Scala in Impatient 习题解答16 XML处理A2

08 June 2014

<fred/>(0)得到什么？<fred/>(0)(0)呢？为什么？

<fred/>是一个Elem，只有一个子元素Node，<fred/>(0)就会返回这个Node 而Node继承了Seq[Node]，它实际上是一个大小是1的数组。那么<fred/>(0)(0)就会返回它本身。以下可以证明

```scala
scala> <fred/>(0) == <fred/>(0)(0)
res17: Boolean = true
```

如下代码的值是什么？

 <ul>
   <li>Opening bracket: [</li>
   <li>Closing bracket: ]</li>
   <li>Opening brace: {</li>
   <li>Opening brace: }</li>
 </ul>

你如何修复它？

解答如下,除了用CDATA，大括号也可以用双大括号的方式。

```scala
<ul>
  <li>Opening bracket: <![CDATA[[]]></li>
  <li>Closing bracket: <![CDATA[]]]></li>
  <li>Opening brace: <![CDATA[{]]></li>
  <li>Opening brace: }}</li>
</ul>
```

比对 <li>Fred</li> match { case <li>{Text(t)}</li> => t } 和 <li>{"Fred"}</li> match { case <li>{Text(t)}</li> 为什么它们的行为不同？

 scala><li>Fred</li> match { case <li>{Text(t)}</li> => t }
 res31: String = Fred

 scala> <li>{"Fred"}</li> match { case <li>{Text(t)}</li> => t }
 scala.MatchError: <li>Fred</li> (of class scala.xml.Elem)
   ... 32 elided

<li>{"Fred"}</li>的内嵌的Fred不会生成Text对象，而是生成Atom[String],所以无法匹配。修改成以下就可以匹配了

```scala
<li>{"Fred"}</li> match { case <li>{t: Atom[String] @unchecked}</li> => t }
```

创建16.xml，内容如下

<root>
  <a><img src="1.jpg"/></a>
  <a><img src="1.jpg" alt="aa"/></a>
  <img alt="bb"/>
  <img src="2.jpg"/>
</root>

import scala.xml._
val root = XML.loadFile("/home/code/scala/16.xml")
(root \\ "img").foreach { case n if (n.attribute("alt") == None) => print(n); case _ => }

沿用上题的16.xml

import scala.xml._
val root = XML.loadFile("/home/code/scala/16.xml")
root \\ "img" \\ "@src" foreach {println _}

import scala.xml._
val root = XML.loadFile("/home/code/scala/16.xml")
root \\ "a" foreach {a => println(a.text + "  " +a.attributes.get("href").getOrElse(""))}

def makeXML(map: Map[String, String]) = {
  <dl>{map.map(p => <dt>{p._1}</dt><dd>{p._2}</dd>)}</dl>
}

import xml.Elem
def makeMap(dl: Elem) = {
  (dl \ "dt" zip dl \ "dd").map(a => a._1.text -> a._2.text).toMap
}

同10

测试用文件page.xml

<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
    <head>
        <title>simple document</title>
    </head>
    <body>
        <p>a simple paragraph
            <img alt="aa"></img>
            <img src="1.jpg"></img>
        </p>
        <img/>
    </body>
    <![CDATA[system.out.println()]]>
    <!-- comments here -->
</html>

代码如下

```scala
import xml._
import xml.transform._
import xml.parsing.ConstructingParser
import xml.dtd.DocType
import java.io.File

//val a = xml.XML.loadFile("/tmp/page.xml")
//save DTD and CDATA
val doc = ConstructingParser.fromFile(new File("/tmp/page.xml"),
  true).document
val a = doc.docElem

val imgRule = new RewriteRule {
  override def transform(n:Node) = n match {
    case i @ <img>{_*}</img> if(i.attribute("alt") == None) => 
    i.asInstanceOf[Elem] % Attribute(null, "alt", "TODO", Null)
    case _ => n
  }
}

val result = new RuleTransformer(imgRule).transform(a)
XML.save("/tmp/page1.xml", result(0), "UTF-8", false,
  DocType("html", doc.dtd.externalID, Nil))
```

use Scala 2.11.1

Scala 22