PS: I'm talking about subsetting matrices using a matrix as an index.
Tony Plate wrote:
Matrix indexing seems to give rather "variable" results when zeros or negative values are included among the indices (in terms of both error messages and in terms of the number of returned values when there is no error message).
Is this the intended behavior?
I couldn't see any comments about zeros or negative values in matrix indices in either ?"[" or Section 3.4.2 "Indexing matrices and arrays" of the "R Language Definition" (Version 2.0.1), or in Section 5.3 "Index arrays" of "An Introduction to R" (Version 2.1.0).
(It looks like the special treatment of zeros and negative indices is being applied to the vector indices after they are derived by standard indexing arithmetic from the matrix indices -- so zeros and negative values in the matrix indices have no special meaning. There does seem to be some range checking, but it's not consistent -- sometimes index values that are out of range get through.)
> x <- matrix(1:6,ncol=2) > dim(x) [1] 3 2 > x [,1] [,2] [1,] 1 4 [2,] 2 5 [3,] 3 6 > x[rbind(c(1,1), c(2,2))] [1] 1 5 > x[rbind(c(1,1), c(2,2), c(0,1))] [1] 1 5 > x[rbind(c(1,1), c(2,2), c(0,0))] Error: only 0's may mix with negative subscripts > x[rbind(c(1,1), c(2,2), c(0,2))] [1] 1 5 3 > x[rbind(c(1,1), c(2,2), c(0,3))] Error: subscript out of bounds > x[rbind(c(1,1), c(2,2), c(1,0))] Error: only 0's may mix with negative subscripts > x[rbind(c(1,1), c(2,2), c(2,0))] Error: only 0's may mix with negative subscripts > x[rbind(c(1,1), c(2,2), c(3,0))] [1] 1 5 > x[rbind(c(1,1), c(2,2), c(1,2))] [1] 1 5 4 > x[rbind(c(1,1), c(2,2), c(-1,2))] [1] 1 5 2 > x[rbind(c(1,1), c(2,2), c(-2,2))] [1] 1 5 1 > x[rbind(c(1,1), c(2,2), c(-3,2))] [1] 1 5 > x[rbind(c(1,1), c(2,2), c(-4,2))] Error: only 0's may mix with negative subscripts > x[rbind(c(1,1), c(2,2), c(-1,-1))] Error: subscript out of bounds > > # range checks are at least sometimes not applied > x <- matrix(1:6, ncol=3) > dim(x) [1] 2 3 > x[rbind(c(1,1), c(2,2), c(-3,3))] [1] 1 4 1 > x[rbind(c(1,1), c(2,2), c(-4,3))] [1] 1 4 >
> version _ platform i386-pc-mingw32 arch i386 os mingw32 system i386, mingw32 status major 2 minor 0.1 year 2004 month 11 day 15 language R >
(I see the same behavior in R 2.1.0 beta of 2005/04/04)
-- Tony Plate
PS: FWIW, S-PLUS 6.2 does something more like one would get when using each row of the index matrix as arguments to a call to "[", except that negative values are not allowed at all (any zero value in a row causes that row to be omitted from the result).
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