R help - Nov 2012 - code optimisation problem

I'd like to write a function that has a vector and a (pos.) number as inputs
and returns what is on the picture below (arithmetic means of (k)
consecutive elements of a given vector). The problem is it works too slow
for long vectors and i know it can be done without "for" loop.
However, i've
got no idea how. Can anyone help me with that? 

f1 <- function(v,k) {
  n <- length(v)
  z <- (n-k+1)
  for (i in k:n) {
    v[i-k+1] <- sum(v[(i-k+1):i])
  }
  v <- v[1:(n-k+1)]
  v <- v/k
  return (v) 
}

<http://r.789695.n4.nabble.com/file/n4651030/Untitled.png> 



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Hello,

Package 'compiler' is good at optimizing for loops. Try the following.


#install.packages('compiler')
library(compiler)
f1.c <- cmpfun(f1)

N <- 1e6
x <- rnorm(N)
system.time(f1(x, 10))
    user  system elapsed
    6.77    0.06    6.83
system.time(f1.c(x, 10))
    user  system elapsed
    2.57    0.00    2.57


Hope this helps,

Rui Barradas

Em 27-11-2012 21:43, 47 escreveu:
> I'd like to write a function that has a vector and a (pos.) number as
inputs
> and returns what is on the picture below (arithmetic means of (k)
> consecutive elements of a given vector). The problem is it works too slow
> for long vectors and i know it can be done without "for" loop.
However, i've
> got no idea how. Can anyone help me with that?
>
> f1 <- function(v,k) {
>    n <- length(v)
>    z <- (n-k+1)
>    for (i in k:n) {
>      v[i-k+1] <- sum(v[(i-k+1):i])
>    }
>    v <- v[1:(n-k+1)]
>    v <- v/k
>    return (v)
> }
>
> <http://r.789695.n4.nabble.com/file/n4651030/Untitled.png>
>
>
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/code-optimisation-problem-tp4651030.html
> Sent from the R help mailing list archive at Nabble.com.
>
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