Array.Exists<T>(T[], Predicate<T>) Method
Definition
Important
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Determines whether the specified array contains elements that match the conditions defined by the specified predicate.
public:
generic <typename T>
static bool Exists(cli::array <T> ^ array, Predicate<T> ^ match);public static bool Exists<T> (T[] array, Predicate<T> match);static member Exists : 'T[] * Predicate<'T> -> boolPublic Shared Function Exists(Of T) (array As T(), match As Predicate(Of T)) As BooleanType Parameters
- T
The type of the elements of the array.
Parameters
- array
- T[]
The one-dimensional, zero-based Array to search.
- match
- Predicate<T>
The Predicate<T> that defines the conditions of the elements to search for.
Returns
true if array contains one or more elements that match the conditions defined by the specified predicate; otherwise, false.
Exceptions
Examples
The following example specifies the match conditions for the Exists method using lambda expressions to check whether a planet starts with a given letter or whether the planet is found on the given array.
using System;
namespace Example
{
class Program
{
static void Main(string[] args)
{
string[] planets = { "Mercury", "Venus",
"Earth", "Mars", "Jupiter",
"Saturn", "Uranus", "Neptune" };
Console.WriteLine("One or more planets begin with 'M': {0}",
Array.Exists(planets, element => element.StartsWith("M")));
Console.WriteLine("One or more planets begin with 'T': {0}",
Array.Exists(planets, element => element.StartsWith("T")));
Console.WriteLine("Is Pluto one of the planets? {0}",
Array.Exists(planets, element => element == "Pluto"));
}
}
}
// The example displays the following output:
// One or more planets begin with 'M': True
// One or more planets begin with 'T': False
// Is Pluto one of the planets? False
open System
let planets =
[| "Mercury"; "Venus"
"Earth"; "Mars"; "Jupiter"
"Saturn"; "Uranus"; "Neptune" |]
Array.Exists(planets, fun element -> element.StartsWith "M")
|> printfn "One or more planets begin with 'M': %O"
Array.Exists(planets, fun element -> element.StartsWith "T")
|> printfn "One or more planets begin with 'T': %O"
Array.Exists(planets, fun element -> element = "Pluto")
|> printfn "Is Pluto one of the planets? %O"
// The example displays the following output:
// One or more planets begin with 'M': True
// One or more planets begin with 'T': False
// Is Pluto one of the planets? False
Module Example
Public Sub Main()
Dim planets() As String = {"Mercury", "Venus",
"Earth", "Mars", "Jupiter",
"Saturn", "Uranus", "Neptune"}
Console.WriteLine("One or more planets begin with 'M': {0}",
Array.Exists(planets, Function(element)
Return element.StartsWith("M")
End Function))
Console.WriteLine("One or more planets begin with 'T': {0}",
Array.Exists(planets, Function(element)
Return element.StartsWith("T")
End Function))
Console.WriteLine("Is Pluto one of the planets? {0}",
Array.Exists(planets, Function(element)
Return element.Equals("Pluto")
End Function))
End Sub
End Module
' The example displays the following output:
' One or more planets begin with 'M': True
' One or more planets begin with 'T': False
' Is Pluto one of the planets? False
The following example uses the Exists method to indicate whether any names in a string array begin with a specified character. The example instantiates a StringSearcher object by passing the string to search for to its class constructor. The StringSearcher.StartsWith method has same signature as the Predicate<T> delegate. When the Exists method is called, each member of the array is passed to the delegate until it returns true or iterates all the elements in the array.
using System;
public class Example
{
public static void Main()
{
String[] names = { "Adam", "Adel", "Bridgette", "Carla",
"Charles", "Daniel", "Elaine", "Frances",
"George", "Gillian", "Henry", "Irving",
"James", "Janae", "Lawrence", "Miguel",
"Nicole", "Oliver", "Paula", "Robert",
"Stephen", "Thomas", "Vanessa",
"Veronica", "Wilberforce" };
Char[] charsToFind = { 'A', 'K', 'W', 'Z' };
foreach (var charToFind in charsToFind)
Console.WriteLine("One or more names begin with '{0}': {1}",
charToFind,
Array.Exists(names, (new StringSearcher(charToFind)).StartsWith));
}
}
public class StringSearcher
{
char firstChar;
public StringSearcher(char firstChar)
{
this.firstChar = char.ToUpper(firstChar);
}
public bool StartsWith(string s)
{
if (string.IsNullOrEmpty(s)) return false;
if(s.Substring(0, 1).ToUpper() == firstChar.ToString())
return true;
else
return false;
}
}
// The example displays the following output:
// One or more names begin with 'A': True
// One or more names begin with 'K': False
// One or more names begin with 'W': True
// One or more names begin with 'Z': False
open System
type StringSearcher(firstChar) =
member _.StartsWith(s) =
if String.IsNullOrEmpty s then
false
else
s.Substring(0, 1).ToUpper() = string firstChar
let names =
[| "Adam"; "Adel"; "Bridgette"; "Carla";
"Charles"; "Daniel"; "Elaine"; "Frances"
"George"; "Gillian"; "Henry"; "Irving"
"James"; "Janae"; "Lawrence"; "Miguel"
