arc length

How would one find the length of an arc using autolisp?

 

I think you need to calculate it using the object data, which is
Radius(group code 40) and Start Angle and End Angle(group code 50).

 

HiHo;
Here is a old one you can use the parts.
;CADENCE Magazine February 1992
;Page 86



;Note: Calculate arc length, included angle, chord, and radius
;en = entity name
;ed = entity definition
;et = entity type
;r = radius
;p1 = end pt of arc at start angle
;p2 = end pt of arc at end angle
;c = chord length
;cb = chord bearing
;sa = start angle
;ea = ending angle
;a = included angle
;al = arc length
;ctr = center point
;answer1 = arc length, included angle, and radius
;answer2 = chord length and bearing
;--------------------
(defun c:ArcLIST
(/ i en ed et r p1 p2 c cb sa ea a al ctr answer1 answer2)
;-----clear user input-----
(setq i 1)
(while i
(setq en (car (entsel "\nPick an arc: ")))
(if en
(progn
(setq ed (entget en))
(setq et (cdr (assoc 0 ed)))
(if (/= et "ARC")
(prompt "\nEntity is not an arc.")
(setq i nil)
);if
);progn
(prompt "\nYou did not pick anything.")
);if
);while
;-----retrieve relevant data-----
(setq ctr (cdr (assoc 10 ed)))
(setq r (cdr (assoc 40 ed)))
(setq sa (cdr (assoc 50 ed)))
(setq ea (cdr (assoc 51 ed)))
(setq p1 (polar ctr sa r))
(setq p2 (polar ctr ea r))
(setq c (distance p1 p2)) ;chord length
(setq cb (angle p1 p2)) ;chord bearing
;-----calculate included angle-----
(if (< sa ea)
(progn
(setq ea (- ea sa))
(setq sa (- sa sa))
(setq a ea)
)
(progn
(setq sa (- sa ea))
(setq ea (- ea ea))
(setq a (abs (- sa (* 2 pi))))
)
);if
;-----calculate arc length-----
(setq al (/ (* pi r a) pi)) ;arc length
(terpri)
(setq answer1
(strcat "Arc length = " (rtos al)
" Included angle = " (angtos a 0)
" Radius = " (rtos r)
)
);setq
(setq answer2
(strcat "Chord length = " (rtos c)
" Chord bearing = " (angtos cb)
)
);setq
(prompt answer1)
(terpri)
(prompt answer2)
(princ)
);defun

 

Thanks, I think I can make that work for what I am doing.
Happy Autolisping

 

This is one lisp available:

;|

DIMARC.LSP - Dimension an arc with length, rather than angle
(c) 1998 Tee Square Graphics

|;

(defun CIMARC (/ arc ent obj l)
(setq cmd (getvar "cmdecho")
arc (entsel "\nPick ARC to dimension: ")
ent (entget (car arc))
obj (cdr (assoc 0 ent)))
(if (= obj "ARC")
(progn
(setvar "cmdecho" 1)
(setq l (* (cdr (assoc 40 ent))
(if (minusp (setq l (- (cdr (assoc 51 ent))
(cdr (assoc 50 ent)))))
(+ pi pi l) l)))
(command "_.dimangular" arc "_t" (rtos l))
(while (= (logand (getvar "cmdactive") 1) 1)
(command pause))
(setvar "cmdecho" cmd))
(alert "Object selected is not an ARC."))
(princ)
)