dfm: create a document-feature matrix

Description

Construct a sparse document-feature matrix, from a character, corpus, or tokens object.

Usage

dfm(x, tolower = TRUE, stem = FALSE, select = NULL, remove = NULL, thesaurus = NULL, dictionary = NULL, valuetype = c("glob", "regex", "fixed"), groups = NULL, verbose = FALSE, ...)

Arguments

character, corpus, or tokens object

tolower

convert all tokens to lowercase

stem

if TRUE, stem words

select

a user supplied regular expression defining which features to keep, while excluding all others. This can be used in lieu of a dictionary if there are only specific features that a user wishes to keep. To extract only Twitter usernames, for example, set select = "@*" and make sure that removeTwitter = FALSE as an additional argument passed to tokenize. Note: select = "^@\\w+\\b" would be the regular expression version of this matching pattern. The pattern matching type will be set by valuetype.

remove

a character vector of user-supplied features to ignore, such as "stop words". To access one possible list (from any list you wish), use stopwords(). The pattern matching type will be set by valuetype. For behaviour of remove with ngrams > 1, see Details.

thesaurus

A list of character vector "thesaurus" entries, in a dictionary list format, which operates as a dictionary but without excluding values not matched from the dictionary. Thesaurus keys are converted to upper case to create a feature label in the dfm, as a reminder that this was not a type found in the text, but rather the label of a thesaurus key. For more fine-grained control over this and other aspects of converting features into dictionary/thesaurus keys from pattern matches to values, you can use dfm_lookup after creating the dfm.

dictionary

A list of character vector dictionary entries, including regular expressions (see examples)

valuetype

how to interpret keyword expressions: "glob" for "glob"-style wildcard expressions; "regex" for regular expressions; or "fixed" for exact matching. See valuetype for details.

groups

character vector containing the names of document variables for aggregating documents; only applies when calling dfm on a corpus object

verbose

display messages if TRUE

...

additional arguments passed to tokens, for character and corpus

Value

a dfm-class object

Details

The default behavior for remove/select when constructing ngrams using dfm(x, ngrams > 1) is to remove/select any ngram constructed from a matching feature. If you wish to remove these before constructing ngrams, you will need to first tokenize the texts with ngrams, then remove the features to be ignored, and then construct the dfm using this modified tokenization object. See the code examples for an illustration.

Examples

Run this code

## for a corpus

corpus_post80inaug <- corpus_subset(data_corpus_inaugural, Year > 1980)
dfm(corpus_post80inaug)
dfm(corpus_post80inaug, tolower = FALSE)

# grouping documents by docvars in a corpus
dfm(corpus_post80inaug, groups = "President", verbose = TRUE)

# with English stopwords and stemming
dfm(corpus_post80inaug, remove = stopwords("english"), stem = TRUE, verbose = TRUE)
# works for both words in ngrams too
dfm("Banking industry", stem = TRUE, ngrams = 2, verbose = FALSE)

# with dictionaries
corpus_post1900inaug <- corpus_subset(data_corpus_inaugural, Year>1900)
mydict <- dictionary(list(christmas = c("Christmas", "Santa", "holiday"),
               opposition = c("Opposition", "reject", "notincorpus"),
               taxing = "taxing",
               taxation = "taxation",
               taxregex = "tax*",
               country = "states"))
dfm(corpus_post1900inaug, dictionary = mydict)

# with the thesaurus feature
mytexts <- c("The new law included a capital gains tax, and an inheritance tax.",
             "New York City has raised a taxes: an income tax and a sales tax.")
mydict <- dictionary(list(tax=c("tax", "income tax", "capital gains tax", "inheritance tax")))
dfm(phrasetotoken(mytexts, mydict), thesaurus = lapply(mydict, function(x) gsub("\\s", "_", x)))
# pick up "taxes" with "tax" as a regex
dfm(phrasetotoken(mytexts, mydict), thesaurus = list(anytax = "tax"), valuetype = "regex")

# removing stopwords
testText <- "The quick brown fox named Seamus jumps over the lazy dog also named Seamus, with
             the newspaper from a boy named Seamus, in his mouth."
testCorpus <- corpus(testText)
# note: "also" is not in the default stopwords("english")
featnames(dfm(testCorpus, select = stopwords("english")))
# for ngrams
featnames(dfm(testCorpus, ngrams = 2, select = stopwords("english"), removePunct = TRUE))
featnames(dfm(testCorpus, ngrams = 1:2, select = stopwords("english"), removePunct = TRUE))

## removing stopwords before constructing ngrams
tokensAll <- tokens(char_tolower(testText), removePunct = TRUE)
tokensNoStopwords <- removeFeatures(tokensAll, stopwords("english"))
tokensNgramsNoStopwords <- tokens_ngrams(tokensNoStopwords, 2)
featnames(dfm(tokensNgramsNoStopwords, verbose = FALSE))

# keep only certain words
dfm(testCorpus, select = "*s", verbose = FALSE)  # keep only words ending in "s"
dfm(testCorpus, select = "s$", valuetype = "regex", verbose = FALSE)

# testing Twitter functions
testTweets <- c("My homie @justinbieber #justinbieber shopping in #LA yesterday #beliebers",
                "2all the ha8ers including my bro #justinbieber #emabiggestfansjustinbieber",
                "Justin Bieber #justinbieber #belieber #fetusjustin #EMABiggestFansJustinBieber")
dfm(testTweets, select = "#*", removeTwitter = FALSE)  # keep only hashtags
dfm(testTweets, select = "^#.*$", valuetype = "regex", removeTwitter = FALSE)