The real weird thing is I wrote "weird" and it comes out "wired"...
The background is that I did a lot of sparse matrices calculations. Therefore need some indices tweaks, e.g subtract a block from a big matrix. Then I just create an indicies matrix and use it directly as a vector since a matrix is just a vector with dimensions attributed in R. I know I can convert them to a vector but I did not do it anyway. Thanks for this information! Feng On Sat, Feb 26, 2011 at 6:29 PM, Sarah Goslee <sarah.gos...@gmail.com>wrote: > Hello, > > On Sat, Feb 26, 2011 at 12:11 PM, Feng Li <m...@feng.li> wrote: > > Dear R, > > > > I found a very wired behavior for a 2-by-2 matrix, see this example > > > >> A <- matrix(1:4, 2) > >> idx4A <- matrix(1:4, 2) > >> A[idx4A] > > Error in A[idx4A] : subscript out of bounds > > This shouldn't work. From the help for "[", > > When indexing arrays by â[â a single argument âiâ can be a > matrix with as many columns as there are dimensions of âxâ; > the result is then a vector with elements corresponding to > the sets of indices in each row of âiâ. > > Since there are two dimensions to A, and two rows to idx4A, then this > form is used. > The first row of idx4A is c(1, 3) so the first value extracted is > A[1, 3] > which doesn't exist, thus the error message. > > > But other matrices are fine, > > > >> B <- matrix(1:9, 3) > >> idx4B <- matrix(1:9, 3) > >> B[idx4B] > > [1] 1 2 3 4 5 6 7 8 9 > > The part that surprises me is that this works. > Apparently there is an implicit conversion to vector here because > dim(B) != ncol(idx4B) > > What you seem to want is > A[as.vector(idx4A)] > and > B[as.vector(idx4B)] > > but you neglected to tell us what you expected the result to be. > > Sarah > -- > Sarah Goslee > http://www.functionaldiversity.org > -- Feng Li Department of Statistics Stockholm University 106 91 Stockholm, Sweden http://feng.li/ [[alternative HTML version deleted]]
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