From: Evan Klinger
Subject: Symbol Frequency
Date: Wed, 27 Nov 2002 19:29:40 +0000
Message-ID: <as36k2$lkf$1@news.service.uci.edu>

Hello,

Is there a standard Lisp function that will take a list and return an 
associative list representing the symbol frequency in the original list? 
For example, (frequency '(a (a (b) a))) => ((a . 3) (b . 1)). Any help 
would be greatly appreciated.

Thank you.

• Re: Symbol Frequency John Gilson
• Re: Symbol Frequency Barry Margolin

From: John Gilson
Subject: Re: Symbol Frequency
Date: Wed, 27 Nov 2002 20:23:13 +0000
Message-ID: <RQ9F9.184496$Up6.34795807@twister.nyc.rr.com>

"Evan Klinger" <········@uci.edu> wrote in message ·················@news.service.uci.edu...
> Hello,
>
> Is there a standard Lisp function that will take a list and return an
> associative list representing the symbol frequency in the original list?
> For example, (frequency '(a (a (b) a))) => ((a . 3) (b . 1)). Any help
> would be greatly appreciated.
>
> Thank you.

Coincidentally, someone else asked the same question earlier today.

(defun occurrences (list &key (test #'eql))
    (if (consp list)
         (let (alist)
            (labels ((occurrences-aux (object)
                           (cond ((null object))
                                      ((consp object)
                                       (occurrences-aux (first object))
                                       (occurrences-aux (rest object)))
                                      (t
                                       (let ((count (assoc object alist :test test)))
                                          (if (null count)
                                               (setf alist (acons object 1 alist))
                                               (incf (rest count))))))))
              (occurrences-aux (first list))
              (occurrences-aux (rest list)))
           alist)))

> (occurrences '(a (a (a b c) b c) b . c))
((C . 3) (B . 3) (A . 3))

Regards,
jag

From: Barry Margolin
Subject: Re: Symbol Frequency
Date: Wed, 27 Nov 2002 20:48:36 +0000
Message-ID: <DcaF9.61$sA6.998@paloalto-snr1.gtei.net>

In article <············@news.service.uci.edu>,
Evan Klinger  <········@uci.edu> wrote:
>Is there a standard Lisp function that will take a list and return an 
>associative list representing the symbol frequency in the original list? 
>For example, (frequency '(a (a (b) a))) => ((a . 3) (b . 1)). Any help 
>would be greatly appreciated.

No, there's no standard function for this; it seems like a pretty unusual,
application-specific need rather than being appropriate for a
general-purpose library.  And it's pretty easy to implement yourself, as in
the other response.

BTW, the values in that alist seem to be the count of occurrences, not the
frequency, where I interpret frequency to mean a ratio, e.g. ((a . 3/4) (b
. 1/4)).

-- 
Barry Margolin, ······@genuity.net
Genuity, Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
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