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<!DOCTYPE html>
<html lang="" xml:lang="">
<head>
<title>Web scraping part I</title>
<meta charset="utf-8" />
<meta name="author" content="Shawn Santo" />
<link rel="stylesheet" href="slides.css" type="text/css" />
</head>
<body>
<textarea id="source">
class: center, middle, inverse, title-slide
# Web scraping part I
## Programming for Statistical Science
### Shawn Santo
---
## Supplementary materials
Full video lecture available in Zoom Cloud Recordings
Additional resources
- [SelectorGadget Vignette](https://cran.r-project.org/web/packages/rvest/vignettes/selectorgadget.html)
- `rvest` [website](https://rvest.tidyverse.org/)
---
class: inverse, center, middle
# Recap
---
## Summary of packages
.small-text[
| Task | Package | Cheat sheet |
|---------------------|-------------|------------------------------------------------------------------------------|
| Visualize data | `ggplot2` | https://github.com/rstudio/cheatsheets/raw/master/data-visualization-2.1.pdf |
| Wrangle data frames | `dplyr` | https://github.com/rstudio/cheatsheets/raw/master/data-transformation.pdf |
| Reshape data frames | `tidyr` | https://github.com/rstudio/cheatsheets/raw/master/data-import.pdf |
| Iterate | `purrr` | https://github.com/rstudio/cheatsheets/raw/master/purrr.pdf |
| Text manipulation | `stringr` | https://github.com/rstudio/cheatsheets/raw/master/strings.pdf |
| Manipulate factors | `forcats` | https://github.com/rstudio/cheatsheets/raw/master/factors.pdf |
| Manipulate dates | `lubridate` | https://github.com/rstudio/cheatsheets/raw/master/lubridate.pdf |
| Spatial data | `sf` | https://github.com/rstudio/cheatsheets/raw/master/sf.pdf |
]
<br/>
<i>
You don't need to memorize every function in these packages. Just know
where you need to look when you come across a specific problem.
</i>
---
class: inverse, center, middle
# HTML
---
## Hypertext Markup Language
- HTML describes the structure of a web page; your browser interprets the
structure and contents and displays the results.
- The basic building blocks include elements, tags, and attributes.
- an element is a component of an HTML document
- elements contain tags (start and end tag)
- attributes provide additional information about HTML elements
<center>
<img src="images/html_structure.png" height="300" width="450">
</center>
---
## Simple HTML document
```html
<html>
<head>
<title>Web Scraping</title>
</head>
<body>
<h1>Using rvest</h1>
<p>To get started...</p>
</body>
</html>
```
<br/><br/>
We can visualize this in a tree-like structure.
---
## HTML tree-like structure
<center>
<img src="images/html_tree.png" height="450" width="550">
</center>
If we have access to an HTML document, then how can we easily
extract information?
---
class: inverse, center, middle
# Package `rvest`
---
## Package `rvest`
`rvest` is a package authored by Hadley Wickham that makes basic processing and
manipulation of HTML data easy.
```r
library(rvest)
```
Core functions:
| Function | Description |
|---------------------|-------------------------------------------------------------------|
| `xml2::read_html()` | read HTML from a character string or connection |
| `html_nodes()` | select specified nodes from the HTML document using CSS selectors |
| `html_table()` | parse an HTML table into a data frame |
| `html_text()` | extract tag pairs' content |
| `html_name()` | extract tags' names |
| `html_attrs()` | extract all of each tag's attributes |
| `html_attr()` | extract tags' attribute value by name |
---
## HTML in R
```r
simple_html <- "<html>
<head>
<title>Web Scraping</title>
</head>
<body>
<h1>Using rvest</h1>
<p>To get started...</p>
</body>
</html>"
```
--
```r
simple_html
```
```
#> [1] "<html>\n <head>\n <title>Web Scraping</title>\n </head>\n <body>\n \n <h1>Using rvest</h1>\n <p>To get started...</p>\n \n </body>\n</html>"
```
---
```r
html_doc <- read_html(simple_html)
attributes(html_doc)
```
```
#> $names
#> [1] "node" "doc"
#>
#> $class
#> [1] "xml_document" "xml_node"
```
--
<br/>
```r
html_doc
```
```
#> {html_document}
#> <html>
#> [1] <head>\n<meta http-equiv="Content-Type" content="text/html; charset=UTF-8 ...
#> [2] <body>\n \n <h1>Using rvest</h1>\n <p>To get started...</p>\n \n ...
```
---
## CSS selectors
To extract components out of HTML documents use `html_nodes()` and CSS selectors.
