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   30   40  
a  T    F    T    F  
b  F    T    F    T  

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

Is there a short way to get a table that would list names of rows and columns where I have true values? That is, I want to get the following table:

a  10, 30
b  20, 40

Something similar can be obtained by which(x, arr.ind = T), which produces

  row col
a   1   1
b   2   2
a   1   3
b   2   4

But I really want to get the first table.

Answer 1

You can directly use apply.

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