bugGNU Octave - Bugs: bug #49015, atan2 singular cases

 
 

bug #49015: atan2 singular cases

Submitter:  Ernst Reissner <ernstreissner>
Submitted:  Tue 06 Sep 2016 01:49:26 PM UTC
   
 
Category:  Documentation Severity:  3 - Normal
Priority:  5 - Normal Item Group:  None
Status:  Wont Fix Assigned to:  None
Originator Name:  Open/Closed:  * Closed
Release:  * dev Operating System:  * Any
Fixed Release:  None Planned Release:  None
* Mandatory Fields

Add a New Comment Rich Markup
   

Jump to the original submission

Wed 07 Sep 2016 04:33:01 PM UTC, comment #6: 

I can reopen it, doesn't change much.

The basis of Octave's atan2 function is C++ std::atan2, the documentation of which is here:

http://en.cppreference.com/w/cpp/numeric/math/atan2

The C++ function contains the following summary description:

> Computes the arc tangent of y/x using the signs of arguments to determine the correct quadrant.


That's pretty much the same wording and spirit as the Octave doc string.

Mike Miller <mtmiller>
Group Member
Wed 07 Sep 2016 08:26:19 AM UTC, comment #5: 

please reopen this bug.

Ernst Reissner <ernstreissner>
Wed 07 Sep 2016 08:25:56 AM UTC, comment #4: 

I do not agree:
one has to state that it is atan(y/x) and then the exceptions.
As it is, it is contradictuous.
atan(y/x) is not equvalent with arg (complex (X, Y)).

As an alternative first sentence one could write that it is
arg (complex (X, Y)).


I am not sure whether this is what is implemented.
I only know that this is what shall be implemented according to the standard.

Ernst Reissner <ernstreissner>
Tue 06 Sep 2016 10:35:58 PM UTC, comment #3: 

The relationship between atan2 and arg is already in the full doc string, so that is done.

The first sentence cannot possibly be a complete description of the function behavior, it is a simple general definition, which the full doc string clarifies.

Mike Miller <mtmiller>
Group Member
Tue 06 Sep 2016 10:31:49 PM UTC, comment #2: 

The first sentence is
Compute atan (Y / X) for corresponding elements of Y and X.

This is definitively wrong
if "...the first argument is @code{+0} and the second argument is negative,".

Then we obtain atan(0)=0.

With the rule i give, the result is pi.

You are right, it suffices to write `arg (complex (X, Y))'.

Correct is to write either a definition with atan(y/x)
and with the exceptions I gave
or if you give a definition in terms of arg,

but not both.

Ernst Reissner <ernstreissner>
Tue 06 Sep 2016 06:48:22 PM UTC, comment #1: 

IMHO these cases are all covered by the sentence


This function is equivalent to 'arg (complex (X, Y))'.


Mike Miller <mtmiller>
Group Member
Tue 06 Sep 2016 01:49:26 PM UTC, original submission:  

i doubt that atan2(y,x) is nothing but atan(y/x).
I think the truth is

%% This is essentially @code{atan(y/z)} with the following exceptions:
%% @itemize
%% @item
%% If the first argument is @code{+0} and the second argument is negative,
%% or the first argument is positive and finite
%% and the second argument is negative infinity,
%% then the result is the highest value less or equal to @code{pi}.
%% @item
%% If the first argument is @code{-0} and the second argument is negative,
%% or the first argument is negative and finite
%% and the second argument is negative infinity,
%% then the result is the lowest value less or equal to @code{-pi}.
%% @item
%% If the first argument is positive and the second argument is @code{0},
%% or the first argument is positive infinity
%% and the second argument is finite,
%% then the result is the double value closest to @code{pi/2}.
%% @item
%% If the first argument is negative and the second argument is @code{0},
%% or the first argument is negative infinity
%% and the second argument is finite,
%% then the result is the double value closest to @code{-pi/2}.
%% @item
%% If both arguments are positive infinity,
%% then the result is the value closest to @code{pi/4}.
%% @item
%% If the first argument is positive infinity
%% and the second argument is negative infinity,
%% then the result is the double value closest to @code{3*pi/4}.
%% @item
%% If the first argument is negative infinity and
%% the second argument is positive infinity,
%% then the result is the double value closest to @code{-pi/4}.
%% @item
%% If both arguments are negative infinity,
%% then the result is the double value closest to @code{-3*pi/4}.
%% @end itemize
%%

Ernst Reissner <ernstreissner>

 

(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

Attach Files:
   
   
Comment:
   

No files currently attached

 

Depends on the following items: None found

Items that depend on this one: None found

 

Carbon-Copy List
  • -email is unavailable- added by mtmiller (Posted a comment)
  • -email is unavailable- added by ernstreissner (Submitted the item)
  • -email is unavailable- added by ernstreissner
  •  

    There are 0 votes so far. Votes easily highlight which items people would like to see resolved in priority, independently of the priority of the item set by tracker managers.

    Only group members can vote.

     

    Follow 6 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2016-11-18 mtmiller CategoryNone Documentation
        Open/ClosedOpen Closed
    2016-09-07 mtmiller Open/ClosedClosed Open
    2016-09-06 mtmiller StatusNone Wont Fix
        Open/ClosedOpen Closed
    2016-09-06 ernstreissner Carbon-Copy- Added -email is unavailable-

    Back to the top

    Powered by Savane 3.13-4448.
    Corresponding source code