"setdiff" operation getting reversed when using "categories" - Why??

I have a table named "data" with one of the variables (categorical) being "Country". I am trying to merge all the categories that are not missing ("N/A") and after executing

land = setdiff(data.Country,"N/A")

in the workspace, "land" is a 9 x 1 categorical variable. This is perfect BUT when I execute

cats = categories(land)

"cats" becomes a 10 x 1 cell which includes the "N/A" that I got rid of using "setdiff" earlier.

WHY??

I want to use the "cats" in "mergecats(data.Country, cats, "Land")". Pls help.

Answer 1

[Walter Roberson]

Each categorical array has two parts: a list of category names, and a list of "codes" (indices) of the current contents of the array.

Taking a subset of a categorical array does not change the categories that are allocated. For example,

A = categorical({'dog', 'human'});
[A(2), A(1)]

The A(1) and A(2) each extract a single element of the categorical array, but doing so does not create a new categorical array in which the respective values are the only possible categories. If it did create a new categorical array restricted to the one category, then [A(2), A(1)] would have to synthesize a new categorical array with new code numbers and category order 'human', 'dog', but that is not what happens: A(1) is a categorical object with a single code but with category names left intact.