[R] rownames of single row matrices

Robin Hankin rksh at soc.soton.ac.uk
Fri Jun 11 10:09:19 CEST 2004


I want to extract rows of a matrix, and preserve rownames even if only
one row is selected.  Toy example:

R> a <- matrix(1:9,3,3)
R> rownames(a) <- letters[1:3]
R> colnames(a) <- LETTERS[1:3]
R> a
   A B C
a 1 4 7
b 2 5 8
c 3 6 9

Extract the first two rows:
R> wanted <- 1:2
R> a[wanted,]
   A B C
a 1 4 7
b 2 5 8

rownames come through fine.  Now extract just
one row:

R> a[1,]
1 4 7

rowname is lost!  (also note that this isn't a 1-by-n matrix as 
needed.  This object
is a vector). How do I get the rowname back?
My best effort is:

extract <- function(a,wanted){
   if(length(wanted)>1) {
   } else {
      out <- t(as.matrix(a[wanted,]))
      rownames(out) <- rownames(a)[wanted]

[note the transpose and as.matrix()].  There must be a better way!
Anyone got any better ideas?

What is the R rationale for treating a[1,] so differently from a[1:2,]  ?

Robin Hankin
Uncertainty Analyst
Southampton Oceanography Centre
SO14 3ZH
tel +44(0)23-8059-7743
initialDOTsurname at soc.soton.ac.uk (edit in obvious way; spam precaution)

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