step_dummy: Create traditional dummy variables

View source: R/dummy.R

step_dummyR Documentation

Create traditional dummy variables


step_dummy() creates a specification of a recipe step that will convert nominal data (e.g. factors) into one or more numeric binary model terms corresponding to the levels of the original data.


  role = "predictor",
  trained = FALSE,
  one_hot = FALSE,
  preserve = deprecated(),
  naming = dummy_names,
  levels = NULL,
  keep_original_cols = FALSE,
  skip = FALSE,
  id = rand_id("dummy")



A recipe object. The step will be added to the sequence of operations for this recipe.


One or more selector functions to choose variables for this step. See selections() for more details. The selected variables must be factors.


For model terms created by this step, what analysis role should they be assigned? By default, the new columns created by this step from the original variables will be used as predictors in a model.


A logical to indicate if the quantities for preprocessing have been estimated.


A logical. For C levels, should C dummy variables be created rather than C-1?


This argument has been deprecated. Please use keep_original_cols instead.


A function that defines the naming convention for new dummy columns. See Details below.


A list that contains the information needed to create dummy variables for each variable contained in terms. This is NULL until the step is trained by prep().


A logical to keep the original variables in the output. Defaults to FALSE.


A logical. Should the step be skipped when the recipe is baked by bake()? While all operations are baked when prep() is run, some operations may not be able to be conducted on new data (e.g. processing the outcome variable(s)). Care should be taken when using skip = TRUE as it may affect the computations for subsequent operations.


A character string that is unique to this step to identify it.


step_dummy() will create a set of binary dummy variables from a factor variable. For example, if an unordered factor column in the data set has levels of "red", "green", "blue", the dummy variable bake will create two additional columns of 0/1 data for two of those three values (and remove the original column). For ordered factors, polynomial contrasts are used to encode the numeric values.

By default, the excluded dummy variable (i.e. the reference cell) will correspond to the first level of the unordered factor being converted. step_relevel() can be used to create a new reference level by setting the ref_level argument.

This recipe step allows for flexible naming of the resulting variables. For an unordered factor named x, with levels "a" and "b", the default naming convention would be to create a new variable called x_b. The naming format can be changed using the naming argument; the function dummy_names() is the default.

To change the type of contrast being used, change the global contrast option via options.

When the factor being converted has a missing value, all of the corresponding dummy variables are also missing. See step_unknown() for a solution.

When data to be processed contains novel levels (i.e., not contained in the training set), a missing value is assigned to the results. See step_other() for an alternative.

If no columns are selected (perhaps due to an earlier step_zv()), bake() will return the data as-is (e.g. with no dummy variables).

Note that, by default, the new dummy variable column names obey the naming rules for columns. If there are levels such as "0", dummy_names() will put a leading "X" in front of the level (since it uses make.names()). This can be changed by passing in a different function to the naming argument for this step.

Also, there are a number of contrast methods that return fractional values. The columns returned by this step are doubles (not integers).

The package vignette for dummy variables and interactions has more information.


An updated version of recipe with the new step added to the sequence of any existing operations.


When you tidy() this step, a tibble is returned with columns terms, columns , and id:


character, the selectors or variables selected


character, names of resulting columns


character, id of this step

Case weights

The underlying operation does not allow for case weights.

See Also


Other dummy variable and encoding steps: step_bin2factor(), step_count(), step_date(), step_dummy_extract(), step_dummy_multi_choice(), step_factor2string(), step_holiday(), step_indicate_na(), step_integer(), step_novel(), step_num2factor(), step_ordinalscore(), step_other(), step_regex(), step_relevel(), step_string2factor(), step_time(), step_unknown(), step_unorder()


data(Sacramento, package = "modeldata")

# Original data: city has 37 levels
unique(Sacramento$city) %>% sort()

rec <- recipe(~ city + sqft + price, data = Sacramento)

# Default dummy coding: 36 dummy variables
dummies <- rec %>%
  step_dummy(city) %>%
  prep(training = Sacramento)

dummy_data <- bake(dummies, new_data = NULL)

dummy_data %>%
  select(starts_with("city")) %>%
  names() # level "anything" is the reference level

# Obtain the full set of 37 dummy variables using `one_hot` option
dummies_one_hot <- rec %>%
  step_dummy(city, one_hot = TRUE) %>%
  prep(training = Sacramento)

dummy_data_one_hot <- bake(dummies_one_hot, new_data = NULL)

dummy_data_one_hot %>%
  select(starts_with("city")) %>%
  names() # no reference level

tidy(dummies, number = 1)
tidy(dummies_one_hot, number = 1)

recipes documentation built on July 4, 2024, 9:06 a.m.