[Maxima-bugs] [ maxima-Bugs-3510983 ] curious evaluation in ifactor.lisp

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Bugs item #3510983, was opened at 2012-03-25 02:57
Message generated for change (Comment added) made by robert_dodier
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Category: Lisp Core
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Volker van Nek (van_nek)
Assigned to: Nobody/Anonymous (nobody)
Summary: curious evaluation in ifactor.lisp

Initial Comment:
With clisp and sbcl:

(%i1) load("/home/volker/Maxima/git/maxima/src/ifactor.lisp")$

(%i2) :lisp (get-factor-list 15)

((5 1) (3 1))
(%i2) :lisp (let* ((fs (get-factor-list 14)) (f (car fs))) (incf (cadr f) 41))

42
(%i2) :lisp (get-factor-list 15)

((5 42) (3 1))

Each call to get-factor-list should cons a new list, so each return value should be referentially independent. 

I tried to find the reason of this error and observed that in ifactor.lisp lines 223-226

	 (when (> (* d d) n)
	   (push `(,n 1) factors)
	   (when $ifactor_verbose  (format t "small prime cofactor: ~A~%" n))
	   (return-from get-small-factors (values 1 factors)))

the backquote-comma-macro-line seems to cause this.

Using 
(push (list n 1) factors)
or
(setq tmp 1)
(push `(,n ,tmp) factors)
instead the results are as expected. 

I don't get the clue what's wrong with (push `(,n 1) factors). 

If I create a new list foo with (copy-tree factors) inside of get-small-factors and do 
(push `(,n 1) foo)
the results for foo are as expected.

So I guess the variable 'factors' has referential problems. Don't know why.


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>Comment By: Robert Dodier (robert_dodier)
Date: 2012-03-27 07:47

Message:
I think you should just go ahead and change the code. Looks to me like you
have identified a problem and a way to fix it, so I think you should ahead
and fix it.

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Comment By: Volker van Nek (van_nek)
Date: 2012-03-25 03:12

Message:
(setq foo (copy-tree factors))

isn't even necessary.

(setq foo factors) 
or
(setf foo factors) 
also works:

	 (when (> (* d d) n)
      (setf foo factors)
      (print foo)
      (push `(,n 1) foo)
      (print foo)
      (print factors)
      (push `(,n 1) factors)
      (print factors)

prints
(%i2) :lisp (get-factor-list 15)

((3 1)) 
((5 1) (3 1)) 
((3 1)) 
((5 42) (3 1)) <- print
((5 42) (3 1)) <- return value

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