[R] sliding window over a large vector

[Veslot Jacques]

> sl <- function(x,z) c(0,cumsum(diff(x)[1:(length(x)-z-1)])) + rep(sum(x[1:z]),length(x)-z)
> x <- rbinom(100000, 1, 0.5)
> system.time(xx1 <- slide(x,12))
utilisateur     système      écoulé 
      36.86        0.45       37.32 
> system.time(xx2 <- sl(x,12))
utilisateur     système      écoulé 
       0.01        0.00        0.02 
> all.equal(xx1,xx2)
[1] TRUE

Jacques VESLOT

CEMAGREF - UR Hydrobiologie

Route de Cézanne - CS 40061      
13182 AIX-EN-PROVENCE Cedex 5, France

Tél.   + 0033   04 42 66 99 76
fax    + 0033   04 42 66 99 34
email   jacques.veslot at cemagref.fr  


>-----Message d'origine-----
>De : r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] De la part
>de Chris Oldmeadow
>Envoyé : mardi 16 décembre 2008 05:20
>À : r-help at r-project.org
>Objet : [R] sliding window over a large vector
>
>Hi all,
>
>I have a very large binary vector, I wish to calculate the number of
>1's  over sliding windows.
>
>this is my very slow function
>
>slide<-function(seq,window){
>   n<-length(seq)-window
>   tot<-c()
>   tot[1]<-sum(seq[1:window])
>   for (i in 2:n) {
>      tot[i]<- tot[i-1]-seq[i-1]+seq[i]
>   }
>   return(tot)
>}
>
>this works well for for reasonably sized vectors. Does anybody know a
>way for large vectors ( length=12 million), im trying to avoid using C.
>
>Thanks,
>Chris
>
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