[R] sliding window over a large vector

[markleeds at verizon.net]

Hi: Veslot:  I'm too tired to even try to figure out why but I think 
that there is something wrong with your sl function. see below for an 
empirical
proof of that statement.  OR maybe you're definition of sliding window 
is different than rollapply's definition but rollapply's answer makes
more sense to me ?

Output

> set.seed(1)
> x <- rbinom(24, 1, 0.5)
> print(x)
  [1] 0 0 1 1 0 1 1 1 1 0 0 0 1 0 1 0 1 1 0 1 1 0 1 0
>
> xx1 <- sl(x,3)
> print(xx1)
  [1] 1 1 2 2 1 2 2 2 2 1 1 1 2 1 2 1 2 2 1 2 2
>
> temp <- zoo(x)
> ans<-rollapply(temp,3,sum)
> print(ans)
  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
  1  2  2  2  2  3  3  2  1  0  1  1  2  1  2  2  2  2  2  2  2  1


On Tue, Dec 16, 2008 at  3:47 AM, Veslot Jacques wrote:

>> 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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>
> ______________________________________________
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