Editing/Adding factor levels in R

April 29, 2013
By

[This article was first published on We think therefore we R, 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.

I was trying to change few levels in my factor variable by simply coercing characters on that factor variable but it dint seem to work.

data(iris)
iris$Species <- rep("Random", 71)
## Warning: invalid factor level, NAs generated
iris$Species
##   [1] setosa    setosa    setosa    setosa    setosa    setosa    setosa   
## [8] setosa setosa setosa setosa setosa setosa setosa
## [15] setosa setosa setosa setosa setosa setosa setosa
## [22] setosa setosa setosa setosa setosa setosa setosa
## [29] setosa setosa setosa setosa setosa setosa setosa
## [36] setosa setosa setosa setosa setosa setosa setosa
## [43] setosa setosa setosa setosa setosa setosa setosa
## [50]

## [57]
## [64]

## [71]
## [78]

## [85]
## [92]

## [99]
## [106]

## [113]
## [120] virginica virginica virginica virginica virginica virginica
## [127] virginica virginica virginica virginica virginica virginica virginica
## [134] virginica virginica virginica virginica virginica virginica virginica
## [141] virginica virginica virginica virginica virginica virginica virginica
## [148] virginica virginica virginica
## Levels: setosa versicolor virginica

Well, I did find a way to find a work around for that by doing this:

iris$Species <- as.character(iris$Species)
iris$Species <- rep("Random", 71)
iris$Species <- as.factor(iris$Species)
iris$Species
##   [1] setosa    setosa    setosa    setosa    setosa    setosa    setosa   
## [8] setosa setosa setosa setosa setosa setosa setosa
## [15] setosa setosa setosa setosa setosa setosa setosa
## [22] setosa setosa setosa setosa setosa setosa setosa
## [29] setosa setosa setosa setosa setosa setosa setosa
## [36] setosa setosa setosa setosa setosa setosa setosa
## [43] setosa setosa setosa setosa setosa setosa setosa
## [50] Random Random Random Random Random Random Random
## [57] Random Random Random Random Random Random Random
## [64] Random Random Random Random Random Random Random
## [71] Random Random Random Random Random Random Random
## [78] Random Random Random Random Random Random Random
## [85] Random Random Random Random Random Random Random
## [92] Random Random Random Random Random Random Random
## [99] Random Random Random Random Random Random Random
## [106] Random Random Random Random Random Random Random
## [113] Random Random Random Random Random Random Random
## [120] Random virginica virginica virginica virginica virginica virginica
## [127] virginica virginica virginica virginica virginica virginica virginica
## [134] virginica virginica virginica virginica virginica virginica virginica
## [141] virginica virginica virginica virginica virginica virginica virginica
## [148] virginica virginica virginica
## Levels: Random setosa virginica

This problem annoyed me at first, “Why would R not allow me to change/add factor [email protected]#[email protected]#?” but then Utkarsh and I had a conversation about this which made me think otherwise.

Excerpts from the conversation:

Utkarsh: It is usually not good to create data on the fly. Besides, when you create a factor variable, you should give the finite set of values it can take. This prevents future mistakes. It is called type checking. Python does not do it. R does it to some extent. C does it to some extent. Haskell does it very very strictly and it prevents about 50% of bugs from appearing. Let's say you misspell one of the levels.

In retrospect, it actually makes sense for us not to be able to add/edit the levels in factor variables. For a simple reason, we “might” make mistake, and misspelling a factor level could cause serious trouble. Lesson learnt!

To leave a comment for the author, please follow the link and comment on their blog: We think therefore we R.

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)