Category: text mining

Regular Expressions in R – Part 1: Introduction and base R functions

The following is the first part of my introduction to regular expression (regex), in general, and the use of regex in R, in specific. It is loosely inspired on the swirl() tutorial by Jon Calder. I created it in R Markdown and uploaded it to RPubs, for an easier read.

Regular expression

A regular expression, regex or regexp (sometimes called a rational expression) is, in theoretical computer science and formal language theory, a sequence of characters that define a search pattern. Usually this pattern is then used by string searching algorithms for “find” or “find and replace” operations on strings (Wikipedia). Regular expressions were originally developed for the Perl language and have since been implemented in many other languages including R.

Regular expressions usually involve two parts: a pattern and a text string. The pattern defines what type and/or sequence of characters to look for whereas the text string represents the content in which to search/match this pattern. Patterns are always strings themselves and thus need to be enclosed in (single or double) quotation marks.

Example

An example: the pattern “stat” will match the occurance of the letters “s”, “t”, “a”, “t” in that specific order. Regardless of where in the content (text string) they occur and what other characters may precede the “s” or follow the last “t”.

Base R’s grepl() function returns a logical value reflecting whether the pattern is matched. The below demonstrates how the pattern “stats” can be found in both “statistics” and “estate” but not in “castrate” (which does include the letters, but with an r in between), in “catalyst” (which does include the letters, but not in the right order), or in “banana” (which does not include all the letters).

words = c("statistics", "estate", "castrate", "catalyst", "banana")
grepl(pattern = "stat", x = words)

## [1]  TRUE  TRUE FALSE FALSE FALSE

Moreover, regular expressions are case sensitive, so “stat” is not found in “Statistics”, unless it is specified that case should be ignored (FALSE by default).

grepl(pattern = "stat", x = "Statistics")

## [1] FALSE

grepl(pattern = "stat", x = "Statistics", ignore.case = TRUE)

## [1] TRUE

Regular Expressions in Base R

Base R includes seven main functions that use regular expressions with different outcomes. These are grep(), grepl(), regexpr(), gregexpr(), regexec(), sub(), and gsub(). Although they require mostly similar inputs, their returned values are quite different.

`grep()` & `grepl()`

grep() examines each element of a character vector and returns the indices where the pattern is matched.

sentences = c("I like statistics", "I like bananas", "Estates and statues are expensive")
grep("stat", sentences)

## [1] 1 3

By setting the value parameter to TRUE, grep() will return the character element instead of its index.

grep("stat", sentences, value = TRUE)

## [1] "I like statistics"                 "Estates and statues are expensive"

It’s logical brother grepl() you’ve seen before. It returns a logical value instead of the index or the element.

grepl("stat", sentences)

## [1]  TRUE FALSE  TRUE

`regexpr()` & `gregexpr()`

regexpr() seeks for a pattern in a text and returns an integer vector with two attributes (also vectors). The main integer vector returned represents the position where the pattern was first matched in the text. Its attribute “match.length” is also an integer vector representing the length of the match (in this case “stat” is always length 4).

If the pattern is not matched, both of the main vector and the length attribute will have a value of -1.

The second attribute (“useBytes”) is always a logical vector of length one. It represents whether matching is done byte-by-byte (TRUE) or character-by-character (FALSE), but you may disregard it for now.

sentences

## [1] "I like statistics"                 "I like bananas"                   
## [3] "Estates and statues are expensive"

regexpr("stat", sentences)

## [1]  8 -1  2
## attr(,"match.length")
## [1]  4 -1  4
## attr(,"useBytes")
## [1] TRUE

Note that, for the third sentence, regexpr() only returns the values for the first match (i.e., “Estate”) but not those of the second match (i.e., “statues”). For this reason, the function has a brother, gregexpr(), which has the same functionality but performs the matching on a global scale (hence the leading g). This means that the algorithm does not stop after its first match, but continues and reports all matches within the content string.

grepexpr() thus does not return a single vector, but a list of vectors. Each of these vectors reflects an input content string as is the length of the number of matches within that content. For example, the “stat” pattern is matched twice in our third sentence, therefore its vector is length 2, with the starting position of each match as well as their lengths.

sentences

## [1] "I like statistics"                 "I like bananas"                   
## [3] "Estates and statues are expensive"

gregexpr("stat", sentences)

## [[1]]
## [1] 8
## attr(,"match.length")
## [1] 4
## attr(,"useBytes")
## [1] TRUE
## 
## [[2]]
## [1] -1
## attr(,"match.length")
## [1] -1
## attr(,"useBytes")
## [1] TRUE
## 
## [[3]]
## [1]  2 13
## attr(,"match.length")
## [1] 4 4
## attr(,"useBytes")
## [1] TRUE

`()`

In order to explain how regexec() differs from gregexpr(), we first need to explain how parentheses in work in regex. Most simply speaking, parentheses or round brackets (()) indicate groups. One of the advantages of groups is that logical tests can thus be conducted within regular expressions.

sentences

## [1] "I like statistics"                 "I like bananas"                   
## [3] "Estates and statues are expensive"

grepl("like", sentences)

## [1]  TRUE  TRUE FALSE

grepl("are", sentences)

## [1] FALSE FALSE  TRUE

grepl("(are|like)", sentences)

## [1] TRUE TRUE TRUE

`regexec()`

However, these groups can also be useful to extract more detailed information from a regular expression. This is where regexec() comes in.

Like gregexpr(), regexec() returns a list of the same length as the content. This list includes vectors that reflect the starting positions of the overall match, as well as the matches corresponding to parenthesized subpatterns. Similarly, attribute “match.length” reflects the lengths of each of the overall and submatches. In case no match is found, a -1 value is again returned.

