Function to change blanks to NA

This worked for me

    df[df == 'NULL'] <- NA

You can directly index fields that match a logical criterion. So you can just write:

df[is_empty(df)] = NA

Where is_empty is your comparison, e.g. df == "":

df[df == ""] = NA

But note that is.null(df) won’t work, and would be weird anyway¹. I would advise against merging the logic for columns of different types, though! Instead, handle them separately.

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¹ You’ll almost never encounter NULL inside a table since that only works if the underlying vector is a list. You can create matrices and data.frames with this constraint, but then is.null(df) will never be TRUE because the NULL values are wrapped inside the list).