> -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > On Behalf Of Bryan Hanson > Sent: Wednesday, February 02, 2011 5:23 AM > To: Petr Savicky > Cc: r-help@r-project.org > Subject: Re: [R] Function to locate points in 3d octants or points on > twoaxes > > Thanks Petr, the sign function will be of help. I was not aware of > it. Bryan > > On Feb 2, 2011, at 3:21 AM, Petr Savicky wrote: > > > On Tue, Feb 01, 2011 at 08:30:22PM -0500, Bryan Hanson wrote: > >> [Sorry, resending with a proper subject line!] > >> > >> Hi Guru's... > >> > >> I have a set of points that may lie along any of the x, y and z axes > >> in a Cartesian coordinate system. I am hoping that a function exists > >> which will determine if any two selected points are on different > >> axes, > >> i.e, if the one of the points is on x and the other on y or z, not > >> elsewhere on the x axis. Put another way, I need to determine if the > >> triangle formed by the two points and the origin lies in the xy, xz > >> or > >> yz planes. This might be as simple as testing if any particular > >> value > >> is zero, i.e. if the x coordinate is zero, then the points must be on > >> the z and y axes and the triangle in the yz plane. But, I'm looking > >> for a fairly general solution, one that also returns the appropriate > >> plane as the answer. Very closely related to this, I could use a > >> function that determines which of the 8 octants a point lies in. > >> Seems > >> like the cross product might be part of this, but I'm a little rusty > >> on how to apply it. > >> > >> I hope this is clear enough, and someone has a suggestion to point me > >> in the right direction. Before writing my own klunky version, I > >> thought I'd ask. > >
For a general solution you also need to consider how you want to deal with "boundary" conditions. For example, if the x and y coordinates are zero for both points then the points lie in both the xz and yz planes. And if one of the coordinates of a point is zero, then how do you decide which quadrant it is in? Dan Daniel Nordlund Bothell, WA USA ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
