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To: python-list@python.org
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
References: <43d7a46e-3f48-4c11-a66b-a47a3d6b9b9d@k16g2000yqm.googlegroups.com> <irvp0e$mc6$2@dough.gmane.org>
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:6669

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