Enumerable.TakeWhile Method

Definition

Namespace:

System.Linq

Assembly:

System.Linq.dll

Returns elements from a sequence as long as a specified condition is true, and then skips the remaining elements.

Overloads

TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource,Int32,Boolean>)	Returns elements from a sequence as long as a specified condition is true. The element's index is used in the logic of the predicate function.
TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)	Returns elements from a sequence as long as a specified condition is true.

TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource,Int32,Boolean>)

Source:

Take.cs

Returns elements from a sequence as long as a specified condition is true. The element's index is used in the logic of the predicate function.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TSource> ^ TakeWhile(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, int, bool> ^ predicate);

public static System.Collections.Generic.IEnumerable<TSource> TakeWhile<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,int,bool> predicate);

static member TakeWhile : seq<'Source> * Func<'Source, int, bool> -> seq<'Source>

<Extension()>
Public Function TakeWhile(Of TSource) (source As IEnumerable(Of TSource), predicate As Func(Of TSource, Integer, Boolean)) As IEnumerable(Of TSource)

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IEnumerable<TSource>

The sequence to return elements from.

predicate

Func<TSource,Int32,Boolean>

A function to test each source element for a condition; the second parameter of the function represents the index of the source element.

Returns

IEnumerable<TSource>

An IEnumerable<T> that contains elements from the input sequence that occur before the element at which the test no longer passes.

Exceptions

ArgumentNullException

source or predicate is null.

Examples

The following code example demonstrates how to use TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource,Int32,Boolean>) to return elements from the start of a sequence as long as a condition that uses the element's index is true.

string[] fruits = { "apple", "passionfruit", "banana", "mango",
                      "orange", "blueberry", "grape", "strawberry" };

IEnumerable<string> query =
    fruits.TakeWhile((fruit, index) => fruit.Length >= index);

foreach (string fruit in query)
{
    Console.WriteLine(fruit);
}

/*
 This code produces the following output:

 apple
 passionfruit
 banana
 mango
 orange
 blueberry
*/

' Create an array of strings.
Dim fruits() As String =
{"apple", "passionfruit", "banana", "mango",
 "orange", "blueberry", "grape", "strawberry"}

' Take strings from the array until one
' of the string's lengths is greater than or
' equal to the string item's index in the array.
Dim query As IEnumerable(Of String) =
fruits.TakeWhile(Function(fruit, index) _
                     fruit.Length >= index)

' Display the results.
Dim output As New System.Text.StringBuilder
For Each fruit As String In query
    output.AppendLine(fruit)
Next
Console.WriteLine(output.ToString())

' This code produces the following output:
'
' apple
' passionfruit
' banana
' mango
' orange
' blueberry

Remarks

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in C# or For Each in Visual Basic.

The TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource,Int32,Boolean>) method tests each element of source by using predicate and yields the element if the result is true. Enumeration stops when the predicate function returns false for an element or when source contains no more elements.

The first argument of predicate represents the element to test. The second argument represents the zero-based index of the element within source.

The TakeWhile and SkipWhile methods are functional complements. Given a collection sequence coll and a pure function p, concatenating the results of coll.TakeWhile(p) and coll.SkipWhile(p) yields the same sequence as coll.

In Visual Basic query expression syntax, a Take While clause translates to an invocation of TakeWhile.

See also

• Take While Clause (Visual Basic)

TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)

Source:

Take.cs

Returns elements from a sequence as long as a specified condition is true.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TSource> ^ TakeWhile(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, bool> ^ predicate);

public static System.Collections.Generic.IEnumerable<TSource> TakeWhile<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);

static member TakeWhile : seq<'Source> * Func<'Source, bool> -> seq<'Source>

<Extension()>
Public Function TakeWhile(Of TSource) (source As IEnumerable(Of TSource), predicate As Func(Of TSource, Boolean)) As IEnumerable(Of TSource)

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IEnumerable<TSource>

A sequence to return elements from.

predicate

Func<TSource,Boolean>

A function to test each element for a condition.

Returns

IEnumerable<TSource>

An IEnumerable<T> that contains the elements from the input sequence that occur before the element at which the test no longer passes.

Exceptions

ArgumentNullException

source or predicate is null.

Examples

The following code example demonstrates how to use TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) to return elements from the start of a sequence as long as a condition is true.

string[] fruits = { "apple", "banana", "mango", "orange",
                      "passionfruit", "grape" };

IEnumerable<string> query =
    fruits.TakeWhile(fruit => String.Compare("orange", fruit, true) != 0);

foreach (string fruit in query)
{
    Console.WriteLine(fruit);
}

/*
 This code produces the following output:

 apple
 banana
 mango
*/

' Create an array of strings.
Dim fruits() As String =
{"apple", "banana", "mango", "orange", "passionfruit", "grape"}

' Take strings from the array until one of
' the strings matches "orange".
Dim query As IEnumerable(Of String) =
fruits.TakeWhile(Function(fruit) _
                     String.Compare("orange", fruit, True) <> 0)

' Display the results.
Dim output As New System.Text.StringBuilder
For Each fruit As String In query
    output.AppendLine(fruit)
Next
Console.WriteLine(output.ToString())

' This code produces the following output:
'
' apple
' banana
' mango

Remarks

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in C# or For Each in Visual Basic.

The TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) method tests each element of source by using predicate and yields the element if the result is true. Enumeration stops when the predicate function returns false for an element or when source contains no more elements.

The TakeWhile and SkipWhile methods are functional complements. Given a collection sequence coll and a pure function p, concatenating the results of coll.TakeWhile(p) and coll.SkipWhile(p) yields the same sequence as coll.

In Visual Basic query expression syntax, a Take While clause translates to an invocation of TakeWhile.

See also

• Take While Clause (Visual Basic)