Building a Point List

I haven't tried this in a while but what I need to do is to build a list of points which can be used to construct a polyline.

(setq pt1 '(0.0 0.0)
       pt1 (append pt1 (list (car pt1) (+ (cadr pt1 12)))
       pt1 (append pt1 (list (+ (car pt1 12) (+ (cadr pt1) 12)))
       )

But I'm not ending up with what I was intending to, which is

((0.0 0.0) (0.0 12.0) (12.0 12.0))

And this then begs the question, is it better to assembly a list of lists or just a list of the car/cadr of each point

(0.0 0.0 0.0 12.0 12.0 12.0)

When it's said and done I want to use entmake to construct the polyline using the points list.

Here are 2 common ways to construct a list:

(defun test ( / p pL ) ; #1
 (while (setq p (getpoint "\nSpecify point: "))
   (setq pL (cons p pL)) ; items are collected in reverse order
 )
 (reverse pL)
)

(defun test ( / p pL ) ; #2
 (while (setq p (getpoint "\nSpecify point: "))
   (setq pL (append (list p) pL)) ; items are collected in order
 )
)

As for the polyline emaking, check this thread.

(append list1 list2) is adding each element of list2 to the end of list1

Observe each or this sample:

(append (list 1) (list 2))                   -> (1 2) 
(append (list 1 2) (list 3 4))               -> (1 2 3 4)
(append (list (list 1 2)) (list (list 3 4))) -> ((1 2) (3 4))

If you want a list of points, you have to feed the append function with lists of points, each point being a list of numbers (X, Y coordinates). In your sample, all lists are list of numbers.

 

These 2 samples are equivalent:

(setq pt1 '((0.0 0.0))
     pt1 (append pt1 (list (list (caar pt1) (+ (cadar pt1) 12))))
     pt1 (append pt1 (list (list (+ (caar pt1) 12) (+ (cadar pt1) 12))))
)

(setq pt1 '((0.0 0.0))
     pt1 (cons (mapcar '+ (car pt1) '(0.0 12.0)) pt1)
     pt1 (cons (mapcar '+ (car pt1) '(12.0 0.0)) pt1)
     pt1 (reverse pt1)
)

The next step if you dont have it pretty sure something like this

 

(command "_pline")
(while (= (getvar "cmdactive") 1 ) 
(repeat (setq x (length lst))
(command (nth (setq x (- x 1)) lst))
)
(command "")
)

IMO prefer this

'((0.0 0.0) (0.0 12.0) (12.0 12.0))

easier to manipulate by

repeat

while

mapcar

nth

foreach

vl-sort

or transpose etc.. (pls name it if missing)

 

LISt Process

in short LeeSP