Unpacking and merging lists in a column in data.frame

Question

I have the following data.frame:

id     name   altNames
1001   Joan   character(0)      
1002   Jane   c("Janie", "Janet", "Jan")
1003   John   Jon
1004   Bill   Will
1005   Tom    character(0)      

The column altNames could be empty (i.e. character(0)), have just one name, or a list of names. What I want is a data.frame (or a list) where each entry from name and/or altNames appears just once along with the corresponding id, like this:

id     name
1001   Joan
1002   Jane
1002   Janie
1002   Janet
1002   Jan
1003   John
1003   Jon
1004   Bill
1004   Will
1005   Tom

What's the most efficient way of doing it? Even better is dplyr is utilized. Thanks

Edit: Here's the data:

df <- data_frame(
  id = c("1001", "1002","1003", "1004", "1005"), 
  name = c("Joan", "Jane", "John", "Bill", "Tom"), 
  altNames = list(character(0), c("Janie", "Janet", "Jan"), "Jon", "Will", character(0))
)

Answer 1

Here's a possible data.table approach

library(data.table)
setDT(dat)[, .(name = c(name, unlist(altNames))), by = id]
#       id  name
#  1: 1001  Joan
#  2: 1002  Jane
#  3: 1002 Janie
#  4: 1002 Janet
#  5: 1002   Jan
#  6: 1003  John
#  7: 1003   Jon
#  8: 1004  Bill
#  9: 1004  Will
# 10: 1005   Tom

Answer 2

A base R version (using the df added by @rawr)

with(df, {
    ns <- mapply(c, name, altNames)
    data.frame(id = rep(id, times=lengths(ns)), name=unlist(ns), row.names=NULL)
})
#     id  name
#1  1001  Joan
#2  1002  Jane
#3  1002 Janie
#4  1002 Janet
#5  1002   Jan
#6  1003  John
#7  1003   Jon
#8  1004  Bill
#9  1004  Will
#10 1005   Tom