[R] Averaging specified array indices

[arun]

HI,

You could try:

res1 <- sapply(seq(dim(edm)[4]),function(i) mean(edm[do.call(rbind,lapply(xyz_indices,function(x) c(x,i)))],na.rm=TRUE)) 

#or
indx <- cbind(matrix(rep(unlist(xyz_indices),50),ncol=3,byrow=TRUE),rep(1:50,each=4))
res2 <-tapply(edm[indx],((seq(200)-1)%/%4)+1,mean)
 dimnames(res2)[[1]] <- NULL
 identical(res1,as.vector(res2))
#[1] TRUE



A.K.





On Friday, December 27, 2013 8:31 AM, "Morway, Eric" <emorway at usgs.gov> wrote:
In the larger problem I'm attempting to solve, I read a 2Gb file
into a 4D array, where the first 3 dimensions are related to space
(x, y, z), and the 4th dimension is time.  My goal is to find the
average of specific x, y, z-indices for each time step.

A small, reproducible example starts like this:

edm <- array(rnorm(20*20*4*50), dim=c(20,20,4,50))
xyz_indices <- list(c(2,10,1), c(4,5,1), c(6,7,1), c(19,13,1))

What's the best way to calculate the average of the x, y, z
indices specified in the "xyz_indices" list for each of the 50 time
steps represented by the 4th dimension of "edm"?  In other words,
I want to end up with a time series of length 50 where each value of
the series is the average of the 4 xyz_indices.


Eric

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