The beauty of regexec() because clear when we split our pattern into two groups using parentheses: “(st)(at)”. As you can see below, both regexpr() and its global brother gregexpr() disregard this grouping and provide the same output as before – as you would expect for the pattern “stat”. In contast, regexec() notes that we now have a global pattern (“stat”)as well as two subpatterns (“st” and “at”). For each of these, the function returns the starting positions as well as the pattern lengths.

sentences

## [1] "I like statistics"                 "I like bananas"                   
## [3] "Estates and statues are expensive"

regexpr("(st)(at)", sentences)

## [1]  8 -1  2
## attr(,"match.length")
## [1]  4 -1  4
## attr(,"useBytes")
## [1] TRUE

gregexpr("(st)(at)", sentences)

## [[1]]
## [1] 8
## attr(,"match.length")
## [1] 4
## attr(,"useBytes")
## [1] TRUE
## 
## [[2]]
## [1] -1
## attr(,"match.length")
## [1] -1
## attr(,"useBytes")
## [1] TRUE
## 
## [[3]]
## [1]  2 13
## attr(,"match.length")
## [1] 4 4
## attr(,"useBytes")
## [1] TRUE

regexec("(st)(at)", sentences)

## [[1]]
## [1]  8  8 10
## attr(,"match.length")
## [1] 4 2 2
## attr(,"useBytes")
## [1] TRUE
## 
## [[2]]
## [1] -1
## attr(,"match.length")
## [1] -1
## attr(,"useBytes")
## [1] TRUE
## 
## [[3]]
## [1] 2 2 4
## attr(,"match.length")
## [1] 4 2 2
## attr(,"useBytes")
## [1] TRUE

`sub()` & `gsub()`

The final two base regex functions are sub() and its global brother gsub(). These, very intiutively, substitute a matched pattern by a specified replacement and then return all inputs. For instance, we could replace “I” with “You” in our example sentences.

sub(pattern = "I", replacement = "You", sentences)

## [1] "You like statistics"               "You like bananas"                 
## [3] "Estates and statues are expensive"

Similarly, we could desire to replace all spaces by underscores. This would require a global search (i.e., gsub()), as sub() would stop after the first match.

sub(pattern = " ", replacement = "_", sentences)

## [1] "I_like statistics"                 "I_like bananas"                   
## [3] "Estates_and statues are expensive"

gsub(pattern = " ", replacement = "_", sentences)

## [1] "I_like_statistics"                 "I_like_bananas"                   
## [3] "Estates_and_statues_are_expensive"

This was the first part of my introduction to Regular Expression in R. For more information detailed information about all input parameters of each function, please consult the base R manual. In subsequent parts, I will introduce you to so-called Anchors, Character Classes, Groups, Ranges, and Quantifiers. These will allow you to perform more advanced searches and matches. Here, we will also elaborate on lazy, greedy, and possesive regular expressions, which further expand our search capability as well as flexibility.

In the end, I hope to provide you with an overview of several Regular Expressions that I have found extremely useful in my personal project, and which should be valuable to anyone who conducts applied research (in organizations).

Text Mining: Pythonic Heavy Metal

This blog summarized work that has been posted here, here, and here.

Iain of degeneratestate.org wrote a three-piece series where he applied text mining to the lyrics of 222,623 songs from 7,364 heavy metal bands spread over 22,314 albums that he scraped from darklyrics.com. He applied a broad range of different analyses in Python, the code of which you can find here on Github.

For example, he starts part 1 by calculated the difficulty/complexity of the lyrics of each band using the Simple Measure of Gobbledygook or SMOG and contrasted this to the number of swearwords used, finding a nice correlation.

Ratio of swear words vs readability — Lyric complexity relates positive to swearwords used.

Furthermore, he ran some word importance analysis, looking at word frequencies, log-likelihood ratios, and TF-IDF scores. This allowed him to contrast the word usage of the different bands, finding, for instance, one heavy metal band that was characterized by the words “oh yeah baby got love“: fans might recognize either Motorhead, Machinehead, or Diamondhead.

Using cosine distance measures, Iain could compare the word vectors of the different bands, ultimately recognizing band similarity, and song representativeness for a band. This allowed interesting analysis, such as a clustering of the various bands:

However, all his analysis worked out nicely. While he also applied t-SNE to visualize band similarity in a two-dimensional space, the solution was uninformative due to low variance in the data.

He could predict the band behind a song by training a one-vs-rest logistic regression classifier based on the reduced lyric space of 150 dimensions after latent semantic analysis. Despite classifying a song to one of 120 different bands, the classifier had a precision and recall both around 0.3, with negligible hyper parameter tuning. He used the classification errors to examine which bands get confused with each other, and visualized this using two network graphs.

In part 2, Iain tried to create a heavy metal lyric generator (which you can now try out).

His first approach was to use probabilistic distributions known as language models. Basically he develops a Markov Chain, in his opinion more of a “unsmoothed maximum-likelihood language model“, which determines the next most probable word based on the previous word(s). This model is based on observed word chains, for instance, those in the first two lines to Iron Maiden’s Number of the Beast:

Another approach would be to train a neural network. Iain used Keras, which ran on an amazon GPU instance. He recognizes the power of neural nets, but says they also come at a cost:

“The maximum likelihood models we saw before took twenty minutes to code from scratch. Even using powerful libraries, it took me a while to understand NNs well enough to use. On top of this, training the models here took days of computer time, plus more of my human time tweeking hyper parameters to get the models to converge. I lack the temporal, financial and computational resources to fully explore the hyperparameter space of these models, so the results presented here should be considered suboptimal.” – Iain

He started out with feed forward networks on a character level. His best try consisted of two feed forward layers of 512 units, followed by a softmax output, with layer normalisation, dropout and tanh activations, which he trained for 20 epochs to minimise the mean cross-entropy. Although it quickly beat the maximum likelihood Markov model, its longer outputs did not look like genuine heavy metal songs.

