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

[Eric Berger]

You are misinterpreting what is going on.
The rbind command includes c(char, char, int) which produces a
character vector of length 3.
This is what you are rbind-ing which changes the type of the RT column.

If you do rbind(df, data.frame(P="in", ANSWER="V>N",
RT=round(runif(1,7000,16000),0)))
you will see that everything is fine. (New factor values are created.)

HTH,
Eric

On Mon, Sep 19, 2022 at 2:14 PM Tibor Kiss via R-help
<r-help using r-project.org> wrote:
>
> 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
>
>
> Tibor
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