Hi, Gerrit:
On 12/5/24 05:04, Gerrit Eichner wrote:
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.)
YOU'VE MISUNDERSTOOD Bert's reply: A matrix is a vector, and Bert
assigned the indices of the vector to the cells of the matrix.
> A <- matrix(1:25, nrow = 5, ncol = 5)
> A[1:25]
[1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25
> A[13]
[1] 13
"if you use different entries for matrix A", the entries are no
longer the indices of A as a vector.
Does this fact make his example work for you?
It does for me.
Hope this helps.
Spencer Graves
I guess, Duncan Murdochs hint is crucial. I'll comment on his asap.
Best regards -- Gerrit
---------------------------------------------------------------------
Dr. Gerrit Eichner Mathematical Institute, Room 215
gerrit.eich...@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.eich...@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.eich...@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
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