step_tf: Term frequency of tokens

Description Usage Arguments Details Value See Also Examples

Description

'step_tf' creates a *specification* of a recipe step that will convert a list of tokens into multiple variables containing the token counts.

Usage

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step_tf(recipe, ..., role = "predictor", trained = FALSE,
  columns = NULL, weight_scheme = "raw count", weight = 0.5,
  vocabulary = NULL, res = NULL, prefix = "tf", skip = FALSE,
  id = rand_id("tf"))

## S3 method for class 'step_tf'
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_tf', 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

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

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()].

weight_scheme

A character determining the weighting scheme for the term frequency calculations. Must be one of "binary", "raw count", "term frequency", "log normalization" or "double normalization". Defaults to "raw count".

weight

A numeric weight used if 'weight_scheme' is set to "double normalization". Defaults to 0.5.

vocabulary

A character vector of strings to be considered.

res

The words that will be used to calculate the term frequency will be stored here once this preprocessing step has be trained by [prep.recipe()].

prefix

A character string that will be the prefix to the resulting new variables. See notes below

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.

id

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

x

A 'step_tf' object.

Details

Term frequency is a weight of how many times each token appear in each observation. There are different ways to calculate the weight and this step can do it in a couple of ways. Setting the argument 'weight_scheme' to "binary" will result in a set of binary variables denoting if a token is present in the observation. "raw count" will count the times a token is present in the observation. "term frequency" will devide the count with the total number of words in the document to limit the effect of the document length as longer documents tends to have the word present more times but not necessarily at a higher procentage. "log normalization" takes the log of 1 plus the count, adding 1 is done to avoid taking log of 0. Finally "double normalization" is the raw frequency divided by the raw frequency of the most occurring term in the document. This is then multiplied by 'weight' and 'weight' is added tot he result. This is again done to prevent a bias towards longer documents.

The new components will have names that begin with 'prefix', then the name of the variable, followed by the tokens all seperated by '-'. The new variables will be created alphabetically according to token.

Value

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

See Also

[step_hashing()] [step_tfidf()] [step_tokenize()]

Examples

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library(recipes)

data(okc_text)

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

tidy(okc_rec, number = 2)
tidy(okc_obj, number = 2)

textrecipes documentation built on May 2, 2019, 1:27 p.m.