash-table, similar to how we can build arbitrary graphs using conses/
> lists and setf?

Yes.


There is not automatic copy ever done in lisp.  You have to
explicitely copy the structures (such as cons, copy-list,
copy-hash-table, copy-my-struct, etc) you use if you want copies (or
use a function that explicitely copies the input data like append does
for all lists but the last).  And for some types, there are even no
predefined copier function!  




-- 
__Pascal Bourguignon__