unlisting a list while keeping the indices

Question

I have a list which can have either empty entries, entries containing one elements and entries containing multiple elements.

l1 = list(integer(0), 11L, integer(         


        

Answer 1

A "low level" solution:

data.frame(entry=rep(seq_along(l1),lengths(l1)),element=unlist(l1))
#  entry element
#1     2      11
#2     5      11
#3     6      11
#4     7       6
#5     7      36
#6     8      16
#7     9      16