Re: Best way to compute length of arbitrary dimension vector?

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From	"Gabriel Genellina" <gagsl-py2@yahoo.com.ar>
Subject	Re: Best way to compute length of arbitrary dimension vector?
Date	Mon, 30 May 2011 16:01:08 -0300
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En Mon, 30 May 2011 06:46:01 -0300, Peter Otten <__peter__@web.de>
escribió:

> Gabriel wrote:
>
>> Well, the subject says it almost all: I'd like to write a small Vector
>> class for arbitrary-dimensional vectors.
>>
>
>>>> class Vector(object):
> ...     def __init__(self, *coords):
> ...             self._coords = coords
> ...     def __abs__(self):
> ...             return math.sqrt(sum(x*x for x in self._coords))
> ...
>>>> import math
>>>> abs(Vector(1,1))
> 1.4142135623730951
>>>> abs(Vector(3,4))
> 5.0

Using math.fsum instead of sum may improve accuracy, specially when  
len(coords)≫2

py> import math
py>
py> def f1(*args):
...   return math.sqrt(sum(x*x for x in args))
...
py> def f2(*args):
...   return math.sqrt(math.fsum(x*x for x in args))
...
py> pi=math.pi
py> args=[pi]*16
py> abs(f1(*args)/4 - pi)
4.4408920985006262e-16
py> abs(f2(*args)/4 - pi)
0.0

-- 
Gabriel Genellina

Thread

Best way to compute length of arbitrary dimension vector? Gabriel <snoopy.67.z@googlemail.com> - 2011-05-30 02:11 -0700
  Re: Best way to compute length of arbitrary dimension vector? Chris Rebert <clp2@rebertia.com> - 2011-05-30 02:24 -0700
  Re: Best way to compute length of arbitrary dimension vector? Peter Otten <__peter__@web.de> - 2011-05-30 11:46 +0200
    Re: Best way to compute length of arbitrary dimension vector? Gabriel <snoopy.67.z@googlemail.com> - 2011-05-30 06:38 -0700
      Re: Best way to compute length of arbitrary dimension vector? Ian Kelly <ian.g.kelly@gmail.com> - 2011-06-02 17:19 -0600
        Re: Best way to compute length of arbitrary dimension vector? Gabriel <snoopy.67.z@googlemail.com> - 2011-06-03 14:53 -0700
          Re: Best way to compute length of arbitrary dimension vector? Ian Kelly <ian.g.kelly@gmail.com> - 2011-06-03 16:17 -0600
          Re: Best way to compute length of arbitrary dimension vector? Robert Kern <robert.kern@gmail.com> - 2011-06-03 18:12 -0500
  Re: Best way to compute length of arbitrary dimension vector? "Gabriel Genellina" <gagsl-py2@yahoo.com.ar> - 2011-05-30 16:01 -0300
    Re: Best way to compute length of arbitrary dimension vector? Gabriel <snoopy.67.z@googlemail.com> - 2011-06-01 12:35 -0700

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