I just wanted to note that Arun's first approach, which uses matrix indexing -- often a very useful way to do these things, btw -- can be simplified a bit.
m <- do.call(rbind,xyz_indices) ## 4x3 matrix ## avoids repeated evaluation. lapply is not needed as it's a list already sapply(seq(dim(edm)[4]), function(i)mean(edm[cbind(m,i)])) does it. Bert Gunter Genentech Nonclinical Biostatistics (650) 467-7374 "Data is not information. Information is not knowledge. And knowledge is certainly not wisdom." H. Gilbert Welch On Fri, Dec 27, 2013 at 6:42 AM, arun <[email protected]> wrote: > 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" <[email protected]> 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 > > [[alternative HTML version deleted]] > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
