Iterating over a list in R

list_iterate.R

# a simple function to create a list of lists:
list_of_lists <- function(...) {
  list(...)
}
my_lol <- list_of_lists(list(cleesh = "krill"), list(nitfol = "rezrov"))
# want to print out "krill" and "rezrov"

# various suggestions for iterating over a list:
for (name in names(my_lol)) {
  message(my_lol[[name]][[1]])
}
# Nothing happens, because:
names(my_lol) # NULL

for (i in 1:length(my_lol)) {
  message(my_lol[[i]])
}
# this works - hurrah!

# but what if the list is empty?
my_empty_lol <- list_of_lists()
for (i in 1:length(my_empty_lol)) {
  message(my_empty_lol[[i]])
}
# Error in my_empty_lol[[i]] : subscript out of bounds
# Boo!

# How about:
for (i in seq_along(my_lol)) {
  message(my_lol[[i]])
}
# Works...
for (i in seq_along(my_empty_lol)) {
  message(my_empty_lol[[i]])
}
# ... also works (in that it prints out nothing and doesn't give an error)

# Moral of the story: use seq_along

Yes, one should avoid iterating with for loops in R lest one seem gauche or improper. But sometimes it's just easier to. With that in mind, there's a lot of advice out there on how to do it. I found that not all of that advice covered the cases where:

• The list might be empty.
• The list might not have names defined.

Using seq_along seems like the best bet.