Re: Hash tables - how to look up values?

[Thorsten Jolitz <tjolitz@googlemail.com>]

"Pascal J. Bourguignon" <pjb@informatimago.com> writes:

>> I could not find such a function - does it exist already?
>
> rassoc returns the first hit, because lists, even association lists are
> ordered.  Hash tables are not ordered.  We can write something similar,
> returning the first hit, but there's no guarantee the first will always
> be the same.
>
>
> (require 'cl)
>
> (defun* getkey (val table &key (test (function eql)) (default nil))
>    ;; (hash-table-test table)  is for keys, not for values…
>    (maphash (lambda (k v) 
>               (when (funcall test val v) (return-from getkey k)))
>             table)
>    default)
>
> (let ((h (make-hash-table)))
>   (setf (gethash :one h) "un"
>         (gethash :two h) "deux"
>         (gethash :a   h) "un")
>   (getkey "un" h :test (function string=)))
> --> :one ; or :a, who knows?
>
> I'd rather write:
>
> (defun* getkeys (val table &key (test (function eql)))
>    ;; (hash-table-test table)  is for keys, not for values…
>    (let ((keys '()))
>      (maphash (lambda (k v) 
>                  (when (funcall test val v) (push k keys)))
>               table)
>      keys))
>
> (let ((h (make-hash-table)))
>   (setf (gethash :one h) "un"
>         (gethash :two h) "deux"
>         (gethash :a   h) "un")
>   (getkeys "un" h :test (function string=)))
> --> (:a :one)

Great! Thats most amazing about Emacs and its community - if you ask for
a functionality that does not yet exists, it might well exist 5 minutes
later just because you asked for it. 

Thanks a lot, exactly what I need. 

-- 
cheers,
Thorsten