So he turned to recurrent neural network (RNN). The RNN Iain used contains two LSTM layers of 512 units each, followed by a fully connected softmax layer. He unrolled the sequence for 32 characters and trained the model by predicting the next 32 characters, given their immediately preceding characters, while minimizing the mean cross-entropy:

“To generate text from the RNN model, we step character-by-character through a sequence. At each step, we feed the current symbol into the model, and the model returns a probability distribution over the next character. We then sample from this distribution to get the next character in the sequence and this character goes on to become the next input to the model. The first character fed into the model at the beginning of generation is always a special start-of-sequence character.” – Iain

This approach worked quite well, and you can compare and contrast it with the earlier models here. If you’d just like to generate some lyrics, the models are hosted online at deepmetal.io.

In part 3, Iain looks into emotional arcs, examining the happiness and metalness of words and lyrics.

When applied to the combined lyrics of albums, you could examine how bands developed their signature sound over time. For example, the lyrics of Metallica’s first few albums seem to be quite heavy metal and unhappy, before moving to a happier place. The Black album is almost sentiment-neutral, but after that they became ever more darker and more metal, moving back to the style to their first few albums. He applied the same analysis on the text of the Harry Potter books, of which especially the first and last appear especially metal.

Analysis of Media Coverage on Refugees

Hannah Yan Han is doing #100dayprojects on data science and visual storytelling and I can only recommend that you take a look yourself. Below you find her R text analysis (#41) of UNHCR speeches and TV coverage on refugees.

Unsurprisingly, nouns like asylum, repatriation, displacement, persecution, plight, and crisis appear significantly more often in UNHCR speeches on refugees than in general English texts. The first visualization below shows the action-oriented verbs most commonly used in combination with these nouns.

This second visualization shows the most occurring verb-noun pairs.

Hannah used newsflash to retrieve the GDELT data on US TV news. Some channels seem to cover refugees more than others. I would have loved to see which topics occurred on each channel, but unfortunately she did not report on this.

Quantifying Gastronomy

A statistical analysis of 4000 recipes and their ingredients: Quantifying Gastronomy

Copy of Quantifying relative perceptions.pptx — Hierarchical clustering of ingredients. The more to the right an ingredient is, the more common it is.

Summarizing our Daily News: Clustering 100.000+ Articles in Python

Andrew Thompson was interested in what 10 topics a computer would identify in our daily news. He gathered over 140.000 new articles from the archives of 10 different sources, as you can see in the figure below.

The sources of the news articles used in the analysis.

In Python, Andrew converted the text of all these articles into a manageable form (tf-idf document term matrix (see also Harry Plotter: Part 2)), reduced these data to 100 dimensions using latent semantic analysis (singular value decomposition), and ran a k-means clustering to retrieve the 10 main clusters. I included his main results below, but I highly suggest you visit the original article on Medium as Andrew used Plotly to generate interactive plots!

newplot — Most important words per topic (interactive visual in original article)

The topics structure seems quite nice! Topic 0 involves legal issues, such as immigration, whereas topic 1 seems to be more about politics. Topic 8 is clearly sports whereas 9 is education. Next, Andres inspected which media outlet covers which topics most. Again, visit the original article for interactive plots!

newplot (1).png — Media outlets and the topics they cover (interactive version in original article)

In light of the fake news crisis and the developments in (internet) media, I believe Andrew’s conclusions on these data are quite interesting.

I suppose different people could interpret this data and these graphs differently, but I interpret them as the following: when forced into groups, the publications sort into Reuters and everything else.

[…]

Every publication in this dataset except Reuters shares some common denominators. They’re entirely funded on ads and/or subscriptions (Vox and BuzzFeed also have VC funding, but they’re ad-based models), and their existence relies on clicks. By contrast, Reuters’s news product is merely the public face of a massive information conglomerate. Perhaps more importantly, it’s a news wire whose coverage includes deep reporting on the affairs of our financial universe, and therefore is charged with a different mandate than the others — arguably more than the New York Times, it must cover all the news, without getting trapped in the character driven reality-TV spectacle that every other citizen of the dataset appears to so heavily relish in doing. Of them all, its voice tends to maintain the most moderate indoor volume, and no single global event provokes larger-than-life outrage, if outrage can be provoked from Reuters at all. Perhaps this is the product of belonging to the financial press and analyzing the world macroscopically; the narrative of the non-financial press fails to accord equal weight to a change in the LIBOR rate and to the policy proposals of a madman, even though it arguably should. Every other publication here seems to bear intimations of utopia, and the subtext of their content is often that a perfect world would materialize if we mixed the right ingredients in the recipe book, and that the thing you’re outraged about is actually the thing standing between us and paradise. In my experience as a reader, I’ve never felt anything of the sort emanate from Reuters.

This should not be interpreted as asserting that the New York Times and Breitbart are therefore identical cauldrons of apoplexy. I read a beautifully designed piece today in the Times about just how common bioluminescence is among deep sea creatures. It goes without saying that the prospect of finding a piece like that in Breitbart is nonexistent, which is one of the things I find so god damned sad about that territory of the political spectrum, as well as in its diametrical opponents a la Talking Points Memo. But this is the whole point: show an algorithm the number of stories you write about deep sea creatures and it’ll show you who you are. At a finer resolution, we would probably find a chasm between the Times and Fox News, or between NPR and the New York Post. See that third cluster up there, where all the words are kind of compressed with lower TfIdf values and nothing sticks out? It’s actually a whole jungle of other topics, and you can run the algorithm on just that cluster and get new groups and distinctions — and one of those clusters will also be a compression of different kinds of stories, and you can do this over and over in a fractal of machine learning. The distinction here is not the only one, but it is, from the aerial perspective of data, the first.

