Is there a built in function to invert a list? i.e. to go from list( a = c("1", "2", "3"), b = c("1"), d = c("2", "4") ) to list( "1" = c("a", "b"), "2" = c("a", "d"), "3" = "a", "4" = "2" ) Hadley -- http://had.co.nz/