Re: [R] How to optimize this loop ? (correction)

From: Berwin A Turlach <statba_at_nus.edu.sg>
Date: Fri 19 Jan 2007 - 03:13:29 GMT

G'day Martin and others,

On Thu, 18 Jan 2007 15:22:07 +0100
Martin Becker <martin.becker@mx.uni-saarland.de> wrote:

> Sorry, I accidentaly lost one line of the function code (result <-0),
> see below...

But even with that line, the code doesn't work properly. :) Since you changed the order of the for loop (from "i in 1:(len-1)" to "i in (len-1):1") you broke the logic of the code and the result returned is wrong (should be something like "result <- len-i" instead of "result <- i-1").

I agree that your function is faster than the one based on rle(), presumably because rle() does just too many additional computations that are not necessary to solve the problem at hand. But I still don't like to see for() loops in R code. :) I would suggest the following solution (which is only marginally slower than yours on these example inputs):

myfun2 <- function(x){
  len <- length(x)
  which.max(c(x[len] <= rev(x[-len]), TRUE) ) - 1 }

The concatenation of TRUE at the end of the logical vector is necessary for the case that all elements before the last are smaller than the last one. It also has the side benefit that myfun(2) will return 0 instead of numeric(0) (or something similar). On my machine:

> system.time(for (i in 1:10000) erg<-my_fun(c(3, 4, 10,14,8,3,4,6,9)))
[1] 2.860 0.016 2.950 0.000 0.000
> system.time(for (i in 1:10000) erg<-myfun1(c(3, 4, 10,14,8,3,4,6,9)))
[1] 0.256 0.000 0.256 0.000 0.000
> system.time(for (i in 1:10000) erg<-myfun2(c(3, 4, 10,14,8,3,4,6,9)))
[1] 0.280 0.000 0.283 0.000 0.000

Cheers,

        Berwin


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