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rvest.Rpres
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<style>
.reveal .slides > sectionx {
top: -70%;
}
.reveal pre code.r {background-color: #ccF}
.section .reveal li {color:white}
.section .reveal em {font-weight: bold; font-style: "none"}
.small-code pre {
margin-left: -1em; margin-right: -1em;
}
.small-code pre code {
font-size: 1em;
}
</style>
Scraping with R: rvest
========================================================
author: Wouter van Atteveldt
date: Web Scraping with RVest
Why rvest?
===
+ Interpreting HTML as text is hard
+ optional elements, variations/errors possible
+ `<a target="_blank" href="...">`
+ `<a href=site.html><p>link</a>`
+ Finding nested information really hard
+ Especially if HTML contains errors, e.g. unbalanced tags
+ Search at level of node structure
HTML as tree structure
===
```
<html>
<head>
<style>...</style>
</head>
<body>
<h1>This is the head</h1>
<p>This is <a href="..">a link</a></p>
</body>
</html
```
HTML as tree structure
===
+ html
+ head
+ style
+ body
+ h1
+ p
+ a [href=..]
RVest
===
+ Easy web scraping
+ Uses httr and xml2
+ Search structure rather than raw html
```{r, eval=F}
install.packages("rvest")
```
```{r}
library(rvest)
test = read_html("http://i.amcat.nl/test/test.html")
paras = html_nodes(test, "p")
links = html_nodes(paras, "a")
html_attr(links, "href")
```
Aside: magrittr
===
Pipeline notation in RVest
===
```{r}
paras = html_nodes(test, "p")
links = html_nodes(paras, "a")
html_attr(links, "href")
```
+ Pipeline notation:
+ x %>% f(y) is same as f(x, y)
+ Simplifies chained function calls
```{r}
test %>% html_nodes("p") %>%
html_nodes("a") %>% html_attr("href")
```
Using RVest
===
+ Search for elements
+ `html_nodes`, `html_node`
+ Use *CSS Selectors*
+ Fill in forms, follow links
+ `html_form`, `submit_form`
+ `follow_link`
+ Extract information
+ `html_attr`, `html_text`, `html_table`
CSS Selectors
===
type:section
+ Web Scraping: principles and challenges
+ *Scraping with rvest*
+ Introduction
+ *CSS Selectors*
+ Forms and links
+ Logging in
+ Lab: scraping wikipedia II
CSS and HTML
===
+ HTML has limited set of tags
+ Cascading Style Sheets (CSS)
+ Specify style (font, color, border, placement)
+ Based on structure, tags, HTML classses
+ Allows for 'semantic markup'
+ HTML specifies structure, CSS layout
CSS Structure
===
```
<selector> {<attr>: <val>}
```
+ Selectors select groups of nodes:
+ Tag name: `p {..}`
+ Class: `.main, p.main {..}`
+ ID: `#story, p#story {..}`
+ Structure
+ direct child: `p > a {..}`
+ indirect child: `p a {..}`
+ Combinations
+ `p.body a`
+ `#mainbody .title > a`
Let's play a game!
====
type: section
http://flukeout.github.io/
Using CSS in rvest
===
+ `html_node` uses css selectors
+ Make selection as complex as you want
+ Select in whole doc or within other nodes
+ E.g.: [test_css.html](http://i.amcat.nl/test/test_css.html)
Using CSS in rvest
===
class: small-code
```{r}
test2 = read_html("test_css.html")
test2 %>% html_nodes("a") %>% html_attr("href")
test2 %>% html_nodes(".main a") %>% html_attr("href")
test2 %>% html_nodes(".main") %>% html_nodes("a") %>% html_attr("href")
test2 %>% html_nodes(".main p a") %>% html_attr("href")
```
XPath as alternative
===
+ Xpath is general xml query language
+ Uses xml structure (not CSS semantics)
+ Less convenient, but more powerful
+ Use file-system like paths:
+ `//h2`: h2 anywhere in file
+ `//p/a`: a directly under any p
+ `./p`: p as direct child of current node
XPath: axes
===
+ Can also look at siblings, ancestors, etc.
