quanteda (version 2.0.1)

tokens: Construct a tokens object

Description

Construct a tokens object, either by importing a named list of characters from an external tokenizer, or by calling the internal quanteda tokenizer.

Usage

tokens(
  x,
  what = "word",
  remove_punct = FALSE,
  remove_symbols = FALSE,
  remove_numbers = FALSE,
  remove_url = FALSE,
  remove_separators = TRUE,
  split_hyphens = FALSE,
  include_docvars = TRUE,
  padding = FALSE,
  verbose = quanteda_options("verbose"),
  ...
)

Arguments

x

the input object to the tokens constructor, one of: a (uniquely) named list of characters; a tokens object; or a corpus or character object that will be tokenized

what

character; which tokenizer to use. The default what = "word" is the version 2 quanteda tokenizer. Legacy tokenizers (version < 2) are also supported, including the default what = "word1". See the Details and quanteda Tokenizer below.

remove_punct

logical; if TRUE remove all characters in the Unicode "Punctuation" [P] class, with exceptions for those used as prefixes for valid social media tags if preserve_tags = TRUE

remove_symbols

logical; if TRUE remove all characters in the Unicode "Symbol" [S] class

remove_numbers

logical; if TRUE remove tokens that consist only of numbers, but not words that start with digits, e.g. 2day

remove_url

logical; if TRUE find and eliminate URLs beginning with http(s)

remove_separators

logical; if TRUE remove separators and separator characters (Unicode "Separator" [Z] and "Control" [C] categories)

split_hyphens

logical; if TRUE, split words that are connected by hyphenation and hyphenation-like characters in between words, e.g. "self-aware" becomes c("self", "-", "aware")

include_docvars

if TRUE, pass docvars through to the tokens object. Does not apply when the input is a character data or a list of characters.

padding

if TRUE, leave an empty string where the removed tokens previously existed. This is useful if a positional match is needed between the pre- and post-selected tokens, for instance if a window of adjacency needs to be computed.

verbose

if TRUE, print timing messages to the console

...

used to pass arguments among the functions

Value

quanteda tokens class object, by default a serialized list of integers corresponding to a vector of types.

quanteda tokenizer

The default word tokenizer what = "word" splits tokens using stri_split_boundaries(x, type = "word") but by default preserves infix hyphens (e.g. "self-funding"), URLs, and social media "tag" characters (#hashtags and @usernames), and email addresses. The rules defining a valid "tag" can be found here for hashtags and here for usernames.

For backward compatibility, the following older tokenizers are also supported through what:

"word1"

(legacy) implements similar behaviour to the version of what = "word" found in pre-version 2. (It preserves social media tags and infix hyphens, but splits URLs.) "word1" is also slower than "word".

"fasterword"

(legacy) splits on whitespace and control characters, using stringi::stri_split_charclass(x, "[\\p{Z}\\p{C}]+")

"fastestword"

(legacy) splits on the space character, using stringi::stri_split_fixed(x, " ")

"character"

tokenization into individual characters

"sentence"

sentence segmenter based on stri_split_boundaries, but with additional rules to avoid splits on words like "Mr." that would otherwise incorrectly be detected as sentence boundaries. For better sentence tokenization, consider using spacyr.

Details

As of version 2, the choice of tokenizer is left more to the user, and tokens() is treated more as a constructor (from a named list) than a tokenizer. This allows users to use any other tokenizer that returns a named list, and to use this as an input to tokens(), with removal and splitting rules applied after this has been constructed (passed as arguments). These removal and splitting rules are conservative and will not remove or split anything, however, unless the user requests it.

Using external tokenizers is best done by piping the output from these other tokenizers into the tokens() constructor, with additional removal and splitting options applied at the construction stage. These will only have an effect, however, if the tokens exist for which removal is specified at in the tokens() call. For instance, it is impossible to remove punctuation if the input list to tokens() already had its punctuation tokens removed at the external tokenization stage.

To construct a tokens object from a list with no additional processing, call as.tokens() instead of tokens().

Recommended tokenizers are those from the tokenizers package, which are generally faster than the default (built-in) tokenizer but always splits infix hyphens, or spacyr.

See Also

tokens_ngrams(), tokens_skipgrams(), as.list.tokens(), as.tokens()

Examples

# NOT RUN {
txt <- c(doc1 = "A sentence, showing how tokens() works.",
         doc2 = "@quantedainit and #textanalysis https://example.com?p=123.",
         doc3 = "Self-documenting code??",
         doc4 = "<U+00A3>1,000,000 for 50<U+00A2> is gr8 4ever \U0001f600")
tokens(txt)
tokens(txt, what = "word1")

# removing punctuation marks but keeping tags and URLs
tokens(txt[1:2], remove_punct = TRUE)

# splitting hyphenated words
tokens(txt[3])
tokens(txt[3], split_hyphens = TRUE)

# symbols and numbers
tokens(txt[4])
tokens(txt[4], remove_numbers = TRUE)
tokens(txt[4], remove_numbers = TRUE, remove_symbols = TRUE)

# }
# NOT RUN {
# using other tokenizers
tokens(tokenizers::tokenize_words(txt[4]), remove_symbols = TRUE)
tokenizers::tokenize_words(txt, lowercase = FALSE, strip_punct = FALSE) %>%
    tokens(remove_symbols = TRUE)
tokenizers::tokenize_characters(txt[3], strip_non_alphanum = FALSE) %>%
    tokens(remove_punct = TRUE)
tokenizers::tokenize_sentences(
    "The quick brown fox.  It jumped over the lazy dog.") %>%
    tokens()
# }
# NOT RUN {
# }