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.
