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Re: a more precise distance algorithm

References <b2e66a94-7a89-4be9-bbf7-9434396cc178@googlegroups.com>
From Ian Kelly <ian.g.kelly@gmail.com>
Date 2015-05-25 22:42 -0600
Subject Re: a more precise distance algorithm
Newsgroups comp.lang.python
Message-ID <mailman.48.1432615414.5151.python-list@python.org> (permalink)

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On Mon, May 25, 2015 at 1:21 PM, ravas <ravas@outlook.com> wrote:
> I read an interesting comment:
> """
> The coolest thing I've ever discovered about Pythagorean's Theorem is an alternate way to calculate it. If you write a program that uses the distance form c = sqrt(a^2 + b^2) you will suffer from the lose of half of your available precision because the square root operation is last. A more accurate calculation is c = a * sqrt(1 + b^2 / a^2). If a is less than b, you should swap them and of course handle the special case of a = 0.
> """
>
> Is this valid? Does it apply to python?
> Any other thoughts? :D
>
> My imagining:
>
> def distance(A, B):
>     """
>     A & B are objects with x and y attributes
>     :return: the distance between A and B
>     """
>     dx = B.x - A.x
>     dy = B.y - A.y
>     a = min(dx, dy)
>     b = max(dx, dy)
>     if a == 0:
>         return b
>     elif b == 0:
>         return a

This branch is incorrect because a could be negative.

You don't need this anyway; the a == 0 branch is only there because of
the division by a in the else branch.

>     else:
>         return a * sqrt(1 + (b / a)**2)

Same issue; if a is negative then the result will have the wrong sign.

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Thread

a more precise distance algorithm ravas <ravas@outlook.com> - 2015-05-25 12:21 -0700
  Re: a more precise distance algorithm felix <felix@epepm.cupet.cu> - 2015-05-25 16:06 -0400
  Re: a more precise distance algorithm Christian Gollwitzer <auriocus@gmx.de> - 2015-05-25 22:27 +0200
    Re: a more precise distance algorithm ravas <ravas@outlook.com> - 2015-05-25 14:03 -0700
  Re: a more precise distance algorithm Gary Herron <gary.herron@islandtraining.com> - 2015-05-25 13:20 -0700
    Re: a more precise distance algorithm ravas <ravas@outlook.com> - 2015-05-25 14:05 -0700
  Re: a more precise distance algorithm Steven D'Aprano <steve@pearwood.info> - 2015-05-26 13:11 +1000
    Re: a more precise distance algorithm ravas <ravas@outlook.com> - 2015-05-25 21:13 -0700
      Re: a more precise distance algorithm Gary Herron <gherron@digipen.edu> - 2015-05-25 22:09 -0700
        Re: a more precise distance algorithm ravas <ravas@outlook.com> - 2015-05-25 22:49 -0700
    Re: a more precise distance algorithm Christian Gollwitzer <auriocus@gmx.de> - 2015-05-26 07:33 +0200
    Re: a more precise distance algorithm Brian Blais <bblais@gmail.com> - 2015-05-27 14:00 -0400
    Re: a more precise distance algorithm Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-05-27 23:03 +0100
    Re: a more precise distance algorithm Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-05-27 23:04 -0400
  Re: a more precise distance algorithm Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-25 22:42 -0600
    Re: a more precise distance algorithm ravas <ravas@outlook.com> - 2015-05-25 21:59 -0700
  Re: a more precise distance algorithm random832@fastmail.us - 2015-05-26 09:40 -0400
  Re: a more precise distance algorithm random832@fastmail.us - 2015-05-26 09:51 -0400
  Re: a more precise distance algorithm Robin Becker <robin@reportlab.com> - 2015-05-27 14:02 +0100

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