R: how to get row and column names of the true elements of a matrix?

Question

I have a logical matrix x with named rows (\'a\' and \'b\') and named columns (\'10\', \'20\', \'30\', \'40\'). Let\'s say, this:

   10   20   3         


        

Answer 1

You didn't specify this, but your desired output will require that we assume that the result is in fact rectangular. Namely, that we don't get 3 column names for a and only 2 column names for b.

I think this should get you started, at least:

m <- structure(c(TRUE, FALSE, FALSE, TRUE, TRUE, FALSE, FALSE, TRUE),
.Dim = c(2L, 4L), .Dimnames = list(c("a", "b"), c("10", "20", "30", "40")))

rr <- rownames(m)[row(m)[which(m)]]
cc <- colnames(m)[col(m)[which(m)]]

dd <- data.frame(rr = rr,cc = cc)
dd

which returns the information you want, but in a safer "long" format, which won't choke on the non-rectangular case. Once there, you could re-organize it as you specified like this:

library(plyr)
ddply(dd,.(rr),function(x){ x$cc })

but frankly that last bit I find really ugly, and I wouldn't be surprised if a better solution pops up if you wait a bit.