Re: hash strangeness

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

Tom <adatgyujto@gmail.com> writes:

> Here's a code which gives me some headache. It's silly, because
> it was shortened from a longer code to demonstrate the problem.
>
> It is supposed to count something in elisp functions, though
> the condition is removed, so with this code the count should be
> 1 for every function.
>
> So the code checks if the function is already in the hash
> and if not then it inserts new info for that function:
>
>
> (let ((h (make-hash-table :test 'equal)))
>   (mapatoms
>    (lambda (s)
>      (let* ((name (symbol-name s))
>             (info (gethash name h)))
>
>        (unless info
>          (setq info '(count 0)))
>
>        (setq info (plist-put info
>                              'count (1+ (plist-get info 'count))))
>                   
>        (puthash name info h))))
>
>   (pop-to-buffer "*testout*")
>   (erase-buffer)
>   (maphash (lambda (name info)
>              (insert (format "%s %s" (plist-get info 'count) name) "\n"))
>            h))
>
>
> The new info is newly created in the lambda function (it is
> a plist, because in the real code there are other fields too),
> yet for some reason the same info structure is used for all
> iterations.
>
> I tried to debug it and at the "(unless info" part info is nil
> and then it is set to the previous value, though it should be a 
> new value.
>
> Am I missing something here?

You've identified the problem with the literal list.  However, you could
write it much simplier:

(require 'cl) ; always
(let ((h (make-hash-table :test 'eql))) 
  (mapatoms (lambda (name)
              (incf (getf (gethash name h) 'count 0))))
  (pop-to-buffer "*testout*")
  (erase-buffer)
  (maphash (lambda (name info)
             (insert (format "%s %s\n" (getf info 'count) name)))
           h))

In the current emacs lisp, there's no point in using symbol-name strings
as key in the hash-table: just use symbols with eql as test.
Notice that in emacs lisp:

     (not (equal (symbol-name :hello) (symbol-name 'hello)))


It would be different if emacs lisp had packages like Common Lisp.

-- 
__Pascal Bourguignon__                 http://www.informatimago.com/
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