Initializing an empty list

September 13, 2019
By

[This article was first published on woodpeckR, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

Problem

How do I initialize an empty list for use in a for-loop or function?

Context

Sometimes I’m writing a for-loop (I know, I know, don’t use for-loops, but sometimes it’s just easier. I’m a little less good at apply functions than I’d like to be) and I know I’ll need to store the output in a list. Once in a while, the new list will be the same size as an existing one, but more often, I just need to start from scratch, knowing only the number of elements I want to include.

This isn’t a totally alien thing to need to do––it’s pretty familiar if you’re used to initializing empty vectors before for-loops. There’s a whole other debate to be had about whether or not it’s acceptable to start with a truly empty vector and append to it on every iteration of the loop or whether you should always know the length beforehand, but I’ll just focus on the latter case for now.

Anyway, initializing a vector of a given length is easy enough; I usually do it like this:

> desired_length <- 10 # or whatever length you want
> empty_vec <- rep(NA, desired_length)

I couldn’t immediately figure out how to replicate this for a list, though. The solution turns out to be relatively simple, but it’s just different enough that I can never seem to remember the syntax. This post is more for my records than anything, then.

Solution

Initializing an empty list turns out to have an added benefit over my rep(NA) method for vectors; namely, the list ends up actually empty, not filled with NA’s. Confusingly, the function to use is “vector,” not “list.”

> desired_length <- 10 # or whatever length you want
> empty_list <- vector(mode = "list", length = desired_length)

> str(empty_list)
List of 10
 $ : NULL
 $ : NULL
 $ : NULL
 $ : NULL
 $ : NULL
 $ : NULL
 $ : NULL
 $ : NULL
 $ : NULL
 $ : NULL

Outcome

Voilà, an empty list. No restrictions on the data type or structure of the individual list elements. Specify the length easily. Useful for loops, primarily, but may have other applications I haven’t come across yet.

To leave a comment for the author, please follow the link and comment on their blog: woodpeckR.

R-bloggers.com offers daily e-mail updates about R news and tutorials about learning R and many other topics. Click here if you're looking to post or find an R/data-science job.
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.



If you got this far, why not subscribe for updates from the site? Choose your flavor: e-mail, twitter, RSS, or facebook...

Comments are closed.

Search R-bloggers

Sponsors

Never miss an update!
Subscribe to R-bloggers to receive
e-mails with the latest R posts.
(You will not see this message again.)

Click here to close (This popup will not appear again)