[R] Strange behavior of atan2

From: Clément Calenge <calenge_at_biomserv.univ-lyon1.fr>
Date: Thu 14 Apr 2005 - 21:31:34 EST

Dear all,

I've got a problem with the function atan2. For a couple of coordinates x and y,
This function returns the angle between the vector of coordinates (x, y) and the
abscissa axis, i.e. it is the same as atan(y/x) (as indicated on the help page).
If we consider the vector with coordinates x = 0 and y = 0, we have the following result:

> atan(0/0)

[1] NaN

This is expected. However:

> atan2(0,0)

[1] 0

Instead of a missing value, the function atan2 returns an angle equal to 0 radians.
I've searched through the help pages, the FAQ and the forum, but I did'nt find
any explanation to this result. Does anyone know if this behavior is expected, and
why ?
Thank you for any clues.

Clément Calenge

LBBE - UMR CNRS 5558 - Université 
Claude Bernard Lyon 1 - FRANCE
tel. (+33)
fax. (+33)

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Received on Thu Apr 14 21:37:26 2005

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