Is the following what you want? It works for vectors or arrays of any number of dimensions. It assumes that the dimensions of the grids are the same.
> grid1 <- c(1,2,NA,3,NA) > grid2 <- c(101,102,103,104,NA) > shouldCopy <- is.na(grid1) & !is.na(grid2) > grid1[shouldCopy] <- grid2[shouldCopy] > grid1 [1] 1 2 103 3 NA You don't really need the ' & !is.na(grid2)': it just stops the copying of NA elements of grid2 to NA elements of grid1. Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On > Behalf > Of Tom Roche > Sent: Wednesday, May 08, 2013 5:38 PM > To: r-help@r-project.org > Subject: [R] [newbie] *apply to matching elements of n arrays? > > > How to apply a function to all elements with the same indices in > multiple arrays? E.g.: > > I have two spatial grids defined the same way (i.e., same number of > rows and columns--and dimensions, both 2D). Wherever both > > * the value of an element i,j in the first grid is NA > * the value of element i,j in the second grid is !NA > > I want to copy the value from grid2[i,j] to grid1[i,j]. These matrices > are not too big, so I'm able to do this with loops, but I know that's > not "the R way." How to parallelize/vectorize this, e.g., with a > single call to an 'apply'-type method? I believe I know how to operate > on a single matrix (e.g., by using `apply` to traverse it by rows or > cols), but not how to operate on multiple matrices. > > Extra points for solutions that generalize to 3D or 4D (i.e., that allow > applying the same function to identical elements in n arrays of n > dimensions), since I will almost certainly need to work on arrays of > those dimensions eventually. > > Apologies if this is a FAQ, but a fair amount of googling via > rseek.org is not finding an answer (perhaps because I'm not using the > correct search terms). Feel free (in fact, be encouraged :-) to reply > directly to me as well as the list (I'm on the digest, which gets huge). > > TIA, Tom Roche <tom_ro...@pobox.com> > > ______________________________________________ > R-help@r-project.org 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. ______________________________________________ R-help@r-project.org 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.
