The following code example demonstrates the Exists<(Of <(T>)>) generic method and several other generic methods that use the Predicate<(Of <(T>)>) generic delegate.
An array of strings is created, containing 8 dinosaur names, two of which (at positions 1 and 5) end with "saurus". The code example also defines a search predicate method named EndsWithSaurus, which accepts a string parameter and returns a Boolean value indicating whether the input string ends in "saurus".
The Exists<(Of <(T>)>) method stops and returns true at the first element for which the predicate method returns true, which in this case is "Amargasaurus".
.gif) Note: |
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In C# and Visual Basic, it is not necessary to create the Predicate<string> delegate (Predicate(Of String) in Visual Basic) explicitly. These languages infer the correct delegate from context and create it automatically. |
The TrueForAll<(Of <(T>)>) generic method stops and returns false at the first element for which the predicate method returns false.
The Find<(Of <(T>)>) generic method traverses the array from the beginning, passing each element in turn to the EndsWithSaurus method. The search stops when the EndsWithSaurus method returns true for the element "Amargasaurus".
The FindLast<(Of <(T>)>) generic method is used to search the array backward from the end. It finds the element "Dilophosaurus" at position 5. The FindAll<(Of <(T>)>) generic method is used to return an array containing all the elements that end in "saurus". The elements are displayed.
Imports System
Public Class Example
Public Shared Sub Main()
Dim dinosaurs() As String = { "Compsognathus", _
"Amargasaurus", "Oviraptor", "Velociraptor", _
"Deinonychus", "Dilophosaurus", "Gallimimus", _
"Triceratops" }
Console.WriteLine()
For Each dinosaur As String In dinosaurs
Console.WriteLine(dinosaur)
Next
Console.WriteLine(vbLf & _
"Array.Exists(dinosaurs, AddressOf EndsWithSaurus): {0}", _
Array.Exists(dinosaurs, AddressOf EndsWithSaurus))
Console.WriteLine(vbLf & _
"Array.TrueForAll(dinosaurs, AddressOf EndsWithSaurus: {0}", _
Array.TrueForAll(dinosaurs, AddressOf EndsWithSaurus))
Console.WriteLine(vbLf & _
"Array.Find(dinosaurs, AddressOf EndsWithSaurus): {0}", _
Array.Find(dinosaurs, AddressOf EndsWithSaurus))
Console.WriteLine(vbLf & _
"Array.FindLast(dinosaurs, AddressOf EndsWithSaurus): {0}", _
Array.FindLast(dinosaurs, AddressOf EndsWithSaurus))
Console.WriteLine(vbLf & _
"Array.FindAll(dinosaurs, AddressOf EndsWithSaurus):")
Dim subArray() As String = _
Array.FindAll(dinosaurs, AddressOf EndsWithSaurus)
For Each dinosaur As String In subArray
Console.WriteLine(dinosaur)
Next
End Sub
' Search predicate returns true if a string ends in "saurus".
Private Shared Function EndsWithSaurus(ByVal s As String) _
As Boolean
' AndAlso prevents evaluation of the second Boolean
' expression if the string is so short that an error
' would occur.
If (s.Length > 5) AndAlso _
(s.Substring(s.Length - 6).ToLower() = "saurus") Then
Return True
Else
Return False
End If
End Function
End Class
' This code example produces the following output:
'
'Compsognathus
'Amargasaurus
'Oviraptor
'Velociraptor
'Deinonychus
'Dilophosaurus
'Gallimimus
'Triceratops
'
'Array.Exists(dinosaurs, AddressOf EndsWithSaurus): True
'
'Array.TrueForAll(dinosaurs, AddressOf EndsWithSaurus: False
'
'Array.Find(dinosaurs, AddressOf EndsWithSaurus): Amargasaurus
'
'Array.FindLast(dinosaurs, AddressOf EndsWithSaurus): Dilophosaurus
'
'Array.FindAll(dinosaurs, AddressOf EndsWithSaurus):
'Amargasaurus
'Dilophosaurus
using System;
public class Example
{
public static void Main()
{
string[] dinosaurs = { "Compsognathus",
"Amargasaurus", "Oviraptor", "Velociraptor",
"Deinonychus", "Dilophosaurus", "Gallimimus",
"Triceratops" };
Console.WriteLine();
foreach(string dinosaur in dinosaurs)
{
Console.WriteLine(dinosaur);
}
Console.WriteLine(
"\nArray.Exists(dinosaurs, EndsWithSaurus): {0}",
Array.Exists(dinosaurs, EndsWithSaurus));
Console.WriteLine(
"\nArray.TrueForAll(dinosaurs, EndsWithSaurus): {0}",
Array.TrueForAll(dinosaurs, EndsWithSaurus));
Console.WriteLine(
"\nArray.Find(dinosaurs, EndsWithSaurus): {0}",
Array.Find(dinosaurs, EndsWithSaurus));
Console.WriteLine(
"\nArray.FindLast(dinosaurs, EndsWithSaurus): {0}",
Array.FindLast(dinosaurs, EndsWithSaurus));
Console.WriteLine(
"\nArray.FindAll(dinosaurs, EndsWithSaurus):");
string[] subArray =
Array.FindAll(dinosaurs, EndsWithSaurus);
foreach(string dinosaur in subArray)
{
Console.WriteLine(dinosaur);
}
}
// Search predicate returns true if a string ends in "saurus".
