Calculate the bulge of an arc.

[The Buzzard]

Hi,

 

AFAIK, there's no need to test if the cosine equals 0 to avoid an error.

The cos LISP function is implemented so that (cos (/ pi 2)) returns 6.12323e-017 and (cos (* pi 1.5)) -1.83697e-016 rather than 0.

 

So I currently use:

(defun tan (a) (/ (sin a) (cos a)))

 

 

Thanks gile,

 

I tested it and it also works great.

[gile]

Thanks gile for the info, any reason why the discrepancy against 0 or just the acad rounding again?

I can't say.

 

On one hand, it looks like the unavoidable rounding numbers while a program (not only AutoCAD) is dealing with pi (or other real numbers):

(sin 0) returns 0 but (sin pi) returns 1.22465e-016 instead of 0.

 

On the other hand some functions seems to be able to return right results in spite of this necessary rounding of pi value:

(= (cos (* 2 pi)) 1) returns T as well as (= -1 (cos pi)) or (= (atan 1 0) (/ pi 2)) or (= (atan -1 0) (/ pi -2))

[wizman]

Thank you gile..'-)

[pratikchirania]

Hi,

I am new to LISP and am not able to make out much of the code that finds the centre from the start point, end point, bulge and radius.

I need to code this in C++. Can anyone help me by giving the logic of the functions(in lisp) used in the previous replys for the same?

 

thanks in advance,

Pratik

[Lee Mac]

Perhaps see this post

[pratikchirania]

Hi Lee Mac,

 

Thanks for the reply.

I did already have a look at the thread you just posted.

I was able to make out the derivation of radius, but the following code was not clear to me:

centre (polar p1 (+ (- (/ pi 2.) theta/2) (angle p1 p2)) radius))

 

I dont understand by the Lisp function "polar".

Would be grateful if you helped me give the logic of the above lisp code

[Lee Mac]

The polar function is identical to the mathematical sense, in that the arguments take the form of a base point, angle (ACW from X-Axis) and radius.

[VVA]

Geometry of the bulge By Stig Madsen