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.

Answer 2

You can use the fact that table objects are converted to "long" format data frames by as.data.frame():

# Create matrix of interest 
mat <- structure(c(TRUE, FALSE, FALSE, TRUE, TRUE, FALSE, FALSE, TRUE), .Dim = c(2L, 4L), .Dimnames = list(c("a", "b"), c("10", "20", "30", "40")))

# Convert to table, then to long data.frame
df <- mat %>% as.table %>% as.data.frame(., stringsAsFactors=FALSE)

The resulting df is the following:

  Var1 Var2  Freq
1    a   10  TRUE
2    b   10 FALSE
3    a   20 FALSE
4    b   20  TRUE
5    a   30  TRUE
6    b   30 FALSE
7    a   40 FALSE
8    b   40  TRUE

Which you can then index to only keep TRUE rows:

df <- df[df$Freq,1:2] %>% sort
df
      Var1 Var2
    1    a   10
    5    a   30
    4    b   20
    8    b   40

You can use dplyr to convert this into exactly the table you want:

library(plyr)
ddply(df, "Var1", function(x) x$Var2)
  Var1 V1 V2
1    a 10 30
2    b 20 40

Answer 3

You can directly use apply.

apply(
  x, 1, 
  function(u) paste( names(which(u)), collapse="," ) 
)