Document-Term Matrix: Text Mining in R and Python

In text mining, it is important to create the document-term matrix (DTM) of the corpus we are interested in. A DTM is basically a matrix, with documents designated by rows and words by columns, that the elements are the counts or the weights (usually by tf-idf). Subsequent analysis is usually based creatively on DTM.

Exploring with DTM therefore becomes an important issues with a good text-mining tool. How do we perform exploratory data analysis on DTM using R and Python? We will demonstrate it using the data set of U. S. Presidents’ Inaugural Address, preprocessed, and can be downloaded here.

R: textmineR

In R, we can use the package textmineR, which has been in introduced in a previous post. Together with other packages such as dplyr (for tidy data analysis) and snowBall (for stemming), load all of them at the beginning:

library(dplyr)
library(textmineR)
library(SnowballC)

Load the datasets:

usprez.df<- read.csv('inaugural.csv', stringsAsFactors = FALSE)

Then we create the DTM, while we remove all digits and punctuations, make all letters lowercase, and stem all words using Porter stemmer.

dtm<- CreateDtm(usprez.df$speech,
                doc_names = usprez.df$yrprez,
                ngram_window = c(1, 1),
                lower = TRUE,
                remove_punctuation = TRUE,
                remove_numbers = TRUE,
                stem_lemma_function = wordStem)

Then defining a set of functions:

get.doc.tokens<- function(dtm, docid)
  dtm[docid, ] %>% as.data.frame() %>% rename(count=".") %>%
  mutate(token=row.names(.)) %>% arrange(-count)

get.token.occurrences<- function(dtm, token)
  dtm[, token] %>% as.data.frame() %>% rename(count=".") %>%
  mutate(token=row.names(.)) %>% arrange(-count)

get.total.freq<- function(dtm, token) dtm[, token] %>% sum

get.doc.freq<- function(dtm, token)
  dtm[, token] %>% as.data.frame() %>% rename(count=".") %>%
  filter(count>0) %>% pull(count) %>% length

Then we can happily extract information. For example, if we want to get the top-most common words in 2009’s Obama’s speech, enter:

dtm %>% get.doc.tokens('2009-Obama') %>% head(10)

Or which speeches have the word “change”: (but need to stem the word before extraction)

dtm %>% get.token.occurrences(wordStem('change')) %>% head(10)

You can also get the total number of occurrence of the words by:

dtm %>% get.doc.freq(wordStem('change'))   # gives 28

Python: shorttext

In Python, similar things can be done using the package shorttext, described in a previous post. It uses other packages such as pandas and stemming. Load all packages first:

import shorttext
import numpy as np
import pandas as pd
from stemming.porter import stem

import re

And define the preprocessing pipelines:

pipeline = [lambda s: re.sub('[^\w\s]', '', s),
            lambda s: re.sub('[\d]', '', s),
            lambda s: s.lower(),
            lambda s: ' '.join(map(stem, shorttext.utils.tokenize(s)))
 ]
txtpreproceesor = shorttext.utils.text_preprocessor(pipeline)

The function <code>txtpreprocessor</code> above perform the functions we talked about in R.

Load the dataset:

usprezdf = pd.read_csv('inaugural.csv')

The corpus needs to be preprocessed before putting into the DTM:

docids = list(usprezdf['yrprez'])    # defining document IDs
corpus = [txtpreproceesor(speech).split(' ') for speech in usprezdf['speech']]

Then create the DTM:

dtm = shorttext.utils.DocumentTermMatrix(corpus, docids=docids, tfidf=False)

Then we do the same thing as we have done above. To get the top-most common words in 2009’s Obama’s speech, enter:

dtm.get_doc_tokens('2009-Obama')

Or we look up which speeches have the word “change”:

dtm.get_token_occurences(stem('change'))

Or to get the document frequency of the word:

dtm.get_doc_frequency(stem('change'))

They Python and R codes give different document frequencies probably because the two stemmers work slightly differently.

Continue reading “Document-Term Matrix: Text Mining in R and Python”

textmineR: a New Text Mining Package for R

Previously, I wrote an entry on text mining on R and Python, and did a comparison. However, the text mining package employed was tm for R. But it has some problems:

  1. The syntax is not natural for an experienced R users.
  2. tm uses simple_triplet_matrix from the slam library for document-term matrix (DTM) and term-occurrence matrix (TCM), which is not as widely used as dgCMatrix from the Matrix library.

Tommy Jones, a Ph.D. student in George Mason University, and a data scientist at Impact Research, developed an alternative text mining package called textmineR. He presented in a Stat Prog DC Meetup on April 27, 2016. It employed a better syntax, and dgCMatrix. All in all, it is a wrapper for a lot of existing R packages to facilitate the text mining process, like creating DTM matrices with stopwords or appropriate stemming/lemmatizing functions. Here is a sample code to create a DTM with the example from the previous entry:

library(tm)
library(textmineR)

texts <- c('I love Python.',
           'R is good for analytics.',
           'Mathematics is fun.')

dtm<-CreateDtm(texts,
               doc_names = c(1:length(texts)),
               ngram_window = c(1, 1),
               stopword_vec = c(tm::stopwords('english'), tm::stopwords('SMART')),
               lower = TRUE,
               remove_punctuation = TRUE,
               remove_numbers = TRUE
               )

The DTM is a sparse matrix:

3 x 6 sparse Matrix of class &amp;quot;dgCMatrix&amp;quot;
  analytics fun mathematics good python love
1         .   .           .    .      1    1
2         1   .           .    1      .    .
3         .   1           1    .      .    .

On the other hand, it wraps text2vec, an R package that wraps the word-embedding algorithm named gloVe. And it wraps a number of topic modeling algorithms, such as latent Dirichlet allocation (LDA) and correlated topic models (CTM).

In addition, it contains a parallel computing loop function called TmParallelApply, analogous to the original R parallel loop function mclapply, but TmParallelApply works on Windows as well.

textmineR is an open-source project, with source code available on github, which contains his example codes.

Continue reading “textmineR: a New Text Mining Package for R”

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