# [R] placing labels in polygon center ?

Richard A. O'Keefe ok at cs.otago.ac.nz
Thu Aug 14 07:27:54 CEST 2003

I wrote:
I found myself wishing for a function to rotate a vector.
Is there one?  I know about ?lag, but help.search("rotate")
didn't find anything to the point.

Here I was regarding a vector as a _sequence_.
The (one-step) rotation of c(u,v,w,x,y,z) is c(v,w,x,y,z,u).
This is pretty much the way APL uses the word "rotate" (the
vertical-bar-overstruck-with-a-circle operator).

Spencer Graves <spencer.graves at PDF.COM> replied:
I didn't study your code, but regarding a function to rotate a
vector:  Multiplication by an orthogonal matrix does that.

This is a misunderstanding.  We were both using the word "rotate" in
a standard way, the problem is that it has more than one "standard" meaning.

As a matter of fact,
/ 0 1 0 0 \  / u \     / v \
| 0 0 1 0 |  | v |     | w |
| 0 0 0 1 |  | w |  =  | x |
\ 1 0 0 0 /  \ x /     \ u /
so you *can* do the kind of rotation I want using a matrix multiplication,
and this is mathematically useful; it's just not a very good way to do
it in a computer.