Indicator Variables via Feature Hashing

step_dummy_hash() creates a specification of a recipe step that will convert factors or character columns into a series of binary (or signed binary) indicator columns.

Usage

step_dummy_hash(
  recipe,
  ...,
  role = "predictor",
  trained = FALSE,
  columns = NULL,
  signed = TRUE,
  num_terms = 32L,
  collapse = FALSE,
  prefix = "dummyhash",
  sparse = "auto",
  keep_original_cols = FALSE,
  skip = FALSE,
  id = rand_id("dummy_hash")
)

Arguments

recipe: A recipes::recipe() object. The step will be added to the sequence of operations for this recipe.
...: One or more selector functions to choose which variables are affected by the step. See recipes::selections() for more details.
role: 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.
trained: A logical to indicate if the quantities for preprocessing have been estimated.
columns: A character vector of the selected variable names. This is NULL until the step is trained by recipes::prep.recipe(), and should not be set by hand.
signed: A logical, indicating whether to use a signed hash-function (generating values of -1, 0, or 1), to reduce collisions when hashing. Defaults to TRUE.
num_terms: An integer, the number of variables to output. Defaults to 32.
collapse: A logical. Should all of the selected columns be collapsed into a single column to create a single set of hashed features? Defaults to FALSE.
prefix: A character string that will be the prefix to the resulting new variables. See notes below.
sparse: A single string. Should the columns produced be sparse vectors? Can take the values "yes", "no", and "auto". If sparse = "auto" then workflows can determine the best option. Defaults to "auto".
keep_original_cols: A logical to keep the original variables in the output. Defaults to FALSE.
skip: A logical. Should the step be skipped when the recipe is baked by recipes::bake()? While all operations are baked when recipes::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.
id: A character string that is unique to this step to identify it.

Value

An updated version of recipe with the new step added to the sequence of existing steps (if any).

Details

Feature hashing, or the hashing trick, is a transformation of a text variable into a new set of numerical variables. This is done by applying a hashing function over the values of the factor levels and using the hash values as feature indices. This allows for a low memory representation of the data and can be very helpful when a qualitative predictor has many levels or is expected to have new levels during prediction. This implementation is done using the MurmurHash3 method via text2vec::hash_vectorizer().

The argument num_terms controls the number of indices that the hashing function will map to. This is the tuning parameter for this transformation. Since the hashing function can map two different tokens to the same index, a higher value of num_terms will result in a lower chance of collision.

The new columns will have names that begin with prefix, then the name of the variable, then the index of the hash, all separated by _. The indices are padded with zeros to the same width. For example, with the default prefix = "dummyhash" and a variable named text, num_terms = 12 gives dummyhash_text_01 to dummyhash_text_12. If collapse = TRUE, the variable name is replaced by the names of all selected variables joined by _, such as dummyhash_text_g_01 when text and g are selected.

Tidying

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

terms: character, the selectors or variables selected
value: logical, whether a signed hashing was performed
num_terms: integer, number of terms
collapse: logical, were the columns collapsed
id: character, id of this step

Tuning Parameters

This step has 2 tuning parameters:

signed: Signed Hash Value (type: logical, default: TRUE)
num_terms: # Hash Features (type: integer, default: 32)

Sparse data

This step produces sparse columns if sparse = "yes" is being set. The default value "auto" won't trigger production of sparse columns if a recipe is recipes::prep()ed, but allows for a workflow to toggle to "yes" or "no" depending on whether the model supports recipes::sparse_data and if the model is expected to run faster with the data.

The mechanism for determining how much sparsity is produced isn't perfect, and there will be times when you want to manually overwrite by setting sparse = "yes" or sparse = "no".

Case weights

The underlying operation does not allow for case weights.

References

Kilian Weinberger; Anirban Dasgupta; John Langford; Alex Smola; Josh Attenberg (2009).

Kuhn and Johnson (2019), Chapter 7, https://feat.engineering/

Examples


library(recipes)
library(modeldata)
data(grants)

grants_rec <- recipe(~sponsor_code, data = grants_other) |>
  step_dummy_hash(sponsor_code)

grants_obj <- grants_rec |>
  prep()

bake(grants_obj, grants_test)
#> # A tibble: 518 × 32
#>    dummyhash_sponsor_co…¹ dummyhash_sponsor_co…² dummyhash_sponsor_co…³
#>                     <int>                  <int>                  <int>
#>  1                      0                      0                      0
#>  2                      0                      0                      0
#>  3                      0                      0                      0
#>  4                      0                      1                      0
#>  5                      0                      0                      0
#>  6                      0                      1                      0
#>  7                      0                      0                      0
#>  8                      0                      0                      0
#>  9                      0                      0                      0
#> 10                      0                      1                      0
#> # ℹ 508 more rows
#> # ℹ abbreviated names: ¹dummyhash_sponsor_code_01,
#> #   ²dummyhash_sponsor_code_02, ³dummyhash_sponsor_code_03
#> # ℹ 29 more variables: dummyhash_sponsor_code_04 <int>,
#> #   dummyhash_sponsor_code_05 <int>, dummyhash_sponsor_code_06 <int>,
#> #   dummyhash_sponsor_code_07 <int>, dummyhash_sponsor_code_08 <int>,
#> #   dummyhash_sponsor_code_09 <int>, …

tidy(grants_rec, number = 1)
#> # A tibble: 1 × 5
#>   terms        value num_terms collapse id              
#>   <chr>        <lgl>     <int> <lgl>    <chr>           
#> 1 sponsor_code NA           NA NA       dummy_hash_8HfdV
tidy(grants_obj, number = 1)
#> # A tibble: 1 × 5
#>   terms        value num_terms collapse id              
#>   <chr>        <lgl>     <int> <lgl>    <chr>           
#> 1 sponsor_code TRUE         32 FALSE    dummy_hash_8HfdV