R tm document names missing - r

Question

Using R{tm} package, i create a corpus, per usual:
mycorpus <- Corpus(DirSource(folder,pattern="txt"))
Please note I am not using an encoding variable. The summary (mycorpus) shows document names listed. However after a series of tm_map transforms:
(content_transformer(tolower),content_transformer(removeWords), stopwords("SMART"),stripWhitespace)
ending with mycorpus<- tm_map(mycorpus, PlainTextDocument) and mydtm <- DocumentTermMatrix(mycorpus, control = list(...))
I get an error with inspect(mydtm[1:10, intersect(colnames(dtm), 'toyota')]) to get my variable of choice:
Terms
Docs toyota
character(0) 0
character(0) 0
character(0) 0
character(0) 0
character(0) 1
character(0) 0
character(0) 0
character(0) 0
character(0) 1
character(0) 0
The file names (doc ids) have disappeared. Any idea what could be causing this error? more importantly, how do i reinstate the document names? Many thanks.

Answer 1

Code below will work for single file. You likely could use something like list.files to read all files in the directory.
First, I would wrap the cleaning functions in a custom function. Note the order matters and you have to use content_transformer if the function is not from tm.
clean.corpus<-function(corpus){
corpus <- tm_map(corpus, removePunctuation)
corpus <- tm_map(corpus, stripWhitespace)
corpus <- tm_map(corpus, removeNumbers)
corpus <- tm_map(corpus, content_transformer(tolower))
corpus <- tm_map(corpus, removeWords, custom.stopwords)
return(corpus)
}
Then concatenate English words with custom words. This is passed as the last part of the custom function above.
custom.stopwords <- c(stopwords('english'), 'lol', 'smh')
doc<-read.csv('coffee.csv', header=TRUE)
The CSV is a data frame with a column of tweets in a text document and another column with an ID for each tweet. The file from my workshop with this file is here.
The csv file is now in memory so next step is to read it in tabular fashion with a specific mapping when making a corpus. Here the content is in a column called text and the unique ID is in a column name "id".
custom.reader <- readTabular(mapping=list(content="text", id="id"))
corpus <- VCorpus(DataframeSource(doc), readerControl=list(reader=custom.reader))
corpus<-clean.corpus(corpus)
The corpus creation uses the readerControl and then once done you can apply the pre-processing steps. Without the reader control the package assigns the 0 character as the name.
The corpus content of document 1 can be accessed here
corpus[[1]][1]
You can review the corpus meta data for the first document with this code
corpus[[1]][2]
So I think you are needing to use readTabular and readerControl in your corpus construction no matter the source.

Answer 2

I was having the same problem and I realized that it was due to tolower. tolower, unlike removeNumbers, removePunctuation, removeWords, stemDocument, stripWhitespace are not tranformations defined in the tm package. To get a list of transformations defined in the tm package that can be directly applied to a corpus, type:
getTransformations()
[1] “removeNumbers” “removePunctuation” “removeWords” “stemDocument” “stripWhitespace”
Thus, in order to use tolower it first must make a transformation for tolower for it to handle corpus objects properly.
docs <- tm_map(docs,content_transformer(tolower))
The above line of code should stop the files from being renamed to character(0)
The same trick can be applied to any R function to work with corpuses. For example for gsub, the following syntax applies:
docs <- tm_map(docs, content_transformer(gsub), pattern = “internt”, replacement = “internet”)