a utils function to add default cat2cat columns to a `data.frame`. It will be useful e.g. for a boarder periods which will not have additional `cat2cat` columns.
dummy_c2c(df, cat_var, ml = NULL)
`data.frame`.
`character(1)` a categorical variable name.
`character` vector of ml models applied, any of `c("knn", "rf", "lda")`.
the provided `data.frame` with additional `cat2cat` like columns.
if (FALSE) {
dummy_c2c(airquality, "Month")
data("occup_small", package = "cat2cat")
occup_old <- occup_small[occup_small$year == 2008, ]
dummy_c2c(occup_old, "code")
dummy_c2c(occup_old, "code", "knn")
}