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Re: hash tables
Thomas Bushnell, BSG
Re: hash tables
30 Jun 2001 08:54:22 -0700
Gnus/5.0808 (Gnus v5.8.8) Emacs/20.7
Alex Shinn <address@hidden> writes:
> fails when someone simply wants an array of alists, and is not
> considering them a hash.
> One approach to solving this is that unlike most Unix scripting
> languages, Scheme distinguishes between vectors and lists. So in your
> Python translator, you're probably representing tuples and (Python)
> lists as one of these, leaving the other type free to uniquely
> identify hashes.
Well, right now I'm identifying Python lists and Scheme lists. I want
to identify Python tuples and Scheme vectors.
Python does not have improper lists. So I'm going to use pairs like
('hash . the-table-itself)
to represent Python objects that don't have easy Scheme counterparts.
That works as long as the cdr of these pairs is never itself a pair;
then it's easy and quick to distinguish these from Python lists.
> Unfortunately this leaves out potential optimizations for knowing when
> a Python tuple/list would be better implemented as a vector or list
> (an approach which could in many cases make the Guile translation run
> much faster than the original Python).
::grin:: Note that the tagging method makes this easy. Now we have
(define (python:hash? obj)
(and (pair? obj) (eq? (car obj?) 'hash)))
> Another approach, which more closely matches Python's dictionaries, is
> to define a hash record (or class), and give it some meta-information.
> Such as the ahash (automatically-resizing hash):
> (make-record-type "ahash" ' (table size elements))
> which could keep track of the current size and number of elements in
> the table and perform a re-hash when the ratio reaches a certain
> limit. The Perl version of this might also include a next slot,
> holding a continuation referencing a hash-fold loop for use in Perl's
So far I've been avoiding records as being non-standard Scheme. Is