R - 我怎样才能使这个循环更快？

Question

Is there some way to make this loop faster in r?

    V=array(NA, dim=c(nrow(pixDF), n))

    for(i in 1:n)
    {
       sdC<-sqrt(det(Cov[,i,]))
       iC<-inv(Cov[,i,])
       V[,i]<-apply(pixDF,1,function(x)(sdC*exp(-0.5*((x-Mean[i,])%*%iC%*%as.matrix((x-Mean[i,]))))))
    }

where, in this case, pixDF is a matrix with 490000 rows and 4 columns filled with doubles. n = 5. Cov is a (4,5,4) array filled with "doubles". Mean is a (5,4) array filled with doubles as well.

This loop was taking about 30min on my computer. (before editing). Right now it's taking 1min.

Answer 1

As Ronak notes, it is hard to help without reproducible example. But, I think that apply could be avoided. Something like this COULD work:

V <- array(NA, dim = c(nrow(pixDF), n))
tpixDF <- t(pixDF)
for (i in 1:n) {
  x <- Cov[, i, ]
  sdC <- sqrt(det(x))
  iC <- solve(x)
  mi <- Mean[i, ]
  k <- t(tpixDF - mi)
  V[, i] <- sdC*exp(-0.5*rowSums(k %*% iC * k))
}

Also, as Roland mentions inv probably is equal solve .