It would be really interesting to see whether more high-quality media outlets, like the New York Times, could be easily distinguished from more sensational outlets, such as Buzzfeed, when more clusters were used, or potentially other text analytics methodology, like latent Dirichlet allocation.

Harry Plotter: Part 2 – Hogwarts Houses and their Stereotypes

Two weeks ago, I started the Harry Plotter project to celebrate the 20th anniversary of the first Harry Potter book. I could not have imagined that the first blog would be so well received. It reached over 4000 views in a matter of days thanks to the lovely people in the data science and #rstats community that were kind enough to share it (special thanks to MaraAverick and DataCamp). The response from the Harry Potter community, for instance on reddit, was also just overwhelming

Part 2: Hogwarts Houses

All in all, I could not resist a sequel and in this second post we will explore the four houses of Hogwarts: Gryffindor, Hufflepuff, Ravenclaw, and Slytherin. At the end of today’s post we will end up with visualizations like this:

Various stereotypes exist regarding these houses and a textual analysis seemed a perfect way to uncover their origins. More specifically, we will try to identify which words are most unique, informative, important or otherwise characteristic for each house by means of ratio and tf-idf statistics. Additionally, we will try to estime a personality profile for each house using these characteristic words and the emotions they relate to. Again, we rely strongly on ggplot2 for our visualizations, but we will also be using the treemaps of treemapify. Moreover, I have a special surprise this second post, as I found the orginal Harry Potter font, which will definately make the visualizations feel more authentic. Of course, we will conduct all analyses in a tidy manner using tidytext and the tidyverse.

I hope you will enjoy this blog and that you’ll be back for more. To be the first to receive new content, please subscribe to my website www.paulvanderlaken.com, follow me on Twitter, or add me on LinkedIn. Additionally, if you would like to contribute to, collaborate on, or need assistance with a data science project or venture, please feel free to reach out.

R Setup

All analysis were performed in RStudio, and knit using rmarkdown so that you can follow my steps.

In term of setup, we will be needing some of the same packages as last time. Bradley Boehmke gathered the text of the Harry Potter books in his harrypotter package. We need devtools to install that package the first time, but from then on can load it in as usual. We need plyr for ldply(). We load in most other tidyverse packages in a single bundle and add tidytext. Finally, I load the Harry Potter font and set some default plotting options.

# SETUP ####
# LOAD IN PACKAGES
# library(devtools)
# devtools::install_github("bradleyboehmke/harrypotter")
library(harrypotter)
library(plyr)
library(tidyverse)
library(tidytext)

# VIZUALIZATION SETTINGS
# custom Harry Potter font
# http://www.fontspace.com/category/harry%20potter
library(extrafont)
font_import(paste0(getwd(),"/fontomen_harry-potter"), prompt = F) # load in custom Harry Potter font
windowsFonts(HP = windowsFont("Harry Potter"))
theme_set(theme_light(base_family = "HP")) # set default ggplot theme to light
default_title = "Harry Plotter: References to the Hogwarts houses" # set default title
default_caption = "www.paulvanderlaken.com" # set default caption
dpi = 600 # set default dpi

Importing and Transforming Data

Before we import and transform the data in one large piping chunk, I need to specify some variables.

First, I tell R the house names, which we are likely to need often, so standardization will help prevent errors. Next, my girlfriend was kind enough to help me (colorblind) select the primary and secondary colors for the four houses. Here, the ggplot2 color guide in my R resources list helped a lot! Finally, I specify the regular expression (tutorials) which we will use a couple of times in order to identify whether text includes either of the four house names.

# DATA PREPARATION ####
houses <- c('gryffindor', 'ravenclaw', 'hufflepuff', 'slytherin') # define house names
houses_colors1 <- c("red3", "yellow2", "blue4", "#006400") # specify primary colors
houses_colors2 <- c("#FFD700", "black", "#B87333", "#BCC6CC") # specify secondary colors
regex_houses <- paste(houses, collapse = "|") # regular expression

Import Data and Tidy

Ok, let’s import the data now. You may recognize pieces of the code below from last time, but this version runs slightly smoother after some optimalization. Have a look at the current data format.

# LOAD IN BOOK TEXT 
houses_sentences <- list(
  `Philosophers Stone` = philosophers_stone,
  `Chamber of Secrets` = chamber_of_secrets,
  `Prisoner of Azkaban` = prisoner_of_azkaban,
  `Goblet of Fire` = goblet_of_fire,
  `Order of the Phoenix` = order_of_the_phoenix,
  `Half Blood Prince` = half_blood_prince,
  `Deathly Hallows` = deathly_hallows
) %>% 
  # TRANSFORM TO TOKENIZED DATASET
  ldply(cbind) %>% # bind all chapters to dataframe
  mutate(.id = factor(.id, levels = unique(.id), ordered = T)) %>% # identify associated book
  unnest_tokens(sentence, `1`, token = 'sentences') %>% # seperate sentences
  filter(grepl(regex_houses, sentence)) %>% # exclude sentences without house reference
  cbind(sapply(houses, function(x) grepl(x, .$sentence)))# identify references
# examine
max.char = 30 # define max sentence length
houses_sentences %>%
  mutate(sentence = ifelse(nchar(sentence) > max.char, # cut off long sentences
                           paste0(substring(sentence, 1, max.char), "..."),
                           sentence)) %>% 
  head(5)

##                  .id                          sentence gryffindor
## 1 Philosophers Stone "well, no one really knows unt...      FALSE
## 2 Philosophers Stone "and what are slytherin and hu...      FALSE
## 3 Philosophers Stone everyone says hufflepuff are a...      FALSE
## 4 Philosophers Stone "better hufflepuff than slythe...      FALSE
## 5 Philosophers Stone "there's not a single witch or...      FALSE
##   ravenclaw hufflepuff slytherin
## 1     FALSE       TRUE      TRUE
## 2     FALSE       TRUE      TRUE
## 3     FALSE       TRUE     FALSE
## 4     FALSE       TRUE      TRUE
## 5     FALSE      FALSE      TRUE

Transform to Long Format

Ok, looking great, but not tidy yet. We need gather the columns and put them in a long dataframe. Thinking ahead, it would be nice to already capitalize the house names for which I wrote a custom Capitalize() function.

