[R] vectors cross-product V1 x V2

[Jeff Newmiller]

RSiteSearch("cross product")
library(pracma)
?cross

Speed is usually desired in the context of many similar computations, and is 
normally achieved in R by vectorizing computation, so storing the large 
number of 3d vectors together in a structure like a Nx3 matrix so the code 
can be vectorized is the logical approach.  The cross() function takes 
inputs in this form, but the current implementation (0.6-3) then fails to 
take advantage of that storage since it iterates with a for loop. A better 
core implementation of cross() might be:

vcrossp <- function( a, b ) {
   result <- matrix( NA, nrow( a ), 3 )
   result[,1] <- a[,2] * b[,3] - a[,3] * b[,2]
   result[,2] <- a[,3] * b[,1] - a[,1] * b[,3]
   result[,3] <- a[,1] * b[,2] - a[,2] * b[,1]
   result
}

which is about 20 times faster than cross() on my machine.

On 07/08/2011 05:52 AM, Eik Vettorazzi wrote:
> Hi,
> how about this:
>
> mm<-cbind(V1,V2)
> xy<-sapply(1:3,function(x)det(mm[-x,])*(2*(x%%2)-1))
>
> #some checks
> all.equal(0,as.vector(xy%*%V1))
> all.equal(0,as.vector(xy%*%V2))
>
>
> Am 08.07.2011 08:27, schrieb Bai:
>> Hi, everyone,
>>
>> I need an efficient way to do vectors cross product in R.
>>
>> Set vectors,
>> V1 = ai + bj + ck
>> V2 = di + ej + fk
>>
>> then the cross product is
>>    V1 x V2    =  (bf - ce) i + (cd - af) j + (ae - bd) k
>>
>>
>> As shown here ( http://en.wikipedia.org/wiki/Cross_product ).
>>
>> Thanks.
>>
>> Best,
>> Bai
>>
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