+ syntax: `axis::nodetest[attributes]`
+ Useful axes:
+ ancestor
+ parent
+ sibling
XPath: example
===
class: small-code
Get all text under 'Education' subheader
```{r}
url = "https://en.wikipedia.org/wiki/Hong_Kong"
s = read_html(url)
headers = s %>% html_nodes("h2")
hist = headers[headers %>% html_text == "Education"]
text = hist %>% html_nodes(xpath="following-sibling::p") %>% html_text
text[1]
```
Useful resources
===
+ Browser developer tools
+ selectorgadget extension
+ http://flukeout.github.io/
+ http://www.w3schools.com/cssref/css_selectors.asp
+ http://www.w3schools.com/xml/xpath_axes.asp
Extracting information from HTML
===
type:section
+ Web Scraping: principles and challenges
+ *Scraping with rvest*
+ CSS Selectors
+ *Extracting Information*
+ Forms and links
+ Logging in
+ Lab: scraping wikipedia II
Extracting information from HTML
===
+ `html_name`: name of tag(s)
+ `html_attr(attr)`: specific attribute (e.g. href)
+ `html_attrs`: all attributes
+ `html_text`: the (plain) text
Extracting information from HTML
===
class: small-code
```{r}
test2 %>% html_nodes("h1") %>% html_text
test2 %>% html_nodes(".main > *") %>% html_name
test2 %>% html_nodes("a") %>% html_attr("href")
test2 %>% html_nodes(".footer a") %>% html_attrs
```
Extracting tabular info
===
class: small-code
```{r}
t = read_html("http://i.amcat.nl/test/test_table.html")
tab = t %>% html_node("table") %>% html_table
class(tab)
head(tab)
```
Show html structure
===
class: small-code
```{r}
html_structure(t)
```
Write HTML to file (useful for debugging)
===
```{r}
write_html(t, file="/tmp/test.html")
```
Following links and Forms
===
type:section
+ Web Scraping: principles and challenges
+ *Scraping with rvest*
+ CSS Selectors
+ Extracting Information
+ *Forms and links*
+ Logging in
+ Lab: scraping wikipedia II
Submitting forms from R
===
class: small-code
```{r}
search_url = "https://en.wikipedia.org/w/index.php?title=Special:Search"
session = html_session(search_url)
form = session %>% html_node("#search") %>% html_form
form = set_values(form, search="obama")
resp = submit_form(session, form=form)
head(resp %>% html_nodes(".mw-search-result-heading") %>% html_text)
head(resp %>% html_nodes(".mw-search-result-heading a") %>% html_attr("href"))
```
Following links
===
class: small-code
```{r}
r2 = follow_link(resp, i="Michelle Obama")
r2 %>% html_nodes("h1,h2") %>% html_text
r2 = follow_link(resp, css=".mw-search-result-heading a")
r2 %>% html_nodes("h1,h2") %>% html_text
r2 = follow_link(resp, i="next 20")
r2 %>% html_nodes(".mw-search-result-heading") %>% html_text
```
Iterating over search results
===
+ Option 1: 'click' next until no more results
+ Option 2: Manually build search result
+ Find out number of pages / results
+ Create for loop over pages
+ Process each page
Iterating over search results: # of pages
===
class: small-code
```{r}
q = '"City University of Hong Kong"'
session = html_session(search_url)
form = session %>% html_node("#search") %>% html_form %>% set_values(search=q)
r = submit_form(session, form)
info = r %>% html_nodes(".results-info strong") %>% html_text
i = as.integer(info[2])
i
```
Iterating over search results: create urls
===
class: small-code
```{r, eval=F}
q = RCurl::curlEscape(q)
maxpage = floor(i / 100)
offsets = (0:maxpage) * 100
template = "https://en.wikipedia.org/w/index.php?title=Special:Search&limit=100&offset=%i&search=%s"
urls = NULL
for(offset in offsets) {
url = sprintf(template, offset, q)
message("Offset:", offset, "; url:", url)
results = read_html(url) %>% html_nodes(".mw-search-result-heading a")
links = results %>% html_attr("href")
urls = c(urls, links)
}
length(urls)
urls[1:5]
```
Login required?
===
type:section
+ Web Scraping: principles and challenges
+ *Scraping with rvest*
+ CSS Selectors
+ Extracting Information
+ Forms and links
+ *Logging in*
+ Lab: scraping wikipedia II
Logging in to sites
===
+ Some sites require registration/login
+ Need to submit login form
+ Request other URLs within that session
Example: Scraping github
===
class: small-code
```{r, echo=F}
password = readRDS("password.rds")
```
```{r}
s = html_session("https://github.com/login")
form = html_form(s)[[1]]
form = set_values(form, login="vanatteveldt", password=password)
s = submit_form(s, form)
r = jump_to(s, "https://github.com/settings/emails")
emails = r %>% html_nodes("ul#settings-emails li") %>% html_text
# remember? :)
stringi::stri_extract_first(emails, regex="[\\w\\.]+@[\\w\\.]+")
```
Logging in to sites: Problems
===
+ Sites can make it difficult to login (e.g. WSJ.com, LexisNexis)
+ Can always be circumvented, but can be difficult
+ On registering, you agreed to their terms
+ This can include a ban on scraping
+ Always check legal issues first!
+ Often, API is better alternative (if offered)