# custom capitalization function
Capitalize = function(text){ 
  paste0(substring(text,1,1) %>% toupper(),
         substring(text,2))
}

# TO LONG FORMAT
houses_long <- houses_sentences %>%
  gather(key = house, value = test, -sentence, -.id) %>% 
  mutate(house = Capitalize(house)) %>% # capitalize names
  filter(test) %>% select(-test) # delete rows where house not referenced
# examine
houses_long %>%
  mutate(sentence = ifelse(nchar(sentence) > max.char, # cut off long sentences
                           paste0(substring(sentence, 1, max.char), "..."),
                           sentence)) %>% 
  head(20)

##                   .id                          sentence      house
## 1  Philosophers Stone i've been asking around, and i... Gryffindor
## 2  Philosophers Stone           "gryffindor," said ron. Gryffindor
## 3  Philosophers Stone "the four houses are called gr... Gryffindor
## 4  Philosophers Stone you might belong in gryffindor... Gryffindor
## 5  Philosophers Stone " brocklehurst, mandy" went to... Gryffindor
## 6  Philosophers Stone "finnigan, seamus," the sandy-... Gryffindor
## 7  Philosophers Stone                     "gryffindor!" Gryffindor
## 8  Philosophers Stone when it finally shouted, "gryf... Gryffindor
## 9  Philosophers Stone well, if you're sure -- better... Gryffindor
## 10 Philosophers Stone he took off the hat and walked... Gryffindor
## 11 Philosophers Stone "thomas, dean," a black boy ev... Gryffindor
## 12 Philosophers Stone harry crossed his fingers unde... Gryffindor
## 13 Philosophers Stone resident ghost of gryffindor t... Gryffindor
## 14 Philosophers Stone looking pleased at the stunned... Gryffindor
## 15 Philosophers Stone gryffindors have never gone so... Gryffindor
## 16 Philosophers Stone the gryffindor first years fol... Gryffindor
## 17 Philosophers Stone they all scrambled through it ... Gryffindor
## 18 Philosophers Stone nearly headless nick was alway... Gryffindor
## 19 Philosophers Stone professor mcgonagall was head ... Gryffindor
## 20 Philosophers Stone over the noise, snape said, "a... Gryffindor

Visualize House References

Woohoo, so tidy! Now comes the fun part: visualization. The following plots how often houses are mentioned overall, and in each book seperately.

# set plot width & height
w = 10; h = 6  

# PLOT REFERENCE FREQUENCY
houses_long %>%
  group_by(house) %>%
  summarize(n = n()) %>% # count sentences per house
  ggplot(aes(x = desc(house), y = n)) +
  geom_bar(aes(fill = house), stat = 'identity') +
  geom_text(aes(y = n / 2, label = house, col = house),  # center text
            size = 8, family = 'HP') +
  scale_fill_manual(values = houses_colors1) +
  scale_color_manual(values = houses_colors2) +
  theme(axis.text.y = element_blank(),
        axis.ticks.y = element_blank(),
        legend.position = 'none') +
  labs(title = default_title,
       subtitle = "Combined references in all Harry Potter books",
       caption = default_caption,
       x = '', y = 'Name occurence') + 
  coord_flip()

# PLOT REFERENCE FREQUENCY OVER TIME 
houses_long %>%
  group_by(.id, house) %>%
  summarize(n = n()) %>% # count sentences per house per book
  ggplot(aes(x = .id, y = n, group = house)) +
  geom_line(aes(col = house), size = 2) +
  scale_color_manual(values = houses_colors1) +
  theme(legend.position = 'bottom',
        axis.text.x = element_text(angle = 15, hjust = 0.5, vjust = 0.5)) + # rotate x axis text
  labs(title = default_title, 
       subtitle = "References throughout the Harry Potter books",
       caption = default_caption,
       x = NULL, y = 'Name occurence', color = 'House')

The Harry Potter font looks wonderful, right?

In terms of the data, Gryffindor and Slytherin definitely play a larger role in the Harry Potter stories. However, as the storyline progresses, Slytherin as a house seems to lose its importance. Their downward trend since the Chamber of Secrets results in Ravenclaw being mentioned more often in the final book (Edit – this is likely due to the diadem horcrux, as you will see later on).

I can’t but feel sorry for house Hufflepuff, which never really gets to involved throughout the saga.

Retrieve Reference Words & Data

Let’s dive into the specific words used in combination with each house. The following code retrieves and counts the single words used in the sentences where houses are mentioned.

