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Linq中的TakeWhile和SkipWhile

更多时间：2015-4-9 类别：编程学习浏览量：498

Linq中的TakeWhile和SkipWhile

Linq中的TakeWhile和SkipWhile

Linq中的SkipWhile

1、含义

(1)、对数据源进行枚举,从第一个枚举得到的元素开始,调用客户端的predicate

(2)、如果返回true,则跳过该元素,继续进行枚举操作.

(3)、但是,如果一旦predicate返回为false,则该元素以后的所有元素,都不会再调用predicate,而全部枚举给客户端.

2、实现

C# 代码复制


public static IEnumerable<TSource> SkipWhile<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
...{
    if (source == null) throw Error.ArgumentNull("source");
    if (predicate == null) throw Error.ArgumentNull("predicate");
    return SkipWhileIterator<TSource>(source, predicate);
}

private static IEnumerable<TSource> SkipWhileIterator<TSource>(IEnumerable<TSource> source, Func<TSource, bool> predicate)
...{
    bool iteratorVariable0 = false;
    foreach (TSource iteratorVariable1 in source)
    ...{
        if (!iteratorVariable0 && !predicate(iteratorVariable1)) iteratorVariable0 = true;
        if (iteratorVariable0)
        ...{
            yield return iteratorVariable1;
        }
    }
}

3、实例

C# 代码复制


            int[] grades = ...{ 59, 82, 70, 56, 92, 98, 85 };

            IEnumerable<int> lowerGrades =
                grades
                .OrderByDescending(grade => grade)
                .SkipWhile(grade => grade >= 80);

            Console.WriteLine("All grades below 80:");
            foreach (int grade in lowerGrades)
            ...{
                Console.WriteLine(grade);
            }

            /**//*
             This code produces the following output:

             All grades below 80:
             70
             59
             56
            */

二、Linq中的TakeWhile

1、含义

(1)、对数据源进行枚举，从第一个枚举得到的元素开始,调用客户端传入的predicate( c.Name == ""woodyN")

(2)、如果这个predicate委托返回true的话,则将该元素作为Current元素返回给客户端,并且,继续进行相同的枚举,判断操作.

(3)、但是,一旦predicate返回false的话,MoveNext()方法将会返回false,枚举就此打住,忽略剩下的所有元素.

2、实例

C# 代码复制


            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
            */

三、通过一个实例说明Linq中的TakeWhile和SkipWhile的区别

C# 代码复制


static List<Customer> customers = new List<Customer> ...{
new Customer ...{ CustomerID=1,Name="woody1"},
new Customer ...{ CustomerID=2,Name="woody2"},
new Customer ...{ CustomerID=3,Name="woody3"},
new Customer ...{ CustomerID=4,Name="woody1"}
};

var cs1 = customers.TakeWhile(c => c.Name == "woody1");
var cs2 = customers.TakeWhile(c => c.Name == "woody2");
var cs3 = customers.SkipWhile(c => c.Name == "woody1");
var cs4 = customers.SkipWhile(c => c.Name == "woody2");

cs1--cs4的结果

cs1 : woody1(CustomerID=1)
cs2 : 没有任何元素
cs3 : woody2 , woody3 , woody1(CustomerID=4)
cs4 : woody1(CustomerID=1),woody2,woody3,woody1(CustomerID=4)

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