CSS 类与 ID

R 语言网页抓取

Timo Grossenbacher

Instructor

类（Class）

.alert {
  color: red;
  font-weight: 800;
}

...
<div>Some text.</div>
<div class = 'alert'>Important text.</div>  
<div>
  Some text with an
  <a href = '#' class = 'alert'>important link</a>.
</div>
...

带类的 HTML

html %>% html_elements('.alert')

{xml_nodeset (2)}
[1] <div class="alert">Important text...
[2] <a href="#" class="alert">important ...

一次选中多个类

.alert {
  color: red;
  font-weight: 800;
}
.emph {
  font-style: italic;
}

...
<div>Some text.</div>
<div class = 'alert emph'>Important text.</div>  
<div>
  Some text with an
  <a href = '#' class = 'alert'>important link</a>.
</div>
...

多类选择

html %>% 
    html_elements('.alert.emph') # not: .alert, .emph

{xml_nodeset (1)}
[1] <div class="alert emph">Important text...

ID

#special {
  color: green;
}
.alert {
  color: red;
  font-weight: 800;
}

...
<div id = 'special'>Some text.</div>
<div class = 'alert'>Important text.</div>  
<div>
  Some text with an
  <a href = '#' class = 'alert'>important link</a>.
</div>
...

带 ID 的 HTML

html %>% 
  html_elements('#special')

{xml_nodeset (1)}
[1] <div id="special">Some text.</div>

用类型进一步缩小选择范围

#special {
  color: green;
}
.alert {
  color: red;
  font-weight: 800;
}

...
<div id = 'special'>Some text.</div>
<div class = 'alert'>Important text.</div>  
<div>
  Some text with an
  <a href = '#' class = 'alert'>important link</a>.
</div>
...

html %>% 
  html_elements('a.alert')

{xml_nodeset (1)}
[1] <a href="#" class="alert">important ...

html %>% 
  html_elements('#special')

is equivalent to...

html %>% 
  html_elements('div#special')

用于选中特定子元素的伪类

li:first-child { color: blue; }

li:nth-child(2) { color: green; }

li:last-child { color: red; }

...
<ol>
  <li>First element.</li>
  <li>Second element.</li>
  <li>Third element.</li>
</ol>
...

CSS 伪类

html %>% html_elements('li:last-child') 
    # or html_elements('li:nth-child(3)')

{xml_nodeset (1)}
[1] <li>Third element.</li>

¹ https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-classes

小结

Selector type	HTML	CSS selector
Type	`<p>...</p>`	`p`
Multiple types	`<p>...</p><div>...</div>`	`p, div`
Class	`<p class = 'x'>...</p>`	`.x`
Multiple classes	`<p class = 'x y'>...</p>`	`.x.y`
Type + Class	`<p class = 'x'>...</p>`	`p.x`
ID	`<p id = 'x'>...</p>`	`#x`
Type + Pseudo-class	`<p>...</p><p>...</p>`	`p:first-child`

¡Vamos a practicar!

R 语言网页抓取