# IDENTIFY WORDS USED IN COMBINATION WITH HOUSES
words_by_houses <- houses_long %>% 
  unnest_tokens(word, sentence, token = 'words') %>% # retrieve words
  mutate(word = gsub("'s", "", word)) %>% # remove possesive determiners
  group_by(house, word) %>% 
  summarize(word_n = n()) # count words per house
# examine
words_by_houses %>% head()

## # A tibble: 6 x 3
## # Groups:   house [1]
##        house        word word_n
##        <chr>       <chr>  <int>
## 1 Gryffindor         104      1
## 2 Gryffindor        22nd      1
## 3 Gryffindor           a    251
## 4 Gryffindor   abandoned      1
## 5 Gryffindor  abandoning      1
## 6 Gryffindor abercrombie      1

Visualize Word-House Combinations

Now we can visualize which words relate to each of the houses. Because facet_wrap() has trouble reordering the axes (because words may related to multiple houses in different frequencies), I needed some custom functionality, which I happily recycled from dgrtwo’s github. With these reorder_within() and scale_x_reordered() we can now make an ordered barplot of the top-20 most frequent words per house.

# custom functions for reordering facet plots
# https://github.com/dgrtwo/drlib/blob/master/R/reorder_within.R
reorder_within <- function(x, by, within, fun = mean, sep = "___", ...) {
  new_x <- paste(x, within, sep = sep)
  reorder(new_x, by, FUN = fun)
}

scale_x_reordered <- function(..., sep = "___") {
  reg <- paste0(sep, ".+$")
  ggplot2::scale_x_discrete(labels = function(x) gsub(reg, "", x), ...)
}

# set plot width & height
w = 10; h = 7; 

# PLOT MOST FREQUENT WORDS PER HOUSE
words_per_house = 20 # set number of top words
words_by_houses %>%
  group_by(house) %>%
  arrange(house, desc(word_n)) %>%
  mutate(top = row_number()) %>% # count word top position
  filter(top <= words_per_house) %>% # retain specified top number
  ggplot(aes(reorder_within(word, -top, house), # reorder by minus top number
             word_n, fill = house)) +
  geom_col(show.legend = F) +
  scale_x_reordered() + # rectify x axis labels 
  scale_fill_manual(values = houses_colors1) +
  scale_color_manual(values = houses_colors2) + 
  facet_wrap(~ house, scales = "free_y") + # facet wrap and free y axis
  coord_flip() +
  labs(title = default_title, 
       subtitle = "Words most commonly used together with houses",
       caption = default_caption,
       x = NULL, y = 'Word Frequency')

Unsurprisingly, several stop words occur most frequently in the data. Intuitively, we would rerun the code but use dplyr::anti_join() on tidytext::stop_words to remove stop words.

# PLOT MOST FREQUENT WORDS PER HOUSE
# EXCLUDING STOPWORDS
words_by_houses %>%
  anti_join(stop_words, 'word') %>% # remove stop words
  group_by(house) %>% 
  arrange(house, desc(word_n)) %>%
  mutate(top = row_number()) %>% # count word top position
  filter(top <= words_per_house) %>% # retain specified top number
  ggplot(aes(reorder_within(word, -top, house), # reorder by minus top number
             word_n, fill = house)) +
  geom_col(show.legend = F) +
  scale_x_reordered() + # rectify x axis labels
  scale_fill_manual(values = houses_colors1) +
  scale_color_manual(values = houses_colors2) + 
  facet_wrap(~ house, scales = "free") + # facet wrap and free scales
  coord_flip() +
  labs(title = default_title, 
       subtitle = "Words most commonly used together with houses, excluding stop words",
       caption = default_caption,
       x = NULL, y = 'Word Frequency')

However, some stop words have a different meaning in the Harry Potter universe. points are for instance quite informative to the Hogwarts houses but included in stop_words.

Moreover, many of the most frequent words above occur in relation to multiple or all houses. Take, for instance, Harry and Ron, which are in the top-10 of each house, or words like table, house, and professor.

We are more interested in words that describe one house, but not another. Similarly, we only want to exclude stop words which are really irrelevant. To this end, we compute a ratio-statistic below. This statistic displays how frequently a word occurs in combination with one house rather than with the others. However, we need to adjust this ratio for how often houses occur in the text as more text (and thus words) is used in reference to house Gryffindor than, for instance, Ravenclaw.

words_by_houses <- words_by_houses %>%
  group_by(word) %>% mutate(word_sum = sum(word_n)) %>% # counts words overall
  group_by(house) %>% mutate(house_n = n()) %>%
  ungroup() %>%
    # compute ratio of usage in combination with house as opposed to overall
  # adjusted for house references frequency as opposed to overall frequency
  mutate(ratio = (word_n / (word_sum - word_n + 1) / (house_n / n()))) 
# examine
words_by_houses %>% select(-word_sum, -house_n) %>% arrange(desc(word_n)) %>% head()

## # A tibble: 6 x 4
##        house       word word_n     ratio
##        <chr>      <chr>  <int>     <dbl>
## 1 Gryffindor        the   1057  2.373115
## 2  Slytherin        the    675  1.467926
## 3 Gryffindor gryffindor    602 13.076218
## 4 Gryffindor        and    477  2.197259
## 5 Gryffindor         to    428  2.830435
## 6 Gryffindor         of    362  2.213186

# PLOT MOST UNIQUE WORDS PER HOUSE BY RATIO
words_by_houses %>%
  group_by(house) %>%
  arrange(house, desc(ratio)) %>%
  mutate(top = row_number()) %>% # count word top position
  filter(top <= words_per_house) %>% # retain specified top number
  ggplot(aes(reorder_within(word, -top, house), # reorder by minus top number
             ratio, fill = house)) +
  geom_col(show.legend = F) +
  scale_x_reordered() + # rectify x axis labels
  scale_fill_manual(values = houses_colors1) +
  scale_color_manual(values = houses_colors2) + 
  facet_wrap(~ house, scales = "free") +  # facet wrap and free scales
  coord_flip() +
  labs(title = default_title, 
       subtitle = "Most informative words per house, by ratio",
       caption = default_caption,
       x = NULL, y = 'Adjusted Frequency Ratio (house vs. non-house)')

# PS. normally I would make a custom ggplot function 
#    when I plot three highly similar graphs

This ratio statistic (x-axis) should be interpreted as follows: night is used 29 times more often in combination with Gryffindor than with the other houses.

