R help - May 2004 - vector normal to a plane

Hi All,

(I have a degree in math, but I am too embarassed to ask my colleagues, 
so here goes:)

I would like to get a vector normal (orthogonal) to a plane formed by 
two other vectors.  In matlab I do this:

v1 = [.4, .6, .8]; v2 = [.9, .7, .2]; nn = cross(v1,v2) (gives ~[-.48, 
.65, -.24]

if I do R> cross(v1, v2), I get .94.  Huh?

Thanks for all your help, again.

W

Hi!
In which library is this function?
I cant find it using help.search() on my R installation.
Eryk

*********** REPLY SEPARATOR  ***********

On 5/28/2004 at 12:22 PM wwsprague at ucdavis.edu wrote:
>Hi All,
>
>(I have a degree in math, but I am too embarassed to ask my colleagues, 
>so here goes:)
>
>I would like to get a vector normal (orthogonal) to a plane formed by 
>two other vectors.  In matlab I do this:
>
>v1 = [.4, .6, .8]; v2 = [.9, .7, .2]; nn = cross(v1,v2) (gives ~[-.48, 
>.65, -.24]
>
>if I do R> cross(v1, v2), I get .94.  Huh?
>
>Thanks for all your help, again.
>
>W
>
>______________________________________________
>R-help at stat.math.ethz.ch mailing list
>https://www.stat.math.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html


Dipl. bio-chem. Eryk Witold Wolski    @    MPI-Moleculare Genetic   
Ihnestrasse 63-73 14195 Berlin       'v'    
tel: 0049-30-83875219               /   \    
mail: wolski at molgen.mpg.de        ---W-W----   
http://www.molgen.mpg.de/~wolski

> 
> if I do R> cross(v1, v2), I get .94.  Huh?
Meant to say "R> crossprod(v1, v2)"

Sorry

wwsprague at ucdavis.edu wrote:
> Hi All,
> 
> (I have a degree in math, but I am too embarassed to ask my colleagues, 
> so here goes:)
> 
> I would like to get a vector normal (orthogonal) to a plane formed by 
> two other vectors.  In matlab I do this:
> 
> v1 = [.4, .6, .8]; v2 = [.9, .7, .2]; nn = cross(v1,v2) (gives ~[-.48, 
> .65, -.24]
Huh?

I don't have access to Matlab. Can you tell me how cross() is defined in 
Matlab (it's not obvious to me - at least not at 10:40 pm - how can 
anything get negative)?

> 
> if I do R> cross(v1, v2), I get .94.  Huh?
Ha! ;-)

crossprod(v1, v2):
0.4*0.9 + 0.6*0.7 + 0.8*0.2 = 0.94

Uwe Ligges


> Thanks for all your help, again.
> 
> W
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html

W is looking for the vector cross product (a specifically
three-dimensional object important in physics and engineering). The
crossproduct() function provides a matrix product so that crossprod(x,y)
is t(x)%*%y, something completely different.

For the three-dimensional case you could define the cross and dot products

 "%x%"<-function(a,b) {c(a[2]*b[3]-a[3]*b[2], -a[1]*b[3]+a[3]*b[1],
a[1]*b[2]-a[2]*b[1])}

 "%.%%<-function(a,b) sum(a*b)


It would make sense, of course, to check that the arguments actually were
vectors of length 3.



	-thomas

Thomas Lumley <tlumley <at> u.washington.edu> writes:

: 
: W is looking for the vector cross product (a specifically
: three-dimensional object important in physics and engineering). The
: crossproduct() function provides a matrix product so that crossprod(x,y)
: is t(x)%*%y, something completely different.
: 
: For the three-dimensional case you could define the cross and dot products
: 
:  "%x%"<-function(a,b) {c(a[2]*b[3]-a[3]*b[2],
-a[1]*b[3]+a[3]*b[1],
: a[1]*b[2]-a[2]*b[1])}


Alternately, you could get the cofactors from solve:

cross3 <- function(a,b) {
	m <- cbind(a,b,1)
	solve(m)[3,]*det(m)
}