"Nicole"; "Oliver"; "Paula"; "Robert"
"Stephen"; "Thomas"; "Vanessa"
"Veronica"; "Wilberforce" |]
let charsToFind = [ 'A'; 'K'; 'W'; 'Z' ]
for char in charsToFind do
let exists = Array.Exists(names, fun x -> StringSearcher(char).StartsWith x)
// let exists = Array.exists (StringSearcher(char).StartsWith) names
printfn $"One or more names begin with '{char}': {exists}"
// The example displays the following output:
// One or more names begin with 'A': True
// One or more names begin with 'K': False
// One or more names begin with 'W': True
// One or more names begin with 'Z': False
Module Example
Public Sub Main()
Dim names() As String = { "Adam", "Adel", "Bridgette", "Carla",
"Charles", "Daniel", "Elaine", "Frances",
"George", "Gillian", "Henry", "Irving",
"James", "Janae", "Lawrence", "Miguel",
"Nicole", "Oliver", "Paula", "Robert",
"Stephen", "Thomas", "Vanessa",
"Veronica", "Wilberforce" }
Dim charsToFind() As Char = { "A"c, "K"c, "W"c, "Z"c }
For Each charToFind In charsToFind
Console.WriteLine("One or more names begin with '{0}': {1}",
charToFind,
Array.Exists(names, AddressOf (New StringSearcher(charToFind)).StartsWith))
Next
End Sub
End Module
Public Class StringSearcher
Dim firstChar As Char
Public Sub New(firstChar As Char)
Me.firstChar = Char.ToUpper(firstChar)
End Sub
Public Function StartsWith(s As String) As Boolean
If String.IsNullOrEmpty(s) Then Return False
If s.Substring(0, 1).ToUpper = firstChar Then
Return True
Else
Return False
End If
End Function
End Class
' The example displays the following output:
' One or more names begin with 'A': True
' One or more names begin with 'K': False
' One or more names begin with 'W': True
' One or more names begin with 'Z': False
You can also use a lambda expression rather than explicitly define a method whose signature corresponds to that of the delegate. The following example replaces the StringSearcher class and its StartsWith method with a lambda expression.
using System;
public class Example
{
public static void Main()
{
String[] names = { "Adam", "Adel", "Bridgette", "Carla",
"Charles", "Daniel", "Elaine", "Frances",
"George", "Gillian", "Henry", "Irving",
"James", "Janae", "Lawrence", "Miguel",
"Nicole", "Oliver", "Paula", "Robert",
"Stephen", "Thomas", "Vanessa",
"Veronica", "Wilberforce" };
Char[] charsToFind = { 'A', 'K', 'W', 'Z' };
foreach (var charToFind in charsToFind)
Console.WriteLine("One or more names begin with '{0}': {1}",
charToFind,
Array.Exists(names,
s => { if (string.IsNullOrEmpty(s))
return false;
if (s.Substring(0, 1).ToUpper() == charToFind.ToString())
return true;
else
return false;
} ));
}
}
// The example displays the following output:
// One or more names begin with 'A': True
// One or more names begin with 'K': False
// One or more names begin with 'W': True
// One or more names begin with 'Z': False
open System
let names =
[| "Adam"; "Adel"; "Bridgette"; "Carla";
"Charles"; "Daniel"; "Elaine"; "Frances"
"George"; "Gillian"; "Henry"; "Irving"
"James"; "Janae"; "Lawrence"; "Miguel"
"Nicole"; "Oliver"; "Paula"; "Robert"
"Stephen"; "Thomas"; "Vanessa"
"Veronica"; "Wilberforce" |]
let charsToFind = [ 'A'; 'K'; 'W'; 'Z' ]
for char in charsToFind do
let exists =
Array.Exists(names, fun s ->
if String.IsNullOrEmpty s then false
else s.Substring(0, 1).ToUpper() = string char)
printfn $"One or more names begin with '{char}': {exists}"
// The example displays the following output:
// One or more names begin with 'A': True
// One or more names begin with 'K': False
// One or more names begin with 'W': True
// One or more names begin with 'Z': False
Module Example
Public Sub Main()
Dim names() As String = { "Adam", "Adel", "Bridgette", "Carla",
"Charles", "Daniel", "Elaine", "Frances",
"George", "Gillian", "Henry", "Irving",
"James", "Janae", "Lawrence", "Miguel",
"Nicole", "Oliver", "Paula", "Robert",
"Stephen", "Thomas", "Vanessa",
"Veronica", "Wilberforce" }
Dim charsToFind() As Char = { "A"c, "K"c, "W"c, "Z"c }
For Each charToFind In charsToFind
Console.WriteLine("One or more names begin with '{0}': {1}",
charToFind,
Array.Exists(names, Function(s)
If String.IsNullOrEmpty(s) Then Return False
If s.Substring(0, 1).ToUpper = charToFind Then
Return True
Else
Return False
End If
End Function ))
Next
End Sub
End Module
' The example displays the following output:
' One or more names begin with 'A': True
' One or more names begin with 'K': False
' One or more names begin with 'W': True
' One or more names begin with 'Z': False
Remarks
The Predicate<T> is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. The elements of array are individually passed to the Predicate<T>, and processing is stopped when a match is found.
Note
In C# and Visual Basic, it is not necessary to create the Predicate<T> delegate explicitly. These languages infer the correct delegate from context and create it automatically. In F#, functions and lambda expressions are implicitly converted.
This method is an O(n) operation, where n is the Length of array.
Applies to
See also
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