Do you think the results make sense:

Gryffindors spent dozens of hours during their afternoons, evenings, and nights in the, often empty, tower room, apparently playing chess? Nevile Longbottom and Hermione Granger are Gryffindors, obviously, and Sirius Black is also on the list. The sword of Gryffindor is no surprise here either.
Hannah Abbot, Ernie Macmillan and Cedric Diggory are Hufflepuffs. Were they mostly hot curly blondes interested in herbology? Nevertheless, wild and aggresive seem unfitting for Hogwarts most boring house.
A lot of names on the list of Helena Ravenclaw’s house. Roger Davies, Padma Patil, Cho Chang, Miss S. Fawcett, Stewart Ackerley, Terry Boot, and Penelope Clearwater are indeed Ravenclaws, I believe. Ravenclaw’s Diadem was one of Voldemort horcruxes. AlectoCarrow, Death Eater by profession, was apparently sent on a mission by Voldemort to surprise Harry in Rawenclaw’s common room (source), which explains what she does on this list. Can anybody tell me what bust, statue and spot have in relation to Ravenclaw?
House Slytherin is best represented by Gregory Goyle, one of the members of Draco Malfoy’s gang along with Vincent Crabbe. Pansy Parkinson also represents house Slytherin. Slytherin are famous for speaking Parseltongue and their house’s gem is an emerald. House Gaunt were pure-blood descendants from Salazar Slytherin and apparently Viktor Krum would not have misrepresented the Slytherin values either. Oh, and only the heir of Slytherin could control the monster in the Chamber of Secrets.

Honestly, I was not expecting such good results! However, there is always room for improvement.

We may want to exclude words that only occur once or twice in the book (e.g., Alecto) as well as the house names. Additionally, these barplots are not the optimal visualization if we would like to include more words per house. Fortunately, Hadley Wickham helped me discover treeplots. Let’s draw one using the ggfittext and the treemapify packages.

# set plot width & height
w = 12; h = 8; 

# PACKAGES FOR TREEMAP
# devtools::install_github("wilkox/ggfittext")
# devtools::install_github("wilkox/treemapify")
library(ggfittext)
library(treemapify)


# PLOT MOST UNIQUE WORDS PER HOUSE BY RATIO
words_by_houses %>%
  filter(word_n > 3) %>% # filter words with few occurances
  filter(!grepl(regex_houses, word)) %>% # exclude house names
  group_by(house) %>%
  arrange(house, desc(ratio), desc(word_n)) %>%
  mutate(top = seq_along(ratio)) %>%
  filter(top <= words_per_house) %>% # filter top n words
  ggplot(aes(area = ratio, label = word, subgroup = house, fill = house)) +
  geom_treemap() + # create treemap
  geom_treemap_text(aes(col = house), family = "HP", place = 'center') + # add text
  geom_treemap_subgroup_text(aes(col = house), # add house names
                             family = "HP", place = 'center', alpha = 0.3, grow = T) +
  geom_treemap_subgroup_border(colour = 'black') +
  scale_fill_manual(values = houses_colors1) +
  scale_color_manual(values = houses_colors2) + 
  theme(legend.position = 'none') +
  labs(title = default_title, 
       subtitle = "Most informative words per house, by ratio",
       caption = default_caption)

treemap_house_word_ratio

A treemap can display more words for each of the houses and displays their relative proportions better. New words regarding the houses include the following, but do you see any others?

Slytherin girls laugh out loud whereas Ravenclaw had a few little, pretty girls?
Gryffindors, at least Harry and his friends, got in trouble often, that is a fact.
Yellow is the color of house Hufflepuff whereas Slytherin is green indeed.
Zacherias Smith joined Hufflepuff and Luna Lovegood Ravenclaw.
Why is Voldemort in camp Ravenclaw?!

In the earlier code, we specified a minimum number of occurances for words to be included, which is a bit hacky but necessary to make the ratio statistic work as intended. Foruntately, there are other ways to estimate how unique or informative words are to houses that do not require such hacks.

TF-IDF

tf-idf similarly estimates how unique / informative words are for a body of text (for more info: Wikipedia). We can calculate a tf-idf score for each word within each document (in our case house texts) by taking the product of two statistics:

TF or term frequency, meaning the number of times the word occurs in a document.
IDF or inverse document frequency, specifically the logarithm of the inverse number of documents the word occurs in.

A high tf-idf score means that a word occurs relatively often in a specific document and not often in other documents. Different weighting schemes can be used to td-idf’s performance in different settings but we used the simple default of tidytext::bind_tf_idf().

An advantage of tf-idf over the earlier ratio statistic is that we no longer need to specify a minimum frequency: low frequency words will have low tf and thus low tf-idf. A disadvantage is that tf-idf will automatically disregard words occur together with each house, be it only once: these words have zero idf (log(4/4)) so zero tf-idf.

