How to keep the beginning and end of sentence mark

问题:

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.

回答1:

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.

回答2:

How about an approach like this:

ngrams(
  tokenize(
    unlist(
      segment(text, what = "other", delimiter = "(?<=\\</s\\>)", perl = TRUE)),
    what = "fastestword", simplify = TRUE),
  n = 3L)

# [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>"     "formatted_properly!</s>_<s>I'm"
# [9] "properly!</s>_<s>I'm_a"         "<s>I'm_a_second"               
#[11] "a_second_sentence</s>"

Or, if you don't want ngrams that cross sentence boundaries:

unlist(
  ngrams(
    tokenize(
      unlist(
        segment(text, what = "other", delimiter = "(?<=\\</s\\>)", perl = TRUE)),
      what = "fastestword"),
    n = 3L))
#[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_a_second"           
#[9] "a_second_sentence</s>" 

I left the customizing options (e.g. removePunct = TRUE, etc.) up to you.