[R] Why R is 200 times slower than Matlab ?

[Erik Iverson]

Zhandong Liu wrote:
> I am switching from Matlab to R, but I found that R is 200 times slower than
> matlab.
> 
> Since I am newbie to R, I must be missing some important programming tips.

The most important tip I would give you is to use the vectorized nature 
of R whenever possible.  This helps avoid messy indexing and 'for' loops.

Look at the following 3 functions.  Yours, Gabor's, and my own (which I 
was about to post when I saw Gabor's nice solution, and is basically the 
same).

Also see the system timings after the definitions.

grw_permute <- function(input_fc){
   fc_vector <- input_fc
   index <- 1
   k <- length(fc_vector)
   fc_matrix <- matrix(0, 2, k^2)

   for(i in 1:k){
     for(j in 1:k){
       fc_matrix[index]  <-  fc_vector[i]
       fc_matrix[index+1]  <-  fc_vector[j]
       index <- index + 2
     }
   }
   return(fc_matrix)
}

grw.permute2 <- function(v) {
   cbind( rep(v, each=length(v)), rep(v, length(v)) )
}


grw_permute3 <- function(input_fc) {
   matrix(c(rep(input_fc, each = length(input_fc)),
            rep.int(input_fc, times = length(input_fc))),
          nrow = 2, byrow = TRUE)
}

 > system.time(p1 <- grw_permute(1:300))
    user  system elapsed
   1.548   0.064   2.341
 > system.time(p2 <- grw_permute2(1:300))
    user  system elapsed
   0.009   0.001   0.010
 > system.time(p3 <- grw_permute3(1:300))
    user  system elapsed
   0.008   0.002   0.010


Erik Iverson