# Editing/Adding factor levels in R

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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$Species[c language="(50:120)"][/c] <- rep("Random", 71) ## Warning: invalid factor level, NAs generated iris$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$Species[c language="(50:120)"][/c] <- rep("Random", 71) iris$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!

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