First 3 elements of list

R2005 Vlisp:

How can I get the first 3 items of a list.

(18.1876 5.3017 -9.86971 0.0)

Besides (list (car lt)(cadr lt)(caddr lt))

TIA

Bill

 

Bill,

Here's one way that works for a list with 4 elements:

(setq a (18.1876 5.3017 -9.86971 0.0))
(reverse (cdr (reverse a)))

But I prefer the example you posted.

Steve Doman

 

I prefer nth per
(list
(nth 0 lt)
(nth 1 lt)
(nth 2 lt)
)

 

Bill,

I agree with Steve. Why not (list (car... ?

An alternate method given a list of unknown length.

(setq lst '(18.1876 5.3017 -9.86971 0.0 0.0 0.0))
(setq newlst nil)
(while (< (length newlst) 4)
(setq newlst (cons (car lst) newlst))
(setq lst (cdr lst))
)
(reverse newlst)

Joe Burke

 

Also

(mapcar '(lambda(e1 e2) e2)'(1 2 3)'(18.1876 5.3017 -9.86971 0.0))

 

That's what I was looking for.

Interesting technique also.


Thanks

Bill

 

Bill,
Usually when I ask here, I find a "slicker" way to do what I thought was the only way to do it, for years....

Bill

 

Although I'd hate to maintain that code. Imagine trying to read that 2 years
from now and figure out what is the *real* intention of the statement.

--
R. Robert Bell


That's what I was looking for.

Interesting technique also.


Thanks

Bill

 

I thought what I posted was the long way around.

Mapcar for something like this? There's probably a few other tortured methods
available. Let's have a contest. The slowest method wins. ;-)

Joe Burke

 

I vote for Steve's; only 24 characters.

 

....for fastest, that is.

 

Although I'd hate to maintain that code. Imagine trying to >>read that 2 years
Good point,
'((car lt)(cadr lt)(caddr lt))
is probably best anyway.
I've always been a sucker for gimmicks.

Bill

 

Don't know about faster, but this is another way:

(defun list-head (lst cnt)
(cond
( (or (zerop cnt) (not lst)) nil)
(t (cons (car lst)
(list-head (cdr lst) (1- cnt))
)
)
)
)



AutoCAD based Security Planning Solutions:
http://www.caddzone.com/securityplanning

 

I rarely use the mapcar and lambda (I know, please no chastisements).
So, this took a while to figure out, but here is another method -

(setq Glist (list "1a" "2b" "3c" "4d" "5e"))
(defun GetList (Nlist Glist /)
(mapcar '(lambda (x) (nth x Glist)) Nlist)
)

Command: (GetList (list 0 1) Glist)
("1a" "2b")

Command: (GetList (list 0 1 2 3) Glist)
("1a" "2b" "3c" "4d")

Command: (GetList (list 1 3) Glist)
("2b" "4d")

Command: (GetList (list 0) Glist)
("1a")

 

How's this?

(defun 3_of_list (flist / nlist)
(repeat 3
(setq nlist (cons (car flist) nlist)
flist (cdr flist))
)
(reverse nlist)
)

--
Ken Alexander
Acad2004
Windows XP

"We can't solve problems by using the same kind
of thinking we used when we created them."
--Albert Einstein

 

How about this
(vl-string-right-trim "0.0" "18.1876 5.3017 -9.86971")

 

Bill,
There's nothing wrong with your first post and Tony's
function is a good way to generalize the sitiuation but
your last post won't do anything. The quote will stop
evaluation.
Regards,
Doug

 

Doug,
thanks.
I am having trouble understanding quote.
Seems like sometimes I can use '(( to list things and sometimes I have to use (list (.

Bill

 

Just remember:
If you don't want the list evaluated, use quote.

'(1 2 3 4) is OK because literal numbers (and strings)
do not require evaluation.

(list 1 2 3 4) is OK also.

(setq a 1 b 2 c 3)
3

'(a b c) returns (a b c)
(list a b c) returns (1 2 3)

(mapcar 'eval '(a b c)) returns (1 2 3) [not recommended, just an illustration]

 

Hey thanks,

I kinda thought that's the way it worked.

Bill