Re: [R] vector normal to a plane

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From: Gabor Grothendieck (ggrothendieck@myway.com)
Date: Sat 29 May 2004 - 09:31:32 EST


Message-id: <loom.20040529T013001-346@post.gmane.org>

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)
}

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