[R] Undocumented behaviour of diag when replacing the diagonal of a matrix?

[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 using 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 using 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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