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.
