LINQ之Take, TakeWhile

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Take()

Take()可以获取序列的指定部分。具体使用请看下面案例。

MSDN public static IEnumerable Take( this IEnumerable source, int count );

代码示例：

public static class Program
{
    static void Main( string[] args )
    {
        int[]       numbers = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        string[]    texts   = new string[] { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

        // 开始的3个
        IEnumerable    takenNumbers    = numbers.Take( 3 );
        // 开始的4个
        IEnumerable takenTexts      = texts.Take( 4 );

        System.Console.WriteLine( "takenNumbers:{0}", takenNumbers.Text() );
        System.Console.WriteLine( "takenTexts  :{0}", takenTexts.Text() );

        System.Console.ReadKey();
    }
    
    public static string Text( this IEnumerable i_source )
    {
        string text = string.Empty;
        foreach( var value in i_source )
        {
            text += string.Format( "[{0}], ", value );
        }
        return text;
    }
}

takenNumbers:[0], [1], [2],
takenTexts :[Sun], [Mon], [Tue], [Wed],

如果将大于元素总数的数字或负数传递给此参数，它也会正常工作。负数的话相当于0。

代码示例：

public static class Program
{
    static void Main( string[] args )
    {
        int[]       numbers = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        string[]    texts   = new string[] { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

        // 超过总数
        IEnumerable    takenNumbers    = numbers.Take( 100 );
        // 负数
        IEnumerable takenTexts      = texts.Take( -5 );

        System.Console.WriteLine( "takenNumbers:{0}", takenNumbers.Text() );
        System.Console.WriteLine( "takenTexts  :{0}", takenTexts.Text() );

        System.Console.ReadKey();
    }

    public static string Text( this IEnumerable i_source )
    {
        string text = string.Empty;
        foreach( var value in i_source )
        {
            text += string.Format( "[{0}], ", value );
        }
        return text;
    }
}

takenNumbers:[0], [1], [2], [3], [4], [5], [6], [7], [8], [9],
takenTexts :

在前一种形式中，Take()所需元素的数量是从头开始指定的。 但是，您的需求可能是指定条件而不是数字。

在这种情况下TakeWhile()。 您可以在参数中描述条件。

MSDN public static IEnumerable TakeWhile( this IEnumerable source, Func predicate );

代码示例：

public static class Program
{
    static void Main( string[] args )
    {
        int[]       numbers = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        string[]    texts   = new string[] { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

        // 小于4的值
        IEnumerable    takenNumbers    = numbers.TakeWhile( value => value  value.EndsWith( "n" )  );

        System.Console.WriteLine( "takenNumbers:{0}", takenNumbers.Text() );
        System.Console.WriteLine( "takenTexts  :{0}", takenTexts.Text() );

        System.Console.ReadKey();
    }

    public static string Text( this IEnumerable i_source )
    {
        string text = string.Empty;
        foreach( var value in i_source )
        {
            text += string.Format( "[{0}], ", value );
        }
        return text;
    }
}

takenNumbers:[0], [1], [2], [3],
takenTexts :[Sun], [Mon],

当TakeWhile()指定条件时，不仅可以获得每个元素的信息，而且可以获得每个元素的索引。 MSDN public static IEnumerable TakeWhile( this IEnumerable source, Func predicate );

在以下示例中，index索引号作为参数在lambda表达式条件语句中传递。

代码示例：

public static class Program
{
    static void Main( string[] args )
    {
        int[]       numbers = new int[] { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 };
        string[]    texts   = new string[] { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

        // 值小于索引
        IEnumerable    takenNumbers    = numbers.TakeWhile( ( value, index ) => value > index );
        // 值的长度大于索引
        IEnumerable takenTexts      = texts.TakeWhile( ( value, index ) => value.Length > index );
        
        System.Console.WriteLine( "takenNumbers:{0}", takenNumbers.Text() );
        System.Console.WriteLine( "takenTexts  :{0}", takenTexts.Text() );
        
        System.Console.ReadKey();
    }

    public static string Text( this IEnumerable i_source )
    {
        string text = string.Empty;
        foreach( var value in i_source )
        {
            text += string.Format( "[{0}], ", value );
        }
        return text;
    }
}

takenNumbers:[9], [8], [7], [6], [5],
takenTexts :[Sun], [Mon], [Tue],

TakeWhile()和Where()的区别

有一个类似的LINQ函数Where()。

MSDN public static IEnumerable Where( this IEnumerable source, Func predicate );

Where()用于获取符合条件的元素。 让我们用Where()与TakeWhile()进行比较。

以下示例描述了获取元素低于指定数字的条件。

public static class Program
{
    static void Main( string[] args )
    {
        int[]   numbers = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

        // 值小于5
        IEnumerable    takenNumbers    = numbers.TakeWhile( value => value  value  value  value