[R] find mean of a list of timeseries

[Spencer Graves]

	  I suggest you not worry about the loops.  A decade ago (e.g., with 
S-Plus 3 or 3.1), loops were a major problem.  Releases of S-Plus and R 
since then have made substantial improvements in loop computations.

	  My preferred solution to your problem, as I understand it, is as 
follows:

a<-ts(matrix(c(1,1,1,10,10,10,20,20,20),nrow=3),names=c('var1','var2','var3'))
b<-ts(matrix(c(2,2,2,11,11,11,21,21,21),nrow=3),names=c('var1','var2','var3'))
c<-ts(matrix(c(3,3,3,12,12,12,22,22,22),nrow=3),names=c('var1','var2','var3'))

data<-list(a,b,c)

mean.list <- function(object){
   n <- length(object)
   a1 <- object[[1]]
   if(n>1)for(i in 2:n){
     a1 <- a1+object[[i]]
   }
   a1/n
}

gc()
start.time <- proc.time()
mean.list(data)
(et <- proc.time()-start.time)

 > mean.list(data)
Time Series:
Start = 1
End = 3
Frequency = 1
   a1.a1.var1 a1.a1.var2 a1.a1.var3
1          2         11         21
2          2         11         21
3          2         11         21
 > (et <- proc.time()-start.time)
[1] 0.04 0.00 0.09   NA   NA

	  My attempt to avoid loops is the following:  	

apply.list <- function(object, FUN){
   dim.list <- sapply(object, dim)
   if(is.list(dim.list))
     stop("attributes of ", deparse(substitute(object)),
          " do not have the same number of dimensions.")
   allEqual <- apply(dim.list, 1, function(x)diff(range(x)))
   if(any(allEqual !=0))
     stop("attributes of ", deparse(substitute(object)),
          " don't all have the same dimensions")
   n <- length(object)
   Dim <- c(n, dim.list[,1])
#
   obj.array <- array(unlist(object), dim=Dim)
   k <- length(dim)
   apply(obj.array, 2:k, FUN)
}

 > start.time <- proc.time()
 > apply.list(data, mean)
      [,1] [,2] [,3]
[1,]    2    2    2
[2,]   11   11   11
[3,]   21   21   21
 > (et <- proc.time()-start.time)
[1] 0.00 0.00 0.07   NA   NA
	
	 This seems to run slightly faster on this miniscule data.  However the 
answer is transposed, and I don't want to take the time to understand 
and fix that problem.

	  hope this helps.
	  spencer graves

tom wright wrote:

> Can someone please give me a clue how to 're'write this so I dont need
> to use loops.
> 
> a<-ts(matrix(c(1,1,1,10,10,10,20,20,20),nrow=3),names=c('var1','var2','var3'))
> b<-ts(matrix(c(2,2,2,11,11,11,21,21,21),nrow=3),names=c('var1','var2','var3'))
> c<-ts(matrix(c(3,3,3,12,12,12,22,22,22),nrow=3),names=c('var1','var2','var3'))
> 
> data<-list(a,b,c)
> 
> I now want to find the means of all vectors var1,var2 and var3
> 
> i.e. I need to end up with a new time series with three data vectors
> (var1, var2 and var3)
> result<-ts(matrix(c(2,2,2,11,11,11,21,21,21),nrow=3),names=c('var1','var2','var3))
> 
> I think its the list thats throwing my use of apply, I might be wrong
> but what other data structure could I use?
> 
> Many thanks
> Tom
> 
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