Last active
December 30, 2015 04:50
-
-
Save jlmelville/0856c2523b6dc3c84ff3 to your computer and use it in GitHub Desktop.
Iterating over a list in R
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
# 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 | |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
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:
Using
seq_along
seems like the best bet.