1. Forum
  2. USENET
  3. comp.lang.scheme

  • Re: Symbol Frequency

    From B. Pym@Nobody447095@here-nor-there.org to comp.lang.lisp,comp.lang.scheme on Sun Sep 15 05:23:30 2024
    From Newsgroup: comp.lang.scheme

    John Gilson 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.

    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))

    Gauche Scheme:

    (define (occurrences tree)
    (rlet1 alist '()
    (let go ((x tree))
    (cond
    ((null? x))
    ((pair? x) (go (car x)) (go (cdr x)))
    (#t (ainc! alist x))))))

    (occurrences '(a (a (a b c) b c) b . c))
    ===>
    ((c . 3) (b . 3) (a . 3))

    Given:

    (define-syntax ainc!
    (syntax-rules ()
    [(_ alist key val func default)
    (let ((pair (assoc key alist)))
    (if pair
    (set-cdr! pair (func val (cdr pair)))
    (set! alist (cons (cons key (func val default)) alist))))]
    [(_ alist key val func)
    (ainc! alist key val func 0)]
    [(_ alist key val)
    (ainc! alist key val +)]
    [(_ alist key)
    (ainc! alist key 1)]))
    --- Synchronet 3.22a-Linux NewsLink 1.2