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

Ebert,Timothy Aaron tebert @end|ng |rom u||@edu
Mon Sep 19 14:22:04 CEST 2022

Sorry, My bad.
A vector must be of a single class. When you declare c("in", "V>N", round(runif(1, 7000, 16000), 0)) R will calculate the random number, but then convert it to a character class to conform with the other two elements in that vector. R then binds this to your original df and finds that it must add a character to a numeric vector. To keep the vector of all the same class it converts everything to character.



From: tibor.kiss using rub.de <tibor.kiss using rub.de>
Sent: Monday, September 19, 2022 8:07 AM
To: Ebert,Timothy Aaron <tebert using ufl.edu>
Cc: r-help using r-project.org
Subject: Re: [R] Question concerning side effects of treating invalid factor levels

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this is a misunderstanding of my question. I wasn't worried about invalid factor levels that produce NA. My question was why a column changes its class, which I thought was a side effect. If you add a vector containing one character string, the class of the whole vector becomes _chr_. And after this element has been added to a column, we have two NAs for the column which are factors, and a character string, which is responsible for the change of a numerical vector into a character string vector (see ?c, where you find: "The output type is determined from the highest type of the components in the hierarchy NULL < raw < logical < integer < double < complex < character < list < expression.").



Am 19.09.2022 um 13:59 schrieb Ebert,Timothy Aaron <tebert using ufl.edu<mailto:tebert using ufl.edu>>:

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<mailto: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<mailto: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))

'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)))

'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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