Slides

Here a link to the lecture slides for this session: LINK

Overview

In this practical you’ll learn how to do natural language processing in R. By the end of this practical you will know how to:

  1. Derive word frequencies.
  2. Determine a term-frequency matrix
  3. Do sentiment analysis.

Functions

Here are some tools:

Function Description
paste(), paste0 Base function for combining strings
str_c() stringr function for combining strings
str_extract_all() stringr function for extracting strings using regular expressions

Here are the main read-in functions:

Function Description
read_file() Read flat csv file
readRDS() Read from R’s RDS format
file(...,'r'), readLines Read from file connection

Here are the main web-scraping functions (package rvest):

Function Description
read_html() Read html from web
read_nodes() Access tagged elements within the html document using, e.g., XPath
html_table() Extract a table from an html document

Here are the main tidytext (& dplyr) functions:

Function Description
unnest_tokens() Split text into words (tokens)
bind_tf_idf() Compute tf_idf weighting
get_sentiments Access sentiment data set
inner_join() Join words with, e.g., sentiments
anti_join() Eliminate words, e.g., stop_words

Tasks

This tutorial begins with an optional task for users who feel confident with programming in R and want to deal with the often complicated but necessary step of bringing the raw text data into the right shape for your analysis tools. As this will involve programming elements that we have not covered yet, I generally recommend to jump to Read in processed text data and to return to this section later if you have the time.

Read in raw text files (Advanced)

  1. Read in the subtitles for each episode of Game of Thrones using read_file() from the readr-package. To do this, first extract the file names of all of the files using list.files(path, full.names = TRUE). One way to achieve this quickly is by first creating a vector containing the subtitle’s folders using paste() and then by using the lapply()-function to iterate over the folders. The lapply()-function such as any other apply()-function (this will be covered in the Programming with R session) iterates the object provided as the first argument and applies a function provided as the second. Thus, you want to run a command similar to files <- lapply(folder_paths, list.files, full.names = TRUE). Note that any third arguments will be passed on to the function specified in the second argument.

  2. Extract text lines from subtitles. Begin by inspecting the text. Use str_sub() to print the first few hundred characters. Try to identify what characters precede the the spoken lines and which succeed. Think about how to build a regular expression that captures the end and stop points of the spoken line that also handles the many lines including not speech but comments. Evaluate the code below (find more info here). Try to understand why the regular expression looks that way. Use it to extract the text

# inspect
str_sub(got[[1]], 1, 1000)
[1] "1\r\n00:01:55,418 --> 00:01:58,420\r\nEasy, boy.\r\n\r\n2\r\n00:02:55,047 --> 00:02:56,881\r\n<i>( Gasps</i>\r\n\r\n3\r\n00:03:11,162 --> 00:03:13,331\r\n<i>What do you expect?</i>\r\n<i>They're savages.</i>\r\n\r\n4\r\n00:03:13,365 --> 00:03:15,700\r\nOne lot steals a goat\r\nfrom another lot,\r\n\r\n5\r\n00:03:15,734 --> 00:03:18,737\r\nbefore you know it they're ripping\r\neach other to pieces.\r\n\r\n6\r\n00:03:18,771 --> 00:03:20,939\r\nI've never seen wildlings\r\ndo a thing like this.\r\n\r\n7\r\n00:03:20,974 --> 00:03:23,275\r\nI never seen a thing like this,\r\nnot ever in my life.\r\n\r\n8\r\n00:03:23,310 --> 00:03:26,179\r\nHow close did you get?\r\n\r\n9\r\n00:03:26,213 --> 00:03:29,148\r\n- Close as any man would.\r\n- We should head back to the wall.\r\n\r\n10\r\n00:03:31,685 --> 00:03:33,319\r\nDo the dead frighten you?\r\n\r\n11\r\n00:03:33,353 --> 00:03:35,788\r\nOur orders were\r\nto track the wildlings.\r\n\r\n12\r\n00:03:35,822 --> 00:03:38,657\r\nWe tracked them.\r\nThey won't trouble us no more.\r\n\r\n13\r\n00:03:38,691 --> 00:03:41,360\r\nYou don't think\r\nhe'll ask us how the"
# extract data
got = str_extract_all(got, '(?<=\n)[^(][<i>]*[:alpha:]+[:control:]*[:print:]+(?=\r*\n)')
  1. Extract episode names from Wikipedia using the code below. Try to understand what the code does.
# define XPath locations of episode tables
paths = paste0('//*[@id="mw-content-text"]/div/table[',2:8,']')

# extract episode names
names = unlist(lapply(paths, function(x) {
  read_html('https://en.wikipedia.org/wiki/List_of_Game_of_Thrones_episodes') %>% 
    html_nodes(xpath = x) %>% 
    html_table() %>% 
    `[[`(1) %>%  
    `[[`(3) %>% 
    str_replace_all('"','')
  }))

4.Combine the extracted text, the episode names, their index in the season, and the season’s index inside a single tibble(). Use the code below. Try to understand what the code does.

