Teradata R Package Function Reference | 17.00 - 17.00 - NGrams - Teradata R Package

Teradata® R Package Function Reference

prodname
Teradata R Package
vrm_release
17.00
created_date
September 2020
category
Programming Reference
featnum
B700-4007-090K

Description

The nGrams function tokenizes (splits) an input stream of text and outputs n multigrams (called n-grams) based on the specified delimiter and reset parameters. nGrams provides more flexibility than standard tokenization when performing text analysis. Many two-word phrases carry important meaning (for example, "machine learning") that unigrams (single-word tokens) do not capture. This, combined with additional analytical techniques, can be useful for performing sentiment analysis, topic identification, and document classification.

Usage

  td_ngrams_mle (
    data = NULL,
      text.column = NULL,
      delimiter = "[\\s]+",
      grams = NULL,
      overlapping = TRUE,
      to.lower.case = TRUE,
      punctuation = "[`~#^&*()-]",
      reset = "[.,?!]",
      total.gram.count = FALSE,
      total.count.column = "totalcnt",
      accumulate = NULL,
      n.gram.column = "ngram",
      num.grams.column = "n",
      frequency.column = "frequency"
  )

Arguments

data

Required Argument. Specifies input table where each row of the input table contains a document to be tokenized. The input table can have additional rows, some or all of which the function returns in the output table.

text.column

Required Argument. Specifies the name of the column that contains the input text. Input columns must contain string SQL types.

delimiter

Optional Argument. A regular expression that specifies the character or string that separates words in the input text. The default value is the set of all whitespace characters which includes the characters for space, tab, newline, carriage return and some others. Default Value: "[\s]+"

grams

Required Argument. A list of integers or ranges of integers that specify the length, in words, of each n-gram (that is, the value of n). A range_of_values has the syntax integer1-integer2, where integer1 <= integer2. The values of n, integer1, and integer2 must be positive.

overlapping

Optional Argument. A Boolean value that specifies whether the function allows overlapping n-grams. When this value is "true" (the default), each word in each sentence starts an n-gram, if enough words follow it (in the same sentence) to form a whole n-gram of the specified size. For information on sentences, see the description of the reset argument. Default Value: TRUE

to.lower.case

Optional Argument. A Boolean value that specifies whether the function converts all letters in the input text to lowercase. Default Value: TRUE

punctuation

Optional Argument. A regular expression that specifies the punctuation characters for the function to remove before evaluating the input text. Default Value: "['~#^&*()-]"

reset

Optional Argument. A regular expression that specifies the character or string that ends a sentence. At the end of a sentence, the function discards any partial n-grams and searches for the next n-gram at the beginning of the next sentence. An n-gram cannot span two sentences. Default Value: "[.,?!]"

total.gram.count

Optional Argument. A Boolean value that specifies whether the function returns the total number of n-grams in the document (that is, in the row). If this value is TRUE, then the name of the returned column is specified by the total.count.column argument. Note: The total number of n-grams is not necessarily the number of unique ngrams. Default Value: FALSE

total.count.column

Optional Argument. Specifies the name of the column to return if the value of the total.gram.count argument is "TRUE". Default Value: "totalcnt"

accumulate

Optional Argument. Specifies the names of the columns to return for each n-gram. These columns cannot have the same names as those specified by the arguments n.gram.column, num.grams.column, and total.count.column. By default, the function returns all input columns for each n-gram.

n.gram.column

Optional Argument. Specifies the name of the column that contains the generated n-grams. Default Value: "ngram"

num.grams.column

Optional Argument. Specifies the name of the column that is to contain the length of n-gram (in words). Default Value: "n"

frequency.column

Optional Argument. Specifies the name of the column that contains the count of each unique n-gram (that is, the number of times that each unique n-gram appears in the document). Default Value: "frequency"

Value

Function returns an object of class "td_ngrams_mle" which is a named list containing Teradata tbl object. Named list member can be referenced directly with the "$" operator using name: result.

Examples

    # Get the current context/connection
    con <- td_get_context()$connection
    
    # Load example data.
    loadExampleData("ngram_example", "paragraphs_input")
    
    # Create remote tibble objects.
    paragraphs_input <- tbl(con, "paragraphs_input")
    
    # Example 1 -
    td_ngrams_out1 <- td_ngrams_mle(data = paragraphs_input,
                       text.column = "paratext",
                       delimiter = " ",
                       grams = c("4-6"),
                       overlapping = TRUE,
                       to.lower.case = TRUE,
                       total.gram.count = TRUE,
                       accumulate = c("paraid","paratopic")
                       )
    
    # Example 2 -
    td_ngrams_out2 <- td_ngrams_mle(data = paragraphs_input,
                       text.column = "paratext",
                       delimiter = " ",
                       grams = c("4-6"),
                       overlapping = FALSE,
                       to.lower.case = TRUE,
                       total.gram.count = FALSE,
                       accumulate = c("paraid","paratopic")
                       )