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

[Rui Barradas]

Às 12:38 de 04/12/2024, Gerrit Eichner escreveu:
> 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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Hello,

I cannot reproduce any error, everything works as expected.
sessionInfo at the end.



A <- matrix(0, nrow = 5, ncol = 5)
diag(A) <- 1; A
#      [,1] [,2] [,3] [,4] [,5]
# [1,]    1    0    0    0    0
# [2,]    0    1    0    0    0
# [3,]    0    0    1    0    0
# [4,]    0    0    0    1    0
# [5,]    0    0    0    0    1

diag(A[-1, ]) <- 2; A
#      [,1] [,2] [,3] [,4] [,5]
# [1,]    1    0    0    0    0
# [2,]    2    1    0    0    0
# [3,]    0    2    1    0    0
# [4,]    0    0    2    1    0
# [5,]    0    0    0    2    1

diag(A[-5, -1]) <- 3; A
#      [,1] [,2] [,3] [,4] [,5]
# [1,]    1    3    0    0    0
# [2,]    2    1    3    0    0
# [3,]    0    2    1    3    0
# [4,]    0    0    2    1    3
# [5,]    0    0    0    2    1

diag(A[-5, -(1:2)]) <- 4; A
#      [,1] [,2] [,3] [,4] [,5]
# [1,]    1    3    4    0    0
# [2,]    2    1    3    4    0
# [3,]    0    2    1    3    4
# [4,]    0    0    2    1    3
# [5,]    0    0    0    2    1

sessionInfo()
# R version 4.4.2 (2024-10-31 ucrt)
# Platform: x86_64-w64-mingw32/x64
# Running under: Windows 11 x64 (build 22631)
#
# Matrix products: default
#
#
# locale:
# [1] LC_COLLATE=Portuguese_Portugal.utf8 
LC_CTYPE=Portuguese_Portugal.utf8
# [3] LC_MONETARY=Portuguese_Portugal.utf8 LC_NUMERIC=C 

# [5] LC_TIME=Portuguese_Portugal.utf8
#
# time zone: Europe/Lisbon
# tzcode source: internal
#
# attached base packages:
# [1] stats     graphics  grDevices utils     datasets  methods   base
#
# loaded via a namespace (and not attached):
# [1] compiler_4.4.2
#



Hope this helps,

Rui Barradas


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