Kevin - 1 month ago 15
R Question

# R - Compute Cross Product of Vectors (Physics)

What am I doing wrong?

``````> crossprod(1:3,4:6)
[,1]
[1,]   32
``````

According to this website:http://onlinemschool.com/math/assistance/vector/multiply1/

It should give:

``````{-3; 6; -3}
``````

Here is a generalized cross product:

``````xprod <- function(...) {
args <- list(...)

# Check for valid arguments

if (length(args) == 0) {
stop("No data supplied")
}
len <- unique(sapply(args, FUN=length))
if (length(len) > 1) {
stop("All vectors must be the same length")
}
if (len != length(args) + 1) {
stop("Must supply N-1 vectors of length N")
}

# Compute generalized cross product by taking the determinant of sub-matricies

m <- do.call(rbind, args)
sapply(seq(len),
FUN=function(i) {
det(m[,-i,drop=FALSE]) * (-1)^(i+1)
})
}
``````

``````> xprod(1:3, 4:6)
[1] -3  6 -3
``````

This works for any dimension:

``````> xprod(c(0,1)) # 2d
[1] 1 0
> xprod(c(1,0,0), c(0,1,0)) # 3d
[1] 0 0 1
> xprod(c(1,0,0,0), c(0,1,0,0), c(0,0,1,0)) # 4d
[1]  0  0  0 -1
``````
Source (Stackoverflow)