Procedura: attraversare un albero binario con attività in parallelo
Nell'esempio seguente vengono mostrati due modi in cui è possibile utilizzare attività parallele per attraversare una struttura di dati ad albero. La creazione della struttura ad albero stessa viene lasciata come esercizio.
Esempio
Imports System.Threading.Tasks
Public Class TreeWalk
Shared Sub Main()
Dim tree As Tree(Of Person) = New Tree(Of Person)()
' ...populate tree (left as an exercise)
' Define the Action to perform on each node.
Dim myAction As Action(Of Person) = New Action(Of Person)(Sub(x)
Console.WriteLine("{0} : {1} ", x.Name, x.Number)
End Sub)
' Traverse the tree with parallel tasks.
DoTree(tree, myAction)
End Sub
Public Class Person
Public Name As String
Public Number As Integer
End Class
Public Class Tree(Of T)
Public Left As Tree(Of T)
Public Right As Tree(Of T)
Public Data As T
End Class
' By using tasks explicitly.
Public Shared Sub DoTree(Of T)(ByVal myTree As Tree(Of T), ByVal a As Action(Of T))
If Not myTree Is Nothing Then
Return
End If
Dim left = Task.Factory.StartNew(Sub() DoTree(myTree.Left, a))
Dim right = Task.Factory.StartNew(Sub() DoTree(myTree.Right, a))
a(myTree.Data)
Try
Task.WaitAll(left, right)
Catch ae As AggregateException
'handle exceptions here
End Try
End Sub
' By using Parallel.Invoke
Public Shared Sub DoTree2(Of T)(ByVal myTree As Tree(Of T), ByVal myAct As Action(Of T))
If Not myTree Is Nothing Then
Return
End If
Parallel.Invoke(
Sub() DoTree2(myTree.Left, myAct),
Sub() DoTree2(myTree.Left, myAct),
Sub() myAct(myTree.Data)
)
End Sub
End Class
public class TreeWalk
{
static void Main()
{
Tree<MyClass> tree = new Tree<MyClass>();
// ...populate tree (left as an exercise)
// Define the Action to perform on each node.
Action<MyClass> myAction = x => Console.WriteLine("{0} : {1}", x.Name, x.Number);
// Traverse the tree with parallel tasks.
DoTree(tree, myAction);
}
public class MyClass
{
public string Name { get; set; }
public int Number { get; set; }
}
public class Tree<T>
{
public Tree<T> Left;
public Tree<T> Right;
public T Data;
}
// By using tasks explcitly.
public static void DoTree<T>(Tree<T> tree, Action<T> action)
{
if (tree == null) return;
var left = Task.Factory.StartNew(() => DoTree(tree.Left, action));
var right = Task.Factory.StartNew(() => DoTree(tree.Right, action));
action(tree.Data);
try
{
Task.WaitAll(left, right);
}
catch (AggregateException )
{
//handle exceptions here
}
}
// By using Parallel.Invoke
public static void DoTree2<T>(Tree<T> tree, Action<T> action)
{
if (tree == null) return;
Parallel.Invoke(
() => DoTree2(tree.Left, action),
() => DoTree2(tree.Right, action),
() => action(tree.Data)
);
}
}
I due metodi mostrati sono equivalenti dal punto di vista funzionale. Se si utilizza il metodo StartNew() per creare ed eseguire le attività, si riceve da queste un handle utilizzabile per restare in attesa delle attività e gestire le eccezioni.