# create tibbles
got = lapply(1:length(got), function(i){
  season = ceiling(i / 10)
  episode = i - ((season-1) * 10)
  tibble(season, episode,  title = names[i], text = got[[i]])
  })

# combine data frames
got = do.call(rbind, got)

Read in processed text data

  1. Read in the processed text data using readRDS() from today’s data sets.
got <- readRDS('data/game_of_thrones.RDS')

Segment and count words

  1. Extract the words from the subtitles using the amazingly fast and convenient unnest_tokens() from the tidytext package (install and load). Try using the pipe %>%. Evaluate the effect on the object. How much bigger have its dimensions become?
# install
# install.packages('tidytext')

# load
library(tidytext)
Warning: package 'tidytext' was built under R version 3.4.4
# tokenize
got_words <- got %>%
  unnest_tokens(word, text) 

# print
got
# A tibble: 63,263 x 4
   season episode title            text                              
    <dbl>   <dbl> <chr>            <chr>                             
 1     1.      1. Winter Is Coming Easy, boy.                        
 2     1.      1. Winter Is Coming <i>( Gasps</i>                    
 3     1.      1. Winter Is Coming <i>What do you expect?</i>        
 4     1.      1. Winter Is Coming <i>They're savages.</i>           
 5     1.      1. Winter Is Coming One lot steals a goat             
 6     1.      1. Winter Is Coming from another lot,                 
 7     1.      1. Winter Is Coming before you know it they're ripping
 8     1.      1. Winter Is Coming each other to pieces.             
 9     1.      1. Winter Is Coming do a thing like this.             
10     1.      1. Winter Is Coming not ever in my life.              
# ... with 63,253 more rows
got_words
# A tibble: 252,945 x 4
   season episode title            word  
    <dbl>   <dbl> <chr>            <chr> 
 1     1.      1. Winter Is Coming easy  
 2     1.      1. Winter Is Coming boy   
 3     1.      1. Winter Is Coming i     
 4     1.      1. Winter Is Coming gasps 
 5     1.      1. Winter Is Coming i     
 6     1.      1. Winter Is Coming i     
 7     1.      1. Winter Is Coming what  
 8     1.      1. Winter Is Coming do    
 9     1.      1. Winter Is Coming you   
10     1.      1. Winter Is Coming expect
# ... with 252,935 more rows
  1. Count words using the count()-function from the dplyr-package. Don’t forget to ungroup().
# count words
got_cnts <- got_words %>%
  count(season, episode, title, word) %>%
  ungroup()
  1. One of the most fundamental findings of Psycholinguistics is that there are few words that occur a lot and many that occur rarely. To be precise, the relationship between the words rank in the frequency distribution (with 1 indicating the rank with the highest frequency) and the actual frequency is commonly found to follow a power law roughly proportional to \(rank^{-alpha}\) where \(alpha\) is some value around 1. This relationship is know as Zipf’s law. Let’s see if Game of Thrones also shows Zipf’s law. Create a new variable that contains the rank of the frequency counts (remember rank 1 must be associated with the highest frequency) using rank(). Then plot the relationship between the log() of the rank and the log() of the frequency. If the plot looks roughly linear then the relationship follows a power law (because we are plotting in a log-log space).
# count words
got_cnts %>%
  mutate(rank = rank(-n)) %>%
  ggplot(aes(log(rank),log(n))) +
  geom_point() + 
  geom_smooth()
`geom_smooth()` using method = 'gam'

  1. Another fundamental finding of Psycholinguistics related to information science is that frequent words should have fewer numbers of characters in order to maximize the efficiency of communication. Analyze this relationship by adding a variable containing the word’s number of character symbols using nchar() and plotting its relationship to the words’ frequencies. Does Game of Thrones communicate efficiently?
# count words
got_cnts %>%
  mutate(nchar = nchar(word)) %>%
  ggplot(aes(nchar,log(n))) +
  geom_point() + 
  geom_smooth()
`geom_smooth()` using method = 'gam'

Term-document matrix

In this subsection we will be looking at the term document matrix. To do this, we will actually not be required to transform our data into a matrix, because of the way how tidyverse works. However, if you wanted to, you cast the table with counts to term-document or document-term matrices using cast_tdm() or cast_dtm(). This will create an object of class TermDocumentMatrix or DocumentTermMatrix and allow you to continue working with the tools included in the tm-package.

  1. Determine the 3 most important words for each episode based on raw word frequencies (using the title variable and top_n()).
# count words
most_frequent <- got_cnts %>%
  group_by(title, season, episode) %>%
  top_n(3) %>%
  ungroup()
Selecting by n
# plot
most_frequent %>% 
  arrange(season, episode, desc(n)) %>%
  mutate(word = as.factor(word),
         title_no = paste0(season,'_',episode,'_',title)) %>%
  ggplot(aes(word, n, fill = season)) +
  geom_col(show.legend = FALSE) +
  labs(x = NULL, y = "tf-idf") +
  facet_wrap(~title_no, ncol = 6, scales = "free") +
  coord_flip()