General indexing in multidimensional arrays
On 11-08-01 5:38 AM, Jannis wrote:
Dear R community, I have a general question regarding indexing in multidiemensional arrays. Imagine I have a three dimensional array and I only want to extract on vector along a single dimension from it: data<- array(rnorm(64),dim=c(4,4,4)) result<- data[1,1,] If I want to extract more than one of these vectors, it would now really help me to supply a logical matrix of the size of the first two dimensions: indices<- matrix(FALSE,ncol=4,nrow=4) indices[1,3]<- TRUE indices[4,1]<- TRUE result<- data[indices,] This, however would give me an error. I am used to this kind of indexing from Matlab and was wonderingt whether there exists an easy way to do this in R without supplying complicated index matrices of all three dimensions or logical vectors of the size of the whole matrix? The only way I could imagine would be to: result<- data[rep(as.vector(indices),times=4)] but this seems rather complicated and also depends on the order of the dimensions I want to extract. I do not want R to copy Matlabs behaviour, I am just wondering whether I missed one concept of indexing in R?
Base R doesn't have anything like that as far as I know. The closest is matrix indexing: you construct a 3 column matrix whose rows are the indices of each element you want to extract. Possibly plyr or some other package has functions to do this. Duncan Murdoch