obtaining row indexes from a list of column indexes

List,

given an arbitrary n x m boolean matrix, say

      [,1]  [,2]  [,3]  [,4]  [,5]
[1,] FALSE FALSE FALSE FALSE FALSE
[2,] FALSE FALSE  TRUE FALSE FALSE
[3,] FALSE FALSE FALSE FALSE FALSE
[4,] FALSE FALSE FALSE  TRUE  TRUE

I have the TRUE entries (indexes) of each column, in a list

[[1]]
integer(0)

[[2]]
integer(0)

[[3]]
[1] 2

[[4]]
[1] 4

[[5]]
[1] 4

How can I, from this list, efficiently obtain the TRUE entries of each
row, in this case

[[1]]
integer(0)

[[2]]
[1] 3

[[3]]
integer(0)

[[4]]
[1] 4 5

without constructing the n x m boolean matrix?

Edzer Pebesma
Institute for Geoinformatics (ifgi), University of M?nster
Weseler Stra?e 253, 48151 M?nster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de