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
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