Levels of a factor

On Jul 24, 2013, at 6:25 AM, Borja Rivier wrote:

Hi all,

I am having a bit of trouble using the levels() function.
I have a factor with many elements, and when I use the function levels() to
extract the list of unique elements, some of the elements returned are not
actually in the factor.

snipped

When you take a subset of a factor vector, the levels are not reduced to the unique values in the new vector. There is droplevels function that would need to be applied if you already have such a vector, and there is a drop argument that you need to set to TRUE in the `[.factors`

call if you want to "attack the problem at the source".

?`[.factor  # missing trailing back-tick

?droplevels

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