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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: count symbols in a list
Date: Mon, 7 Jul 2025 12:44:04 -0000 (UTC)
Organization: A noiseless patient Spider
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B. Pym wrote:

> B. Pym wrote:
> 
> > B. Pym wrote:
> > 
> > > Erik Naggum wrote:
> > > 
> > > > >  I want to write a function that takes a list of symbols k and and lisp
> > > > >  expression l and counts the number of times each symbol in k occurs in
> > > > >  the lisp expression. It should return an alist binding each symbol to its
> > > > >  count.  I want to do this without flattening the list before I go through
> > > > >  it looking for symbols.
> > > > 
> > > >   Look for two things in this code: How it is formatted, and how it does
> > > >   its work.  (The way you have formatted your code annoys people.)  Explain
> > > >   to me why this works and gives the right answer when you have ascertained
> > > >   that it does.  Explain why it is efficient in both time and space.
> > > > 
> > > > (defun count-member (symbols tree)
> > > >   (let* ((counts (loop for symbol in symbols collect (cons symbol 0)))
> > > 
> > > Why didn't he use the simpler "mapcar" instead of "loop"?
> > > Example:
> > > 
> > > (mapcar (lambda(s) (cons s 0)) '(a b c))
> > >   ===>
> > > ((A . 0) (B . 0) (C . 0))
> > > 
> > > 
> > > >          (lists (list tree))
> > > >          (tail lists))
> > > >     (dolist (list lists)
> > > >       (dolist (element list)
> > > >         (cond ((consp element)
> > > >                (setf tail (setf (cdr tail) (list element))))
> > > >               ((member element symbols :test #'eq)
> > > >                (incf (cdr (assoc element counts :test #'eq)))))))
> > > >     counts))
> > > 
> > > 
> > > Testing:
> > > 
> > > * (count-member '(w x y z) '(a x (b y y (z) z)))
> > > 
> > > ((W . 0) (X . 1) (Y . 0) (Z . 0))
> > > 
> > > It only counts the top-level symbols!
> > 
> > 
> > The testing was done under SBCL.  Perhaps the function
> > will work correctly under another version of CL.
> > In any case, this is questionable code.
> 
> Here's a version in CL that follows Naggum's lead
> by avoiding recursion.
> When an item in the list that is currently being
> processed is found to be a list, it is pushed
> onto the list of trees.
> 
> (defun count-member (symbols tree)
>   (let ((counts (mapcar (lambda(s) (cons s 0)) symbols))
>         (trees (list tree)))
>     (do () ((null trees)) ;; Until trees is empty.
>       (dolist (x (pop trees))
>         (cond ((consp x) (push x trees))
>               (t (let ((found (assoc x counts)))
>                     (if found (incf (cdr found))))))))
>     counts))
> 
> Under SBCL:
> 
> * (count-member '(w x y z) '(a x (b y y (z) z)))
> 
> ((W . 0) (X . 1) (Y . 2) (Z . 2))
>  

In Gauche Scheme:

(define (count-member symbols tree)
  (let ((counts (map (cut  cons <> 0) symbols))
        (trees (list tree)))
    (do () ((null? trees)) ;; Until trees is empty.
      (dolist (x (pop! trees))
        (cond ((pair? x) (push! trees x)) ;; Arg. order different from CL.
              ((assoc x counts) =>
               (lambda(f) (inc! (cdr f)))))))
    counts))