How to check if a variable is categorical with R?

Question

I have an R data frame and some of the variables are categorical. For example sex is "male" or "female" and "do you smoke" is 0 or 1. Others variables instead are continuous. I would like to know if there is any way to decide if a variable is categorical or not and in case compute its frequencies.

I think in my case a good test would be to check if the variable takes less than k=4 values.

Answer 1

While you should use factors for categorical variables, you can find the unique values in a vector x with unique, and count them:

length(unique(x))

Answer 2

You can use class(dataframe$variable) to know the class of a variable within a data frame as well as determine whether the variable is a factor or not.