In CSS, selectors are patterns used to select elements you want to style.
- CSS stands for Cascading Style Sheets.
- CSS describes how HTML elements are to be displayed on screen, paper, or
in other media.
- CSS can be added to HTML elements in 3 ways:
- Inline - by using the style attribute in HTML elements
- Internal - by using a style element in the head section
- External - by using an external CSS file
---
## More on CSS
.small-text[
Selector | Example | Description
:-----------------|:-----------------|:--------------------------------------------------
element | `p` | Select all &lt;p&gt; elements
element element | `div p` | Select all &lt;p&gt; elements inside a &lt;div&gt; element
element>element | `div > p` | Select all &lt;p&gt; elements with &lt;div&gt; as a parent
.class | `.title` | Select all elements with class="title"
#id | `#name` | Select all elements with id="name"
[attribute] | `[class]` | Select all elements with a class attribute
[attribute=value] | `[class=title]` | Select all elements with class="title"
]
For more CSS selector references click [here](https://www.w3schools.com/cssref/css_selectors.asp).
--
<br/>
Fortunately, we can determine the necessary CSS selectors we need via the
point-and-click tool [selector gadget](https://selectorgadget.com/).
More on this in a moment.
---
## Example
URL: https://raysnotebook.info/ows/schedules/The%20Whole%20Shebang.html
.tiny[
```html
<html lang=en>
<head>
<title>Rays Notebook: Open Water Swims 2020 — The Whole Shebang</title>
</head>
<body>
<main class=schedule>
<h1>The Whole Shebang</h1>
<p>This schedule lists every swim in the database. 383 events.</p>
<table class=schedule>
<thead><tr><th>Date</th><th>Location</th><th>Name</th><th>Distance</th><th>More</th></tr></thead>
<tbody>
<tr id=January>
<td class=date>Jan 12, Sun</td>
<td class=where>
<a class=mapq href="http://www.google.com/maps/?q=27.865501,-82.631997">Petersburg, FL</a>
<span class=more>
Gandy Beach, Gandy Blvd N St, Petersburg, FL
</span>
</td>
<td class=name><a href="http://tampabayfrogman.com/">Tampa Bay Frogman</a></td>
<td class=distance>5 km</td>
<td class=more><span class=time>7:15 AM</span>, Old Tampa Bay.</td>
</tr>
</body>
</html>
```
]
---
Suppose we want to extract and parse the information highlighted below in
yellow.
.tiny[
```r
<html lang=en>
<head>
<title>Rays Notebook: Open Water Swims 2020 — The Whole Shebang</title>
</head>
<body>
<main class=schedule>
<h1>The Whole Shebang</h1>
*<p>This schedule lists every swim in the database. 383 events.</p>
<table class=schedule>
<thead><tr><th>Date</th><th>Location</th><th>Name</th><th>Distance</th><th>More</th></tr></thead>
<tbody>
<tr id=January>
<td class=date>Jan 12, Sun</td>
<td class=where>
<a class=mapq href="http://www.google.com/maps/?q=27.865501,-82.631997">Petersburg, FL</a>
<span class=more>
Gandy Beach, Gandy Blvd N St, Petersburg, FL
</span>
</td>
<td class=name><a href="http://tampabayfrogman.com/">Tampa Bay Frogman</a></td>
<td class=distance>5 km</td>
<td class=more><span class=time>7:15 AM</span>, Old Tampa Bay.</td>
</tr>
</body>
</html>
```
]
---
**Step 1**
Save the HTML as a character object named `html_swim`.
```r
html_swim <- "<html lang=en> ... </body></html>"
```
--
**Step 2**
To extract all `<p>` elements:
```r
html_swim %>%
read_html() %>%
* html_nodes(css = "p")
```
```
#> {xml_nodeset (1)}
#> [1] <p>This schedule lists every swim in the database. 383 events.</p>
```
--
**Step 3**
To extract the contents between the tags:
```r
html_swim %>%
read_html() %>%
html_nodes(css = "p") %>%
* html_text()
```
```
#> [1] "This schedule lists every swim in the database. 383 events."
```
---
Suppose we want to extract and parse pieces of the information highlighted
below in yellow.
