(defun sumlist (lst)
(remove nil lst)
(mapcar #''+ lst))
Lisp question
Author
What exactly is wrong with this function...
I''m really at a loss in terms of using the provided debugger/tracing etc as yet in the LispWorks Personal software so I thought I''d toy about a bit with the simpler things first and get comfortable with it.
In the event it doesn''t make much sense I''m trying to return the sum of numeric elements in a list. Thanks much in advance.
You're trying to program imperatively. Don't. Very few Lisp functions actually modify their arguments, because that is a no-no in functional programming.
Just so you know, that code only removes nils, not all non-numerical elements in the list.
You can get the results you need with
[edited by - micepick on May 26, 2003 5:45:03 PM]
(defun sumlist (lst) (mapcar #'+ (remove nil lst))) Just so you know, that code only removes nils, not all non-numerical elements in the list.
You can get the results you need with
remove-if, numberp, and complement. Have fun. ^_^[edited by - micepick on May 26, 2003 5:45:03 PM]
Author
quote:Original post by micepick
You''re trying to program imperatively. Don''t. Very few Lisp functions actually modify their arguments, because that is a no-no in functional programming.
I think my biggest stumbling block so far, as I''ve only been doing this for a few hours now
is trying to equate things in a C(++)/Java etc fashion which really just doesn''t work.quote:Original post by micepick
(defun sumlist (lst)
(mapcar #''+ (remove nil lst)))
Thanks!
quote:Original post by micepick
Just so you know, that code only removes nils, not all non-numerical elements in the list.
Yeah I realized as much. I''m just working with available code snippets from ANSI Common Lisp to get a better feel for this "lisp" mystery. Thanks again and I''m sure I''ll have some more obvious questions soon enough. Cheers.
Author
Hmmm...
I could''ve sworn that worked two days ago...
CL-USER 1 > (defun sumlist (lst) (mapcar #''+ (remove nil lst)))SUMLISTCL-USER 2 > (sumlist ''(1 2))(1 2)CL-USER 3 > I could''ve sworn that worked two days ago...
... Don't know what to tell you. Try making one that actually sums numerical elements. It might work better.
EDIT: Wait, what am I thinking, of course that won't work! I must have been high when I was helping you.
Explanation:
[edited by - micepick on May 29, 2003 10:23:42 PM]
EDIT: Wait, what am I thinking, of course that won't work! I must have been high when I was helping you.
Explanation:
mapcar applies a function that takes one argument (try evaluating (+ 1)) to each element of the list consecutively. That's obviously not what you need: You need a function that takes elements of the list two at a time and feeds them to a function that collapses them to one. The function you need is reduce.[edited by - micepick on May 29, 2003 10:23:42 PM]
Anyway, sorry for the confusion! In case you're not clear on what
Is that clear?
The great thing about Lisp is, you can implement most of its functions yourself in a couple of lines of code.
[edited by - micepick on May 29, 2003 10:33:21 PM]
reduce does, I wrote a simple implementation for you:(defun reduce (func lst) (if (= (length lst) 1) ; if there's only one element (car lst) ; just return it (funcall func (car lst) (reduce func (cdr lst))))) ; otherwise, recurse and reduceIs that clear?
The great thing about Lisp is, you can implement most of its functions yourself in a couple of lines of code.
[edited by - micepick on May 29, 2003 10:33:21 PM]
Author
Hey look at me I'm smart. Anyway, apply works Sorry for wasting your time once again!
Edit: ah, and thanks for that as well, I'm trying to still get over the c mentality which doesn't seem to want to go away.
I have noticed that and I like the possibilities it offers.
[edited by - kordova on May 29, 2003 10:36:08 PM]
Sorry for wasting your time once again!Edit: ah, and thanks for that as well, I'm trying to still get over the c mentality which doesn't seem to want to go away.
quote:Original post by micepick
The great thing about Lisp is, you can implement most of its functions yourself in a couple of lines of code.
I have noticed that and I like the possibilities it offers.
[edited by - kordova on May 29, 2003 10:36:08 PM]
Yeah, apply is fine too, since + takes any number of arguments.
is equivalent to
is equivalent to
Enjoying yourself?
[edited by - micepick on May 29, 2003 10:36:54 PM]
(apply #'+ '(1 2 3))is equivalent to
(+ 1 2 3)is equivalent to
(funcall #'+ 1 2 3)Enjoying yourself?
[edited by - micepick on May 29, 2003 10:36:54 PM]
This topic is closed to new replies.
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