Finding ngrams in R and comparing ngrams across corpora

Question

I'm getting started with the tm package in R, so please bear with me and apologies for the big ol' wall of text. I have created a fairly large corpus of Socialist/Communist propaganda and would like to extract newly coined political terms (multiple words, e.g. "struggle-criticism-transformation movement").

This is a two-step question, one regarding my code so far and one regarding how I should go on.

Step 1: To do this, I wanted to identify some common ngrams first. But I get stuck very early on. Here is what I've been doing:

library(tm)
library(RWeka)

a  <-Corpus(DirSource("/mycorpora/1965"), readerControl = list(language="lat")) # that dir is full of txt files
summary(a)  
a <- tm_map(a, removeNumbers)
a <- tm_map(a, removePunctuation)
a <- tm_map(a , stripWhitespace)
a <- tm_map(a, tolower)
a <- tm_map(a, removeWords, stopwords("english")) 
a <- tm_map(a, stemDocument, language = "english") 
# everything works fine so far, so I start playing around with what I have
adtm <-DocumentTermMatrix(a) 
adtm <- removeSparseTerms(adtm, 0.75)

inspect(adtm) 

findFreqTerms(adtm, lowfreq=10) # find terms with a frequency higher than 10

findAssocs(adtm, "usa",.5) # just looking for some associations  
findAssocs(adtm, "china",.5)

# ... and so on, and so forth, all of this works fine

The corpus I load into R works fine with most functions I throw at it. I haven't had any problems creating TDMs from my corpus, finding frequent words, associations, creating word clouds and so on. But when I try to use identify ngrams using the approach outlined in the tm FAQ, I'm apparently making some mistake with the tdm-constructor:

# Trigram

TrigramTokenizer <- function(x) NGramTokenizer(x, 
                                Weka_control(min = 3, max = 3))

tdm <- TermDocumentMatrix(a, control = list(tokenize = TrigramTokenizer))

inspect(tdm)

I get this error message:

Error in rep(seq_along(x), sapply(tflist, length)) : 
invalid 'times' argument
In addition: Warning message:
In is.na(x) : is.na() applied to non-(list or vector) of type 'NULL'

Any ideas? Is "a" not the right class/object? I'm confused. I assume there's a fundamental mistake here, but I'm not seeing it. :(

Step 2: Then I would like to identify ngrams that are significantly overrepresented, when I compare the corpus against other corpora. For example I could compare my corpus against a large standard english corpus. Or I create subsets that I can compare against each other (e.g. Soviet vs. a Chinese Communist terminology). Do you have any suggestions how I should go about doing this? Any scripts/functions I should look into? Just some ideas or pointers would be great.

Thanks for your patience!

Answer 1

I could not reproduce your problem, are you using the latest versions of R, tm, RWeka, etc.?

require(tm)
a <- Corpus(DirSource("C:\\Downloads\\Only1965\\Only1965"))
summary(a)  
a <- tm_map(a, removeNumbers)
a <- tm_map(a, removePunctuation)
a <- tm_map(a , stripWhitespace)
a <- tm_map(a, tolower)
a <- tm_map(a, removeWords, stopwords("english")) 
# a <- tm_map(a, stemDocument, language = "english") 
# I also got it to work with stemming, but it takes so long...
adtm <-DocumentTermMatrix(a) 
adtm <- removeSparseTerms(adtm, 0.75)

inspect(adtm) 

findFreqTerms(adtm, lowfreq=10) # find terms with a frequency higher than 10
findAssocs(adtm, "usa",.5) # just looking for some associations  
findAssocs(adtm, "china",.5)

# Trigrams
require(RWeka)
TrigramTokenizer <- function(x) NGramTokenizer(x, Weka_control(min = 3, max = 3))
tdm <- TermDocumentMatrix(a, control = list(tokenize = TrigramTokenizer))
tdm <- removeSparseTerms(tdm, 0.75)
inspect(tdm[1:5,1:5])

And here's what I get

A term-document matrix (5 terms, 5 documents)

Non-/sparse entries: 11/14
Sparsity           : 56%
Maximal term length: 28 
Weighting          : term frequency (tf)

                                   Docs
Terms                               PR1965-01.txt PR1965-02.txt PR1965-03.txt
  †chinese press                              0             0             0
  †renmin ribao                               0             1             1
  — renmin ribao                              2             5             2
  “ chinese people                            0             0             0
  “renmin ribaoâ€\u009d editorial             0             1             0
  etc. 

Regarding your step two, here are some pointers to useful starts:

http://quantifyingmemory.blogspot.com/2013/02/mapping-significant-textual-differences.html http://tedunderwood.com/2012/08/14/where-to-start-with-text-mining/ and here's his code https://dl.dropboxusercontent.com/u/4713959/Neuchatel/NassrProgram.R