step_stem: Stemming of list-column variables

Description

`step_stem` creates a *specification* of a recipe step that will convert a list of tokens into a list of stemmed tokens.

Usage

step_stem(recipe, ..., role = NA, trained = FALSE, columns = NULL,
  options = list(), custom_stemmer = NULL, skip = FALSE,
  id = rand_id("stem"))
# S3 method for step_stem
tidy(x, ...)

Arguments

recipe

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 `step_stem`, this indicates the variables to be encoded into a list column. See [recipes::selections()] for more details. For the `tidy` method, these are not currently used.

role

Not used by this step since no new variables are created.

trained

A logical to indicate if the recipe has been baked.

columns

A list of tibble results that define the encoding. This is `NULL` until the step is trained by [recipes::prep.recipe()].

options

A list of options passed to the stemmer function.

custom_stemmer

A custom stemming function. If none is provided it will default to "SnowballC".

skip

A logical. Should the step be skipped when the recipe is baked by [recipes::bake.recipe()]? While all operations are baked when [recipes::prep.recipe()] 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.

A `step_stem` object.

Value

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

Details

Words tend to have different forms depending on context, such as organize, organizes, and organizing. In many situations it is beneficial to have these words condensed into one to allow for a smaller pool of words. Stemming is the act of choping off the end of words using a set of heuristics.

Note that the steming will only be done at the end of the string and will therefore not work reliably on ngrams or sentences.

Examples

Run this code

# NOT RUN {
library(recipes)

data(okc_text)

okc_rec <- recipe(~ ., data = okc_text) %>%
  step_tokenize(essay0) %>%
  step_stem(essay0)
  
okc_obj <- okc_rec %>%
  prep(training = okc_text, retain = TRUE)

juice(okc_obj, essay0) %>% 
  slice(1:2)

juice(okc_obj) %>% 
  slice(2) %>% 
  pull(essay0) 
  
tidy(okc_rec, number = 2)
tidy(okc_obj, number = 2)

# Using custom stemmer. Here a custom stemmer that removes the last letter
# if it is a s.
remove_s <- function(x) gsub("s$", "", x)

okc_rec <- recipe(~ ., data = okc_text) %>%
  step_tokenize(essay0) %>%
  step_stem(essay0, custom_stemmer = remove_s)
  
okc_obj <- okc_rec %>%
  prep(training = okc_text, retain = TRUE)

juice(okc_obj, essay0) %>% 
  slice(1:2)

juice(okc_obj) %>% 
  slice(2) %>% 
  pull(essay0) 
# }