[R] Remove empty levels in subset

[svo]

Hi,

I have exactly the same question (how to remove empty levels in my subset),
but in my case the factor command does not work, because my dataframe is not
atomic

> Try this:
>
> test2$a <- factor(test2$a)
>

R gives me the error message:

Error in sort.list(y) : 'x' must be atomic for 'sort.list'
Have you called 'sort' on a list?

Do you have advice?

Thank you

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