[Rd] Inquiry about the behaviour of subsetting and names in matrices

[Karolis Koncevičius]

Hello,

I have stumbled upon a few cases where the behaviour of naming and subsetting in matrices seems unintuitive.
All those look related so wanted to put everything in one message.


1. Why row/col selection by names with NAs is not allowed?

  x <- setNames(1:10, letters[1:10])
  X <- matrix(x, nrow=2, dimnames = list(letters[1:2], LETTERS[1:5]))

  x[c(1, NA, 3)]       # vector: works and adds "NA"
  x[c("a", NA, "c")]   # vector: works and adds "NA"
  X[,c(1, NA, 3)]      # works and selects "NA" column
  X[,c("A", NA, "C")]  # <error>


2. Should setting names() for a matrix be allowed?

  names(X) <- paste0("e", 1:length(X))
  X["e4"]  # works

  # but any operation on a matrix drops the names
  X <- X[,-1]  # all names are gone
  X["e4"]      # <error>

  Maybe names() should not be allowed on a matrix?


3. Should selection of non-existent dimension names really be an error?

  x[22]   # works on a vector - gives "NA"
  X[,22]  # <error>


  A potential useful use-case is matching a smaller matrix to a larger one:
  A <- matrix(rnorm(10), nrow=2, dimnames = list(c("a","c")))
  B <- matrix(rnorm(20), nrow=4, dimnames = list(c("a", "b", "c", "d")))

  # matching larger matrix to the smaller one <works>
  B[rownames(A),]

  # matching smaller matrix to the larger one <error>
  A[rownames(B),]


These also doesn't seem to be documented in '[', 'names', 'rownames’.

Interested if there specific reasons for this behaviour, or could these potentially be adjusted?

Kind regards,
Karolis K.