[Ecls-list] map iterator

[Marco Antoniotti]

Shameless plug. :)

http://common-lisp.net/project/cl-enumeration

Marco

On Nov 9, 2005, at 3:35 AM, Dean O'Connor wrote:

>
> Hi
>
> I am having trouble finding a way to accomplish something in Lisp, 
> perhaps someone has ideas ....
>
> I am trying to implement the functional equivalent an C++ iterator to 
> an STL map class, in Lisp.
> I need this as I have a C++ application function that needs to call a 
> lisp function iteratively to retrieve all key/value mappings.
> So I need to able to call a Lisp function that will return be the next 
> map pair.
>
> From my limited experience in Lisp, hash tables seemed to be the 
> obvious choice. (This could be my first wrong assumption !)
> So I created a class containing a hash table and methods to add, show 
> all and an attempt at a iterator for retrieval.
>
> Now perhaps I am stupidly overcomplicated the matter, but reading all 
> I could on hash-tables, I didn't see an easy way to index or remember 
> where I was up to iterating thru the hash over seperate function 
> calls.
>
> I did find a link that encouraged me to use closures (still quite 
> voodoo to me :) to accomplish it.
> However on this link (below), it did warn about the following:
>
> http://www.cs.queensu.ca/software_docs/gnudev/gcl-ansi/gcl_1059.html
>
> >> "It is unspecified what happens if any of the implicit interior 
> state of an iteration is returned outside the dynamic extent of the 
> *with-hash-table-iterator* /form/ such as by returning some /closure/ 
> over the invocation /form/."
>
> Now my example nearly works (incorrectly returns FIRST pair for both 
> calls)  but its pretty clear to me my closure is not remembering the 
> state of the "with-hash-table-iterator".
> I am pretty sure I haven't done enough syntactically for it to work 
> poperly, but also I was wondering if there is any official word of the 
> behaviour of this form in ELCS ?
>
> If my approach is all wrong, does anyone have any suggestions.
>
> ;---------------------------------------------------------------------
> ; Class: mapper-class
> ;---------------------------------------------------------------------
>
> (defclass mapper-class ()
>  ((hash-table
>    :initform (make-hash-table)
>    :accessor table)
>   (iterator
>   :initform nil
>   :accessor it)))
>
> ;---------------------------------------------------------------------
> ; Methods: mapper-class
> ;---------------------------------------------------------------------
>
> (defmethod show ((m mapper-class))
>
>  (with-accessors ((ht table)) m
>          (maphash #'(lambda (k v) (format t "~a => ~a~%" k v)) ht)))
>
> (defmethod add ((m mapper-class) key value)
>  (with-accessors ((ht table)) m
>          (setf (gethash key ht) value)))
>
> (defmethod clear ((m mapper-class))
>  (with-accessors ((ht table)) m
>          (clrhash ht)))
>
> (defmethod set-start ((m mapper-class))
>  (with-accessors ((ht table) (it it)) m
>          (setf it #'(lambda ()
>                   (with-hash-table-iterator (next-entry ht)
>                             (next-entry))))))
>
> (defmethod get-next ((m mapper-class))
>  (with-accessors ((it it)) m
>          (multiple-value-bind (more key value)
>              (funcall it)
>            (if more
>            (cons key value)
>              nil))))
>
> ;---------------------------------------------------------------------
> ; Example of populating and retrieving key/values
> ;---------------------------------------------------------------------
>
> (defparameter *map1* (make-instance 'mapper-class))
>
> (add *map1* "key1" "val1")
> (add *map1* "key2" "val2")
>
> (show *map1*)
> (set-start *map1*)
>
> (setf key-val (get-next *map1*))
> (format t "~a => ~a~%"  (first key-val) (rest key-val))
>
> (setf key-val (get-next *map1*))
> (format t "~a => ~a~%"  (first key-val) (rest key-val))
>
>
> Cheers
> Dean.
>
>
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--
Marco Antoniotti					http://bioinformatics.nyu.edu/~marcoxa
NYU Courant Bioinformatics Group		tel. +1 - 212 - 998 3488
715 Broadway 10th FL				fax. +1 - 212 - 998 3484
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