Folks: This is sufficiently "tricky" that a word of explanation might be helpful to reveal the magic.
> > Rates <- c(0.006280048, 0.330934659) > Index.Matrix <- matrix(c(1, 2, 1, 2), 2, 2) > >> Rates > [1] 0.006280048 0.330934659 > >> Index.Matrix > [,1] [,2] > [1,] 1 1 > [2,] 2 2 > > > Just index 'Rates' by the values in Index.Matrix: > >> Rates[Index.Matrix] > [1] 0.006280048 0.330934659 0.006280048 0.330934659 > > NewMatrix <- matrix(Rates[Index.Matrix], dim(Index.Matrix)) > >> NewMatrix > [,1] [,2] > [1,] 0.006280048 0.006280048 > [2,] 0.330934659 0.330934659 > The key here is to understand that matrices (arrays of any dimension actually) are just vectors stored in column major order (column 1 on top of column 2 ....) . So Rates[Index.Matrix] is just a length 4 vector concatenating two copies of Rates when you think of Index.matrix as a vector, which it must be here to index a vector (There's also matrix indexing of arrays, but that's something else again). The matrix() function then just makes it a matrix with the desired dimension. So no longer tricky, right? Cheers, Bert Bert Gunter Genentech ______________________________________________ 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.
