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)]))