.tiny[
```r
<html lang=en>
<head>
<title>Rays Notebook: Open Water Swims 2020 — The Whole Shebang</title>
</head>
<body>
<main class=schedule>
<h1>The Whole Shebang</h1>
<p>This schedule lists every swim in the database. 383 events.</p>
<table class=schedule>
<thead><tr><th>Date</th><th>Location</th><th>Name</th><th>Distance</th><th>More</th></tr></thead>
<tbody>
<tr id=January>
<td class=date>Jan 12, Sun</td>
*<td class=where>
* <a class=mapq href="http://www.google.com/maps/?q=27.865501,-82.631997">Petersburg, FL</a>
* <span class=more>
* Gandy Beach, Gandy Blvd N St, Petersburg, FL
* </span>
*</td>
<td class=name><a href="http://tampabayfrogman.com/">Tampa Bay Frogman</a></td>
<td class=distance>5 km</td>
<td class=more><span class=time>7:15 AM</span>, Old Tampa Bay.</td>
</tr>
</body>
</html>
```
]
---
To select all elements with `class="where"`:
```r
html_swim %>%
read_html() %>%
* html_nodes(css = "[class=where]")
```
```
#> {xml_nodeset (1)}
#> [1] <td class="where">\n <a class="mapq" href="http://www.google.com/maps/? ...
```
--
To extract the text:
```r
html_swim %>%
read_html() %>%
html_nodes(css = "[class=where]") %>%
* html_text()
```
```
#> [1] "\n Petersburg, FL\n \n Gandy Beach, Gandy Blvd N St, Petersburg, FL\n \n"
```
--
To extract the attributes:
```r
html_swim %>%
read_html() %>%
html_nodes(css = "[class=where]") %>%
* html_attrs()
```
```
#> [[1]]
#> class
#> "where"
```
---
Suppose we want to extract and parse the information highlighted below in
yellow.
.tiny[
```r
<html lang=en>
<head>
<title>Rays Notebook: Open Water Swims 2020 — The Whole Shebang</title>
</head>
<body>
<main class=schedule>
<h1>The Whole Shebang</h1>
<p>This schedule lists every swim in the database. 383 events.</p>
<table class=schedule>
<thead><tr><th>Date</th><th>Location</th><th>Name</th><th>Distance</th><th>More</th></tr></thead>
<tbody>
<tr id=January>
<td class=date>Jan 12, Sun</td>
<td class=where>
* <a class=mapq href="http://www.google.com/maps/?q=27.865501,-82.631997">Petersburg, FL</a>
<span class=more>
Gandy Beach, Gandy Blvd N St, Petersburg, FL
</span>
</td>
*<td class=name><a href="http://tampabayfrogman.com/">Tampa Bay Frogman</a></td>
<td class=distance>5 km</td>
<td class=more><span class=time>7:15 AM</span>, Old Tampa Bay.</td>
</tr>
</body>
</html>
```
]
---
To extract the links (those with an `href` attribute):
```r
html_swim %>%
read_html() %>%
* html_nodes(css = "[href]")
```
```
#> {xml_nodeset (2)}
#> [1] <a class="mapq" href="http://www.google.com/maps/?q=27.865501,-82.631997" ...
#> [2] <a href="http://tampabayfrogman.com/">Tampa Bay Frogman</a>
```
--
To get only the URLs (value of the `href` attribute):
```r
html_swim %>%
read_html() %>%
html_nodes(css = "[href]") %>%
* html_attr("href")
```
```
#> [1] "http://www.google.com/maps/?q=27.865501,-82.631997"
#> [2] "http://tampabayfrogman.com/"
```
---
## SelectorGadget
[SelectorGadget](https://selectorgadget.com/) makes identifying the CSS
selector you need by easily clicking on items on a webpage.
<center>
<iframe title="vimeo-player" src="https://player.vimeo.com/video/52055686" width="800" height="400" frameborder="0" allowfullscreen></iframe>
</center>
---
class: inverse, center, middle
# Live demo
---
## Exercise
Go to http://books.toscrape.com/catalogue/page-1.html and scrape the first
five pages of data on books with regards to their
1. title
2. price
3. star rating
Organize your results in a neatly formatted tibble similar to below.
```r
# A tibble: 100 x 3
title price rating
<chr> <chr> <chr>
1 A Light in the Attic £51.… Three
2 Tipping the Velvet £53.… One
3 Soumission £50.… One
4 Sharp Objects £47.… Four
5 Sapiens: A Brief History of Humankind £54.… Five
6 The Requiem Red £22.… One
7 The Dirty Little Secrets of Getting Your Dream J… £33.… Four
8 The Coming Woman: A Novel Based on the Life of t… £17.… Three
9 The Boys in the Boat: Nine Americans and Their E… £22.… Four
10 The Black Maria £52.… One
# … with 90 more rows
```
???
