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Re: 'length' function for lists and cons cells?
From: |
duthen . cnv |
Subject: |
Re: 'length' function for lists and cons cells? |
Date: |
Mon, 25 Mar 2013 08:24:05 -0700 (PDT) |
User-agent: |
G2/1.0 |
Le jeudi 21 mars 2013 21:10:10 UTC+1, Drew Adams a écrit :
> > which function could I use when I map an alist e.g. with dolist,
> > that contains both types of associations as shown below: cons
> > cells, or lists with 3 or more elements?
> > 'length' doesn't work on cons cells:
>
> > Debugger entered--Lisp error: (wrong-type-argument listp "c")
> > length(("a" . "c"))
>
> It's not clear to me what you're asking.
>
> Are you looking for a "length" function that works for both true lists
> (but why do you mention 3 or more elements?) and a cons whose last cdr
> is a non-nil atom?
>
> If so then it is up to you to define what you want such a "length" to be/mean.
>
> Perhaps what you want is something like this?
>
> (defun thorsten-len (xs)
> (cond ((null xs) 0)
> ((null (cdr xs)) 1)
> ((atom (cdr xs)) 2)
> (t (1+ (thorsten-len (cdr xs))))))
>
> (thorsten-len '(1 2 3 4 . 5)) => 5
>
> (setq foo '((a . 1) (b 2) (c (3 3 3)) (d 4 4 4 . 4)))
> (mapcar #'thorsten-len foo) => (2 2 2 5)
>
> (setq bar ())
> (dolist (ff foo) (push ff bar))
> (reverse bar) => ((a . 1) (b 2) (c (3 3 3)) (d 4 4 4 . 4))
Considering that:
(a b) contains 2 cons cells and 2 values (a and b),
(a b . c) contains 2 cons cells and 3 values (a b and c),
(a b c) contains 3 cons cells and 3 values (a b and c).
So, one possible point of vue (not necessarily mine, though!)
could consider that (a b . c) is "a little bit longer" than (a b)
and "a little bit shorter" than (a b c)!
Hence, the function:
(defun semi-length (xs)
(cond ((null xs) 0)
((atom xs) .5)
(t (1+ (semi-length (cdr xs))))))
(mapcar (lambda (x) (cons (semi-length x) x))
'(()
a
(a)
(a . b)
(a b)
(a b . c)
(a b c)))
((0) (0.5 . a) (1 a) (1.5 a . b) (2 a b) (2.5 a b . c) (3 a b c))
A true list is just one whith an integer semi-length
whereas a dotted-paired list is one with a fractional semi-length ! :)
- Re: 'length' function for lists and cons cells?, Pascal J. Bourguignon, 2013/03/21
- Re: 'length' function for lists and cons cells?, Thorsten Jolitz, 2013/03/21
- Re: 'length' function for lists and cons cells?, Mark Skilbeck, 2013/03/22
- Re: 'length' function for lists and cons cells?, Thorsten Jolitz, 2013/03/22
- Message not available
- Re: 'length' function for lists and cons cells?, Pascal J. Bourguignon, 2013/03/23
- RE: 'length' function for lists and cons cells?, Drew Adams, 2013/03/23
- Message not available
- Re: 'length' function for lists and cons cells?, Pascal J. Bourguignon, 2013/03/23
- RE: 'length' function for lists and cons cells?, Drew Adams, 2013/03/24
Message not available