I'm trying to create 3-grams using R's quanteda package.
I'm struggling to find a way to keep in the n-grams beginning and end of sentence markers, the <s> and </s> as in the code below.
I thought that using the keptFeatures with a regular expression that matched those should maintain them but the chevron markers are always removed.
How can I keep the chevron markers from being removed or what is the best way to delimit beginning and end of sentence with quanteda?
As a bonus question what is the advantage of docfreq(mydfm) over colSums(mydfm), the result of str(colSums(mydfm)) and str(docfreq(mydfm)) is almost identical (Named num [1:n] the former, Named int [1:n] the latter)?
library(quanteda)
text <- "<s>I'm a sentence and I'd better be formatted properly!</s><s>I'm a second sentence</s>"
qc <- corpus(text)
mydfm <- dfm(qc, ngram=3, removeNumbers = F, stem=T, keptFeatures="\\</?s\\>")
names(colSums(mydfm))
# Output:
# [1] "s_i'm_a" "i'm_a_sentenc" "a_sentenc_and" "sentenc_and_i'd"
# [2] "and_i'd_better" "i'd_better_be" "better_be_format"
# [3] "be_format_proper" "format_proper_s" "proper_s_s" "s_s_i'm"
# [4] "i'm_a_second" "a_second_sentenc" "second_sentenc_s"
EDIT:
Corrected keepFeatures to keptFeatures in code snippet.
To return a simple vector, just unlist the tokenizedText" object returned fromtokenize()(which is a specially classed list, with additional attributes). Here I used thewhat = "fasterword"which splits on "\\s" -- it's a tiny bit smarter thanwhat = "fastestword"which splits on" "`.
# how to not remove the <s>, and return a vector
unlist(toks <- tokenize(text, ngrams = 3, what = "fasterword"))
## [1] "<s>I'm_a_sentence" "a_sentence_and"
## [3] "sentence_and_I'd" "and_I'd_better"
## [5] "I'd_better_be" "better_be_formatted"
## [7] "be_formatted_properly!</s><s>I'm" "formatted_properly!</s><s>I'm_a"
## [9] "properly!</s><s>I'm_a_second" "a_second_sentence</s>"
To keep it within sentence, tokenise the object twice, the first time by sentence, the second time by fasterword.
# keep it within sentence
(sents <- unlist(tokenize(text, what = "sentence")))
## [1] "<s>I'm a sentence and I'd better be formatted properly!"
## [2] "</s><s>I'm a second sentence</s>"
tokenize(sents, ngrams = 3, what = "fasterword")
## tokenizedText object from 2 documents.
## Component 1 :
## [1] "<s>I'm_a_sentence" "a_sentence_and" "sentence_and_I'd" "and_I'd_better"
## [5] "I'd_better_be" "better_be_formatted" "be_formatted_properly!"
##
## Component 2 :
## [1] "</s><s>I'm_a_second" "a_second_sentence</s>"
To preserve the chevron markers in a dfm, you can pass through the same options that you used above in the tokenize() call, since dfm() calls tokenize() but with different defaults -- it uses the ones most users will probably want, whereas tokenize() is much more conservative.
# Bonus questions:
myDfm <- dfm(text, verbose = FALSE, what = "fasterword", removePunct = FALSE)
# "chevron" markers are not removed
features(myDfm)
## [1] "<s>i'm" "a" "sentence" "and" "i'd"
## [6] "better" "be" "formatted" "properly!</s><s>i'm" "second"
## [11] "sentence</s>"
Final part of the bonus question was the difference between docfreq() and colSums(). The former returns the count of documents in which a term occurs, the latter sums the columns to get a total term frequency across documents. See below how different these are for the term "representatives".
# Difference between docfreq() and colSums():
myDfm2 <- dfm(inaugTexts[1:4], verbose = FALSE)
myDfm2[, "representatives"]
docfreq(myDfm2)["representatives"]
colSums(myDfm2)["representatives"]
## Document-feature matrix of: 4 documents, 1 feature.
## 4 x 1 sparse Matrix of class "dfmSparse"
## features
## docs representatives
## 1789-Washington 2
## 1793-Washington 0
## 1797-Adams 2
## 1801-Jefferson 0
docfreq(myDfm2)["representatives"]
## representatives
## 2
colSums(myDfm2)["representatives"]
## representatives
## 4
Update: Some commands and behaviours have changed in quanteda v0.9.9:
Return a simple vector, retaining chevrons:
as.character(toks <- tokens(text, ngrams = 3, what = "fasterword"))
# [1] "<s>I'm_a_sentence" "a_sentence_and" "sentence_and_I'd"
# [4] "and_I'd_better" "I'd_better_be" "better_be_formatted"
# [7] "be_formatted_properly!</s><s>I'm" "formatted_properly!</s><s>I'm_a" "properly!</s><s>I'm_a_second"
# [10] "a_second_sentence</s>"
Keeping within sentence:
(sents <- as.character(tokens(text, what = "sentence")))
# [1] "<s>I'm a sentence and I'd better be formatted properly!" "</s><s>I'm a second sentence</s>"
tokens(sents, ngrams = 3, what = "fasterword")
# tokens from 2 documents.
# Component 1 :
# [1] "<s>I'm_a_sentence" "a_sentence_and" "sentence_and_I'd" "and_I'd_better" "I'd_better_be"
# [6] "better_be_formatted" "be_formatted_properly!"
#
# Component 2 :
# [1] "</s><s>I'm_a_second" "a_second_sentence</s>"
Bonus question part 1:
featnames(dfm(text, verbose = FALSE, what = "fasterword", removePunct = FALSE))
# [1] "<s>i'm" "a" "sentence" "and" "i'd"
# [6] "better" "be" "formatted" "properly!</s><s>i'm" "second"
# [11] "sentence</s>"
Bonus question part 2 is unchanged.