Bert Gunter
2024-Dec-04 13:38 UTC
[R] Undocumented behaviour of diag when replacing the diagonal of a matrix?
matrices are vectors with a "dim" attribute. So what I think is happening is:> A <- matrix(1:25, nrow = 5, ncol = 5) > diag(A[-1,]) <- 0 > A[,1] [,2] [,3] [,4] [,5] [1,] 1 6 11 16 21 [2,] 0 7 12 17 22 [3,] 3 0 13 18 23 [4,] 4 9 0 19 24 [5,] 5 10 15 0 25> > ## is equivqalent to: > > A <- matrix(1:25, nrow = 5, ncol = 5) > wh <- c(diag(A[-1,])) # A's vector indices of diag(A[-1,]) > A[wh] <- 0 > A[,1] [,2] [,3] [,4] [,5] [1,] 1 6 11 16 21 [2,] 0 7 12 17 22 [3,] 3 0 13 18 23 [4,] 4 9 0 19 24 [5,] 5 10 15 0 25 I didn't check, but I assume your other examples would work similarly. Please repost if I am wrong. Cheers, Bert On Wed, Dec 4, 2024 at 4:39?AM Gerrit Eichner <gerrit.eichner at math.uni-giessen.de> wrote:> > Dear list, > > is anyone aware of the following behavious of diag when used to replace > diagonals (plural!) of a matrix? > > Small example: The following is documented and clearly to be expected: > > A <- matrix(0, nrow = 5, ncol = 5) > diag(A) <- 1; A > > > BUT, what about the following? When executing the code of `diag<-` line > by line, it throws errors. So why does it work? > > diag(A[-1, ]) <- 2; A > > diag(A[-5, -1]) <- 3; A > > diag(A[-5, -(1:2)]) <- 4; A > > > Any ideas? > > TIA and best regards -- Gerrit > > --------------------------------------------------------------------- > Dr. Gerrit Eichner Mathematical Institute, Room 215 > gerrit.eichner at math.uni-giessen.de Justus-Liebig-University Giessen > Tel: +49-(0)641-99-32104 Arndtstr. 2, 35392 Giessen, Germany > https://www.uni-giessen.de/math/eichner > > ______________________________________________ > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide https://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.
Gerrit Eichner
2024-Dec-05 11:04 UTC
[R] Undocumented behaviour of diag when replacing the diagonal of a matrix?
Hi, Bert, thx, but I don't think so, because your example fails, if you use different entries for matrix A than your vector 1:25. (Try A filled with only zeroes, e.g.) I guess, Duncan Murdochs hint is crucial. I'll comment on his asap. Best regards -- Gerrit --------------------------------------------------------------------- Dr. Gerrit Eichner Mathematical Institute, Room 215 gerrit.eichner at math.uni-giessen.de Justus-Liebig-University Giessen Tel: +49-(0)641-99-32104 Arndtstr. 2, 35392 Giessen, Germany https://www.uni-giessen.de/math/eichner --------------------------------------------------------------------- Am 04.12.2024 um 14:38 schrieb Bert Gunter:> matrices are vectors with a "dim" attribute. > So what I think is happening is: > >> A <- matrix(1:25, nrow = 5, ncol = 5) >> diag(A[-1,]) <- 0 >> A > [,1] [,2] [,3] [,4] [,5] > [1,] 1 6 11 16 21 > [2,] 0 7 12 17 22 > [3,] 3 0 13 18 23 > [4,] 4 9 0 19 24 > [5,] 5 10 15 0 25 >> >> ## is equivqalent to: >> >> A <- matrix(1:25, nrow = 5, ncol = 5) >> wh <- c(diag(A[-1,])) # A's vector indices of diag(A[-1,]) >> A[wh] <- 0 >> A > [,1] [,2] [,3] [,4] [,5] > [1,] 1 6 11 16 21 > [2,] 0 7 12 17 22 > [3,] 3 0 13 18 23 > [4,] 4 9 0 19 24 > [5,] 5 10 15 0 25 > > I didn't check, but I assume your other examples would work similarly. > Please repost if I am wrong. > > Cheers, > Bert > > On Wed, Dec 4, 2024 at 4:39?AM Gerrit Eichner > <gerrit.eichner at math.uni-giessen.de> wrote: >> >> Dear list, >> >> is anyone aware of the following behavious of diag when used to replace >> diagonals (plural!) of a matrix? >> >> Small example: The following is documented and clearly to be expected: >> >> A <- matrix(0, nrow = 5, ncol = 5) >> diag(A) <- 1; A >> >> >> BUT, what about the following? When executing the code of `diag<-` line >> by line, it throws errors. So why does it work? >> >> diag(A[-1, ]) <- 2; A >> >> diag(A[-5, -1]) <- 3; A >> >> diag(A[-5, -(1:2)]) <- 4; A >> >> >> Any ideas? >> >> TIA and best regards -- Gerrit >> >> --------------------------------------------------------------------- >> Dr. Gerrit Eichner Mathematical Institute, Room 215 >> gerrit.eichner at math.uni-giessen.de Justus-Liebig-University Giessen >> Tel: +49-(0)641-99-32104 Arndtstr. 2, 35392 Giessen, Germany >> https://www.uni-giessen.de/math/eichner >> >> ______________________________________________ >> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide https://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code.