Re: Hash tables - how to look up values?

["Pascal J. Bourguignon" <pjb@informatimago.com>]

Thorsten Jolitz <tjolitz@googlemail.com> writes:

> Hi List, 
> I would need a function that does the opposite of 
>
> ,--------------------
> | (gethash key table)
> `--------------------
>
> i.e. something like 
>
> ,-------------------
> | (getkey val table)
> `-------------------
>
> that returns the corresponding key of a known (string) value in a hash
> table (similar to 'rassoc' for alists). 
>
> 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)


-- 
__Pascal Bourguignon__                     http://www.informatimago.com/
A bad day in () is better than a good day in {}.