Lisp HUG Maillist Archive

RE: another newbie question

> -----Ursprüngliche Nachricht-----
> Von: owner-lisp-hug@lispworks.com
> [mailto:owner-lisp-hug@lispworks.com] Im Auftrag von Jacek Generowicz
> Gesendet: Freitag, 22. Juli 2005 10:14
> An: Carlos Bruguera
> Cc: lisp-hug@lispworks.com
> Betreff: Re: another newbie question
> 
> 
> 
> On Jul 22, 2005, at 8:56 AM, Carlos Bruguera wrote:
> 
> > Hello guys... I need to finish my genethic algorithm today
> and i found
> > some little problem, I'm doing:
> >
> > (+ (* 2 0.0001) 4.1)
> >
> > my interpreter prints this:
> > 4.1001997
> >
> > Why??? shouldn't it be just 4.1002 ?
> 
> The accuracy of floating point numbers is limited. For example, when
> you evaluate 0.0001 you get a number which is close to 0.0001, but  
> not exacly 0.0001.
> 
> Google for
> 
>      floating point computation
> 
> or
> 
>      what every computer scientist should know about floating point
> 

I second this.

Perhaps something like

(format nil "~8,4f" (+ (* 2 1.0001) 4.1))

is helpful.

Ooops, I just wandered about the use of lisps ROUND? Perhaps anybody can
give a hint on the idiomatic use of ROUND.

You can use the reader to force a certain number of digits:

(read-from-string (format nil "~8,4f" (+ (* 2 1.0001) 4.1)))

You can use this to define a helping function

(defun dround (num n)
  "round num to n digits"
  (let ((format-string (concatenate 'string "~," (format nil "~a" n)
"f")))
    (read-from-string (format nil format-string num))))

not very elegant, just a fast hack :))


Andreas

Re: another newbie question

"Andreas Thiele" <andreas@atp-media.de> writes:

> (defun dround (num n)
>   "round num to n digits"
>   (let ((format-string (concatenate 'string "~," (format nil "~a" n)
> "f")))
>     (read-from-string (format nil format-string num))))
> 
> not very elegant, just a fast hack :))
> 
> 
> Andreas

or 

(defun dround (num x)
  (let ((by (expt 10 x)))
    (float (/ (floor num (/ 1 by)) by)))) 

* (dround (+ (* 2 1.0001) 4.1) 2)
=> 6.1



Sean

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Updated at: 2020-12-10 08:51 UTC