Let’s run the treemap gain, but not on the computed tf-idf scores.

words_by_houses <- words_by_houses %>%
  # compute term frequency and inverse document frequency
  bind_tf_idf(word, house, word_n)
# examine
words_by_houses %>% select(-house_n) %>% head()

## # A tibble: 6 x 8
##        house        word word_n word_sum    ratio           tf       idf
##        <chr>       <chr>  <int>    <int>    <dbl>        <dbl>     <dbl>
## 1 Gryffindor         104      1        1 2.671719 6.488872e-05 1.3862944
## 2 Gryffindor        22nd      1        1 2.671719 6.488872e-05 1.3862944
## 3 Gryffindor           a    251      628 1.774078 1.628707e-02 0.0000000
## 4 Gryffindor   abandoned      1        1 2.671719 6.488872e-05 1.3862944
## 5 Gryffindor  abandoning      1        2 1.335860 6.488872e-05 0.6931472
## 6 Gryffindor abercrombie      1        1 2.671719 6.488872e-05 1.3862944
## # ... with 1 more variables: tf_idf <dbl>

# PLOT MOST UNIQUE WORDS PER HOUSE BY TF_IDF
words_per_house = 30
words_by_houses %>%
  filter(tf_idf > 0) %>% # filter for zero tf_idf
  group_by(house) %>%
  arrange(house, desc(tf_idf), desc(word_n)) %>%
  mutate(top = seq_along(tf_idf)) %>%
  filter(top <= words_per_house) %>%
  ggplot(aes(area = tf_idf, label = word, subgroup = house, fill = house)) +
  geom_treemap() + # create treemap
  geom_treemap_text(aes(col = house), family = "HP", place = 'center') + # add text
  geom_treemap_subgroup_text(aes(col = house), # add house names
                             family = "HP", place = 'center', alpha = 0.3, grow = T) +
  geom_treemap_subgroup_border(colour = 'black') +
  scale_fill_manual(values = houses_colors1) +
  scale_color_manual(values = houses_colors2) + 
  theme(legend.position = 'none') +
  labs(title = default_title, 
       subtitle = "Most informative words per house, by tf-idf",
       caption = default_caption)

This plot looks quite different from its predecessor. For instance, Marcus Flint and Adrian Pucey are added to house Slytherin and Hufflepuff’s main color is indeed not just yellow, but canary yellow. Severus Snape’s dual role is also nicely depicted now, with him in both house Slytherin and house Gryffindor. Do you notice any other important differences? Did we lose any important words because they occured in each of our four documents?

House Personality Profiles (by NRC Sentiment Analysis)

We end this second Harry Plotter blog by examining to what the extent the stereotypes that exist of the Hogwarts Houses can be traced back to the books. To this end, we use the NRC sentiment dictionary, see also the the previous blog, with which we can estimate to what extent the most informative words for houses (we have over a thousand for each house) relate to emotions such as anger, fear, or trust.

The code below retains only the emotion words in our words_by_houses dataset and multiplies their tf-idf scores by their relative frequency, so that we retrieve one score per house per sentiment.

# PLOT SENTIMENT OF INFORMATIVE WORDS (TFIDF)
words_by_houses %>%
  inner_join(get_sentiments("nrc"), by = 'word') %>%
  group_by(house, sentiment) %>%
  summarize(score = sum(word_n / house_n * tf_idf)) %>% # compute emotion score
  ggplot(aes(x = house, y = score, group = house)) +
  geom_col(aes(fill = house)) + # create barplots
  geom_text(aes(y = score / 2, label = substring(house, 1, 1), col = house), 
            family = "HP", vjust = 0.5) + # add house letter in middle
  facet_wrap(~ Capitalize(sentiment), scales = 'free_y') + # facet and free y axis
  scale_fill_manual(values = houses_colors1) +
  scale_color_manual(values = houses_colors2) + 
  theme(legend.position = 'none', # tidy dataviz
        axis.text.y = element_blank(),
        axis.ticks.y = element_blank(),
        axis.text.x = element_blank(),
        axis.ticks.x = element_blank(),
        strip.text.x = element_text(colour = 'black', size = 12)) +
  labs(title = default_title, 
       subtitle = "Sentiment (nrc) related to houses' informative words (tf-idf)",
       caption = default_caption,
       y = "Sentiment score", x = NULL)

The results to a large extent confirm the stereotypes that exist regarding the Hogwarts houses:

Gryffindors are full of anticipation and the most positive and trustworthy.
Hufflepuffs are the most joyous but not extraordinary on any other front.
Ravenclaws are distinguished by their low scores. They are super not-angry and relatively not-anticipating, not-negative, and not-sad.
Slytherins are the angriest, the saddest, and the most feared and disgusting. However, they are also relatively joyous (optimistic?) and very surprising (shocking?).

Conclusion and future work

With this we have come to the end of the second part of the Harry Plotter project, in which we used tf-idf and ratio statistics to examine which words were most informative / unique to each of the houses of Hogwarts. The data was retrieved using the harrypotter package and transformed using tidytext and the tidyverse. Visualizations were made with ggplot2 and treemapify, using a Harry Potter font.

I have several ideas for subsequent posts and I’d love to hear your preferences or suggestions:

I would like to demonstrate how regular expressions can be used to retrieve (sub)strings that follow a specific format. We could use regex to examine, for instance, when, and by whom, which magical spells are cast.
I would like to use network analysis to examine the interactions between the characters. We could retrieve networks from the books and conduct sentiment analysis to establish the nature of relationships. Similarly, we could use unsupervised learning / clustering to explore character groups.
I would like to use topic models, such as latent dirichlet allocation, to identify the main topics in the books. We could, for instance, try to summarize each book chapter in single sentence, or examine how topics (e.g., love or death) build or fall over time.
Finally, I would like to build an interactive application / dashboard in Shiny (another hobby of mine) so that readers like you can explore patterns in the books yourself. Unfortunately, the free on shinyapps.io only 25 hosting hours per month : (

For now, I hope you enjoyed this blog and that you’ll be back for more. To receive new content first, please subscribe to my website www.paulvanderlaken.com, follow me on Twitter, or add me on LinkedIn.

If you would like to contribute to, collaborate on, or need assistance with a data science project or venture, please feel free to reach out