Re: bigfloats?

Wednesday, 4 February 2009

 Ar an ceathrú lá de mí Feabhra, scríobh Giacomo Boffi: 

...
 Aidan Kehoe writes:
  > 
  >  Ar an ceathrú lá de mí Feabhra, scríobh Giacomo Boffi: 
  > 
  >  > my question: how can i use bigfloats in my computations? i have to say
  >  > that the xemacs user manual was slightly unhelpful...
  > 
  > It seems buggy to me,

 smells of bit-rot... does "make check" test bignums? 
Yes, but it doesn’t test the bigfloat functions. 

...
  > and it certainly doesn’t have the nice
  > convert-to-bignum-on-overflow behaviour that integer computations have. 
  > For your particular example there, you can do:
  > 
  >   (insert (format "%g" (expt (coerce e 'bigfloat) 710)))
  > 
  > That gives me 8221840746155466228226, instead of the 308-digit number I
  > expected.

 for me, it is 2223399476616162667098  
Whoops, yes, me too; (expt (coerce e 'bigfloat) 709) gives the number
starting with 8 that should print as 308 digits long. 

If I replace the implementation of bigfloat_to_string in number-gmp.c with
that of bigf_to_string from SXEmacs, then I get the 308-digit number; it
doesn’t have a trailing .0, though, for some reason. It is then read as a
bignum. SXEmacs has the same behaviour. I don’t know what Common Lisp
specifies.

...
  > If I set default-float-precision to some higher value, I get
  > constantly-increasing numbers printed. 

 this also is nice:

 (coerce 710 'bigfloat) 
 71 
 (coerce 711 'bigfloat) 
 711.  
Ouch. 

...
 also
 (expt (coerce e 'bigfloat) (coerce 711 'bigfloat)) 
 gives me an arithmetic range error... and finally 
SXEmacs tells me “Operation undefined over domain”. 

...
 (let ((a (coerce 1E100 'bigfloat))) 
   (print (* a a)) 
   (print (+ a a)) 
   (print (* 10 a)) 
   (print (+ a a a a a a a a a a)) ; (* 10 a) 
   nil 
   ) 
 1000000000000000003181 
 2000000000000000003181 
 100000000000000000159 
 100000000000000000159 
 nil  
They give better behaviour with the replaced implementation of
bigfloat_to_string. 

...
 ok, for now i'll forget my experiments with bigfloats

 thank you Aidan 
You’re welcome; sorry I don’t have better news.

-- 
¿Dónde estará ahora mi sobrino Yoghurtu Nghe, que tuvo que huir
precipitadamente de la aldea por culpa de la escasez de rinocerontes?

_______________________________________________
XEmacs-Beta mailing list
XEmacs-Beta(a)xemacs.org
http://calypso.tux.org/cgi-bin/mailman/listinfo/xemacs-beta

2024

2023

2022

2021

2020

2019

2018

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998