How does autocad/lisp round numbers?

I am wondering because I created a routine that calculates square
footages/acreages and then creates a string of text in the editor. I was
rounding the number down to 2 places after the decimal but I was getting
some incorrect totals when I added a few together. Just for kicks I changed
the rounding to 4 places and found the following:

One area calc created an area of 9.1264 acres. When it was rounded to 2
places this was represented as 9.12 acres. I think it should be 9.13. Does
anyone know why this is? Is there a way to account for it?

TIA,

Jeff

 

Apply the rounding *after* doing the addition.
___

 

Hmmm, this little bit of code seems to show correct rounding:
(setq count 0)
(repeat 9
(princ (strcat "\n" (rtos 9.1264573249 2 count)))
(setq count (1+ count))
)

returns:
9
9.1
9.13
9.126
9.1265
9.12646
9.126457
9.1264573
9.12645732

 

Here is the routine I spoke of:

(defun c:act (/ b1 acre p1 h1)
(setq b1 (entsel))
(command ".area" "e" b1)
(setq b1 (getvar "area"))
(setq acre 43560)
(setq b1 (/ b1 acre))
(setq b1 (rtos b1 2 2))
(setq b1 (strcat b1 " AC"))
(setq p1 (getpoint "\nPick start point for CENTER Justified text: "))
(setq h1 (getdist "\nText height: "))
(command ".text" "j" "c" p1 h1 "0" b1)
(princ)
)

I use it to select a polygon and then do the calcs. Could you give it a go
and see if I'm doing something wrong?

I appreciate any help you can provide.

Jeff

 

First: LISP allows you to use other symbol names besides b1 too

Second: what does (setq b1 (getvar "area")) return in your case? Use
(rtos b1 2 20) to tell us the number in full accuracy.

I did never find an error with AutoCAD's way to round up real numbers.
Jeff already showed you that your initial statement is not true.

Tom Berger

 

Jeff,
I cannot duplicate your rounding error. Maybe if you use vlide and place a
break point after the entsel, then use F8 to step thru the routine. Make
sure to have the watch window open and observe the "LAST-VALUE" to see if
you can tell where the error gets introduced.

Jeff

 

I will give that a try. Thank you for the info.

Jeff

 

Jeff,

This probably doesn't apply to your question. Take care when dividing by integers.
The divide function is a bit bizarre in that regard.

Command: (/ 5 3)
1
Command: (/ 5 3.0)
1.66667

Given one or both numbers is a real, the divide function returns the result you'd
expect. Two integers... avoid it.

So I'd suggest (setq acre 43560.0). Or see help regarding the float function.

Joe Burke

 

Thank you Joe. I will alter my routine and see what happens.