[R] Question concerning side effects of treating invalid factor levels

Ebert,Timothy Aaron tebert @end|ng |rom u||@edu
Mon Sep 19 13:59:02 CEST 2022

In your example code, the variable remains a class factor, and all entries are valid. The variables will behave as expected given the factor levels in the original dataframe.

(At least on my system R 4.2, in RStudio, in Windows) R returns a couple of error messages warning me that I was bad.
What you get is NA for "not available", or "not appropriate" or a missing value. You gave the system an invalid factor level so it was entered as missing. If you get data that has a new factor level, you need to tell R to expect a new factor level first.

levels(f1) <- c(levels(f1),"New Level")
levels(f1) <- c(levels(f1),c("NL1","NL2"))

-----Original Message-----
From: R-help <r-help-bounces using r-project.org> On Behalf Of Tibor Kiss via R-help
Sent: Monday, September 19, 2022 6:11 AM
To: r-help using r-project.org
Subject: [R] Question concerning side effects of treating invalid factor levels

[External Email]

Dear List members,

I have tried now for several times to find out about a side effect of treating invalid factor levels, but did not find an answer. Various answers on stackexchange etc. produce the stuff that irritates me without even mentioning it.
So I am asking the list (apologies if this has been treated in the past).

If you add an invalid factor level to a column in a data frame, this has the side effect of turning a numerical column into a column with character strings. Here is a simple example:

> df <- data.frame(
        P = factor(c("mittels", "mit", "mittels", "ueber", "mit", "mit")),
        ANSWER = factor(c(rep("PP>OBJ", 4), rep("OBJ>PP", 2))),
        RT = round(runif(6, 7000, 16000), 0))

> str(df)
'data.frame':   6 obs. of  3 variables:
$ P     : Factor w/ 3 levels "mit","mittels",..: 2 1 2 3 1 1
$ ANSWER: Factor w/ 2 levels "OBJ>PP","PP>OBJ": 2 2 2 2 1 1
$ RT    : num  11157 13719 14388 14527 14686 ..

> df <- rbind(df, c("in", "V>N", round(runif(1, 7000, 16000), 0)))

> str(df)
'data.frame':   7 obs. of  3 variables:
$ P     : Factor w/ 3 levels "mit","mittels",..: 2 1 2 3 1 1 NA
$ ANSWER: Factor w/ 2 levels "OBJ>PP","PP>OBJ": 2 2 2 2 1 1 NA
$ RT    : chr  "11478" "15819" "8305" "8852" ...

You see that RT has changed from _num_ to _chr_ as a side effect of adding the invalid factor level as NA. I would appreciate understanding what the purpose of the type coercion is.

Thanks in advance

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