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.

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.

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

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. +

Thanks.

On 9/15/07, Bruce Momjian <bruce@momjian.us> wrote:

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/
___________________________________________