## Solution
```r
# example for page 1, see how everything works
url <- "http://books.toscrape.com/catalogue/page-1.html"
read_html(url) %>%
html_nodes(css = ".price_color") %>%
html_text()
read_html(url) %>%
html_nodes(css = ".product_pod a") %>%
html_attr("title") %>%
.[!is.na(.)]
read_html(url) %>%
html_nodes(css = ".star-rating") %>%
html_attr(name = "class") %>%
str_remove(pattern = "star-rating ")
# turn our code into a function
get_books <- function(page) {
base_url <- "http://books.toscrape.com/catalogue/page-"
url <- str_c(base_url, page, ".html")
books_html <- read_html(url)
prices <- books_html %>%
html_nodes(css = ".price_color") %>%
html_text()
titles <- books_html %>%
html_nodes(css = ".product_pod a") %>%
html_attr("title") %>%
.[!is.na(.)]
ratings <- books_html %>%
html_nodes(css = ".star-rating") %>%
html_attr(name = "class") %>%
str_remove(pattern = "star-rating ")
books_df <- tibble(
title = titles,
price = prices,
rating = ratings
)
return(books_df)
}
# iterate across pages
pages <- 1:5
books <- map_df(pages, get_books)
```
---
## References
1. Easily Harvest (Scrape) Web Pages. (2020). Rvest.tidyverse.org.
https://rvest.tidyverse.org/.
2. W3Schools Online Web Tutorials. (2020). W3schools.com.
https://www.w3schools.com/.
3. SelectorGadget: point and click CSS selectors. (2020). Selectorgadget.com.
https://selectorgadget.com/.
</textarea>
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if (node.dataset["target"] !== "print-only") continue;
node.parentNode.removeChild(node);
}
deleted = true;
});
})();
(function() {
"use strict"
// Replace <script> tags in slides area to make them executable
var scripts = document.querySelectorAll(
'.remark-slides-area .remark-slide-container script'
);
if (!scripts.length) return;
for (var i = 0; i < scripts.length; i++) {
var s = document.createElement('script');
var code = document.createTextNode(scripts[i].textContent);
s.appendChild(code);
var scriptAttrs = scripts[i].attributes;
for (var j = 0; j < scriptAttrs.length; j++) {
s.setAttribute(scriptAttrs[j].name, scriptAttrs[j].value);
}
scripts[i].parentElement.replaceChild(s, scripts[i]);
}
})();
(function() {
var links = document.getElementsByTagName('a');
for (var i = 0; i < links.length; i++) {
if (/^(https?:)?\/\//.test(links[i].getAttribute('href'))) {
links[i].target = '_blank';
}
}
})();
// adds .remark-code-has-line-highlighted class to <pre> parent elements
// of code chunks containing highlighted lines with class .remark-code-line-highlighted
(function(d) {
const hlines = d.querySelectorAll('.remark-code-line-highlighted');
const preParents = [];
const findPreParent = function(line, p = 0) {
if (p > 1) return null; // traverse up no further than grandparent
const el = line.parentElement;
return el.tagName === "PRE" ? el : findPreParent(el, ++p);
};
for (let line of hlines) {
let pre = findPreParent(line);
if (pre && !preParents.includes(pre)) preParents.push(pre);
}
preParents.forEach(p => p.classList.add("remark-code-has-line-highlighted"));
})(document);</script>
<script>
slideshow._releaseMath = function(el) {
var i, text, code, codes = el.getElementsByTagName('code');
for (i = 0; i < codes.length;) {
code = codes[i];
if (code.parentNode.tagName !== 'PRE' && code.childElementCount === 0) {
text = code.textContent;
if (/^\\\((.|\s)+\\\)$/.test(text) || /^\\\[(.|\s)+\\\]$/.test(text) ||
/^\$\$(.|\s)+\$\$$/.test(text) ||
/^\\begin\{([^}]+)\}(.|\s)+\\end\{[^}]+\}$/.test(text)) {
code.outerHTML = code.innerHTML; // remove <code></code>
continue;
}
}
i++;
}
};
slideshow._releaseMath(document);
</script>
<!-- dynamically load mathjax for compatibility with self-contained -->
<script>
(function () {
var script = document.createElement('script');
script.type = 'text/javascript';
script.src = 'https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-MML-AM_CHTML';
if (location.protocol !== 'file:' && /^https?:/.test(script.src))
script.src = script.src.replace(/^https?:/, '');
document.getElementsByTagName('head')[0].appendChild(script);
})();
</script>
</body>
</html>