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Expanding on a question I asked earlier about how to iterate over a collection of nodes in scala.xml.Node found here

I wanted to take this 1 step further and ask how I could look up to a previous child inside a recursive function to get a value once I hit a specific situation

For example (the markup is here)

<html> <head class="foo"> <title>Welcome</title> </head> <body> <div> <p>Foo</p> </div> </body> </html>

With my current implementation (thanks to @knut-arne-vedaa)

def processNode(node: Node) { if (node.isInstanceOf[Text]) { if (node.text.contains("Welcome")) { //then inside here I want to go up to the prev element (head) and pull the class } } node.child foreach processNode }

I want to add another conditional to get the text "foo" from inside the class section

Any idea what I could add inside this if statement to pull this value directly? Also how can I return the String value from this fx? a break w/ a simple return line or ?

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3 Answers 3

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Note that the parent of the Text node in your example is actually the <title> node.

Since Nodes don't have references to their parents, one way to access them is just to pass them with you when walking down the tree, like this:

def processNode(node: Node, parent: Option[Node]) { if (node.isInstanceOf[Text]) { if (node.text.contains("Welcome")) { println("Node: " + node) println("Parent: " + parent.getOrElse("[toplevel]")) } } node.child foreach { n: Node => processNode(n, Some(node)) } } processNode(xml, None)

Of course, this becomes unwieldy when you want to climb back up the tree to an arbitrary level. One approach to this is to wrap your nodes in a SuperNode that has an optional parent reference.

case class SuperNode(current: Node, parent: Option[SuperNode] = None)

For convenience, make an implicit function to convert nodes to SuperNodes in the default case of no parent.

implicit def nodeMakeSuper(n: Node) = SuperNode(n)

Now you can navigate up the tree an arbitrary number of times, like this:

def processNode(node: SuperNode) { node.current match { case n @ Text(t) if t.contains("Welcome") => { println("Node: " + n) node.parent match { case Some(p) => { println("Parent: " + p.current) p.parent match { case Some(gp) => println("GrandParent: " + gp.current) case None => println("No grandparent!") } } case None => println("No parent!") } } case _ => // these aren't the droids you're looking for } node.current.child foreach { child: Node => processNode(SuperNode(child, Some(node))) } }
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2

If you keep going in this direction, you should use an XML Zipper. I think Scalaz has one (it has one, but I'm not sure if you can use it with XML).

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If you want to do something with the value only if it is inside a node with a certain attribute, you can do it the other way around: first find all nodes with the given attribute, then do the relevant processing on them.

You can find all nodes with a "class" attribute of "foo" like this:

xml \\ "_" filter { _.attribute("class") == Some(Text("foo")) }

EDIT: You use it like this:

val markup = <html>...etc val filtered = markup \\ "_" filter { _.attribute("class") == Some(Text("foo")) } filtered map processNode

If you want to return some value from the processing you need to do it differently, but your question is not clear in regards to that.

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How could I use this w/ the above processNode method or would this be a method all its own? Also how could I print out the filtered list? When I do the filter (as you have it above) and do a println(xml) I still get the same markup returned (appearing that no filter was applied)

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