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

[Gerrit Eichner]

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 using math.uni-giessen.de   Justus-Liebig-University Giessen
Tel: +49-(0)641-99-32104          Arndtstr. 2, 35392 Giessen, Germany
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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 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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