Thread: Documentation fix regarding atan2
| inverse tangent of |
| inverse tangent of y/x |
Thanks
--
___________________________________________
Andrew J. P. Maclean
Centre for Autonomous Systems
The Rose Street Building J04
The University of Sydney 2006 NSW
AUSTRALIA
Ph: +61 2 9351 3283
Fax: +61 2 9351 7474
URL: http://www.acfr.usyd.edu.au/
___________________________________________
On Wed, Sep 05, 2007 at 10:37:18 +1000, Andrew Maclean <andrew.amaclean@gmail.com> wrote: > In Table 9.4 of the documentation atan2 is described as follows: > atan2(*x*, *y*) inverse tangent of *x*/*y* > > I am sure it should read as: > atan2(*y*, x) inverse tangent of y/x Aren't those two statements sayiong the same thing? You've just switched the names 'x' and 'y' and not changed their relationships. > > > You can easily test this: > If y = 2, x = 1, then degrees(atan(y/x)) =63.4 but if we proceed according > to the documentation; degrees(atan2(x,y))=degrees(atan2(1,2))=25.6 which is > not the same as degrees(atan(y/x)). In this example you switched things around part way thorugh. atan2(1,2) is the atan of (1/2), not atan(2/1) as used at the beginning of the example. > So it must be degrees(atan2(y,x))=degrees(atan2(2,1))=63.4.
Bruno Wolff III <bruno@wolff.to> writes:
> Andrew Maclean <andrew.amaclean@gmail.com> wrote:
>> In Table 9.4 of the documentation atan2 is described as follows:
>> atan2(*x*, *y*) inverse tangent of *x*/*y*
>>
>> I am sure it should read as:
>> atan2(*y*, x) inverse tangent of y/x
> Aren't those two statements sayiong the same thing?
They're logically equivalent but I think Andrew is right that y/x
corresponds to the usual interpretation of X and Y directions in
trigonometry.
regards, tom lane
A Cartesian coordinate system is generally assumed i.e there exists an x-y coordinate system so there is an inherent ordering property here.
Regarding atan2, this makes interesting reading: http://en.wikipedia.org/wiki/Atan2
All I am asking is the documentation for atan2 conform with the correct definition. You are actually using atan2(y,x) in postgresql.
The inverse tangent is defined as arctan(y/x). Hence atan2 should be atan2(y,x) to be consistent with this definition. This conforms with C++, C usage.
Andrew
-----Original Message-----
From: Bruno Wolff III [mailto:bruno@wolff.to]
Sent: Wednesday, 5 September 2007 12:34
To: a.maclean@cas.edu.au
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Documentation fix regarding atan2
On Wed, Sep 05, 2007 at 10:37:18 +1000,
Andrew Maclean <andrew.amaclean@gmail.com> wrote:
> In Table 9.4 of the documentation atan2 is described as follows:
> atan2(*x*, *y*) inverse tangent of *x*/*y*
>
> I am sure it should read as:
> atan2(*y*, x) inverse tangent of y/x
Aren't those two statements sayiong the same thing?
You've just switched the names 'x' and 'y' and not changed their relationships.
>
>
> You can easily test this:
> If y = 2, x = 1, then degrees(atan(y/x)) =63.4 but if we proceed according
> to the documentation; degrees(atan2(x,y))=degrees(atan2(1,2))=25.6 which is
> not the same as degrees(atan(y/x)).
In this example you switched things around part way thorugh. atan2(1,2)
is the atan of (1/2), not atan(2/1) as used at the beginning of the example.
> So it must be degrees(atan2(y,x))=degrees(atan2(2,1))=63.4.
Change made. Thanks. Your documentation changes can be viewed in five minutes using links on the developer's page, http://www.postgresql.org/developer/testing. --------------------------------------------------------------------------- Andrew Maclean wrote: > In Table 9.4 of the documentation atan2 is described as follows: > atan2(*x*, *y*) inverse tangent of *x*/*y* > > I am sure it should read as: > atan2(*y*, x) inverse tangent of y/x > > This looks to be the standard C++/c atan2(y,x) function. > > You can easily test this: > If y = 2, x = 1, then degrees(atan(y/x)) =63.4 but if we proceed according > to the documentation; degrees(atan2(x,y))=degrees(atan2(1,2))=25.6 which is > not the same as degrees(atan(y/x)). > So it must be degrees(atan2(y,x))=degrees(atan2(2,1))=63.4. > > > Thanks > Andrew > > -- > ___________________________________________ > Andrew J. P. Maclean > Centre for Autonomous Systems > The Rose Street Building J04 > The University of Sydney 2006 NSW > AUSTRALIA > Ph: +61 2 9351 3283 > Fax: +61 2 9351 7474 > URL: http://www.acfr.usyd.edu.au/ > ___________________________________________ -- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. +
Change made. Thanks. Your documentation changes can be viewed in five
minutes using links on the developer's page,
http://www.postgresql.org/developer/testing.
---------------------------------------------------------------------------
Andrew Maclean wrote:
> In Table 9.4 of the documentation atan2 is described as follows:
> atan2(*x*, *y*) inverse tangent of *x*/*y*
>
> I am sure it should read as:
> atan2(*y*, x) inverse tangent of y/x
>
> This looks to be the standard C++/c atan2(y,x) function.
>
> You can easily test this:
> If y = 2, x = 1, then degrees(atan(y/x)) =63.4 but if we proceed according
> to the documentation; degrees(atan2(x,y))=degrees(atan2(1,2))= 25.6 which is
> not the same as degrees(atan(y/x)).
> So it must be degrees(atan2(y,x))=degrees(atan2(2,1))=63.4.
>
>
> Thanks
> Andrew
>
> --
> ___________________________________________
> Andrew J. P. Maclean
> Centre for Autonomous Systems
> The Rose Street Building J04
> The University of Sydney 2006 NSW
> AUSTRALIA
> Ph: +61 2 9351 3283
> Fax: +61 2 9351 7474
> URL: http://www.acfr.usyd.edu.au/
> ___________________________________________
--
Bruce Momjian <bruce@momjian.us > http://momjian.us
EnterpriseDB http://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
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--
___________________________________________
Andrew J. P. Maclean
Centre for Autonomous Systems
The Rose Street Building J04
The University of Sydney 2006 NSW
AUSTRALIA
Ph: +61 2 9351 3283
Fax: +61 2 9351 7474
URL: http://www.acfr.usyd.edu.au/
___________________________________________