private static bool EndsWithSaurus(String s)
{
if ((s.Length > 5) &&
(s.Substring(s.Length - 6).ToLower() == "saurus"))
{
return true;
}
else
{
return false;
}
}
}
/* This code example produces the following output:
Compsognathus
Amargasaurus
Oviraptor
Velociraptor
Deinonychus
Dilophosaurus
Gallimimus
Triceratops
Array.Exists(dinosaurs, EndsWithSaurus): True
Array.TrueForAll(dinosaurs, EndsWithSaurus): False
Array.Find(dinosaurs, EndsWithSaurus): Amargasaurus
Array.FindLast(dinosaurs, EndsWithSaurus): Dilophosaurus
Array.FindAll(dinosaurs, EndsWithSaurus):
Amargasaurus
Dilophosaurus
*/
using namespace System;
// Search predicate returns true if a string ends in "saurus".
bool EndsWithSaurus(String^ s)
{
if ((s->Length > 5) &&
(s->Substring(s->Length - 6)->ToLower() == "saurus"))
{
return true;
}
else
{
return false;
}
};
void main()
{
array<String^>^ dinosaurs = { "Compsognathus",
"Amargasaurus", "Oviraptor", "Velociraptor",
"Deinonychus", "Dilophosaurus", "Gallimimus",
"Triceratops" };
Console::WriteLine();
for each(String^ dinosaur in dinosaurs )
{
Console::WriteLine(dinosaur);
}
Console::WriteLine("\nArray::Exists(dinosaurs, EndsWithSaurus): {0}",
Array::Exists(dinosaurs, gcnew Predicate<String^>(EndsWithSaurus)));
Console::WriteLine("\nArray::TrueForAll(dinosaurs, EndsWithSaurus): {0}",
Array::TrueForAll(dinosaurs, gcnew Predicate<String^>(EndsWithSaurus)));
Console::WriteLine("\nArray::Find(dinosaurs, EndsWithSaurus): {0}",
Array::Find(dinosaurs, gcnew Predicate<String^>(EndsWithSaurus)));
Console::WriteLine("\nArray::FindLast(dinosaurs, EndsWithSaurus): {0}",
Array::FindLast(dinosaurs, gcnew Predicate<String^>(EndsWithSaurus)));
Console::WriteLine("\nArray::FindAll(dinosaurs, EndsWithSaurus):");
array<String^>^ subArray =
Array::FindAll(dinosaurs, gcnew Predicate<String^>(EndsWithSaurus));
for each(String^ dinosaur in subArray)
{
Console::WriteLine(dinosaur);
}
}
/* This code example produces the following output:
Compsognathus
Amargasaurus
Oviraptor
Velociraptor
Deinonychus
Dilophosaurus
Gallimimus
Triceratops
Array::Exists(dinosaurs, EndsWithSaurus): True
Array::TrueForAll(dinosaurs, EndsWithSaurus): False
Array::Find(dinosaurs, EndsWithSaurus): Amargasaurus
Array::FindLast(dinosaurs, EndsWithSaurus): Dilophosaurus
Array::FindAll(dinosaurs, EndsWithSaurus):
Amargasaurus
Dilophosaurus
*/