sliding window over a large vector

set.seed(1)
x <- rbinom(24, 1, 0.5)
print(x)

xx1 <- sl(x,3)
print(xx1)

temp <- zoo(x)
ans<-rollapply(temp,3,sum)
print(ans)

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     syst?me      ?coul?       36.86        0.45 
37.32

system.time(xx2 <- sl(x,12))

utilisateur     syst?me      ?coul?        0.01        0.00 
0.02

all.equal(xx1,xx2)

[1] TRUE

Jacques VESLOT

CEMAGREF - UR Hydrobiologie

Route de C?zanne - CS 40061      13182 AIX-EN-PROVENCE Cedex 5, France

T?l.   + 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
Envoy??: mardi 16 d?cembre 2008 05:20
??: 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