C#4.0新特性之(四)新的LINQ扩展方法-Zip()
1.简介
所谓zip（中文有拉链的意思），就是像拉链一样，把两个list缝合在一起。Python中有个zip函数可以用来方便的合并两个或者多个集合，例如：
>>> firstName=['Freesc','Joshua','Ken']
>>> lastName=['Huang','Guan','Wang']
>>> for f,l in zip(firstName,lastName):
print('{0} {1}'.format(f,l))
>>> lastName=['Huang','Guan','Wang']
>>> for f,l in zip(firstName,lastName):
print('{0} {1}'.format(f,l))
以上代码会打印出
Freesc Huang
Joshua Guan
Ken Wang
在C#4.0中，我们可以看到一个类似的扩展函数[1]：
//
// Summary:
// Merges two sequences by using the specified predicate function.
//
// Parameters:
// first:
// The first sequence to merge.
//
// second:
// The second sequence to merge.
//
// resultSelector:
// A function that specifies how to merge the elements from the two sequences.
//
// Type parameters:
// TFirst:
// The type of the elements of the first input sequence.
//
// TSecond:
// The type of the elements of the second input sequence.
//
// TResult:
// The type of the elements of the result sequence.
//
// Returns:
// An System.Collections.Generic.IEnumerable<T> that contains merged elements
// of two input sequences.
public static IEnumerable<TResult> Zip<TFirst, TSecond, TResult>(this IEnumerable<TFirst> first,
IEnumerable<TSecond> second,
Func<TFirst, TSecond, TResult> resultSelector);
// Summary:
// Merges two sequences by using the specified predicate function.
//
// Parameters:
// first:
// The first sequence to merge.
//
// second:
// The second sequence to merge.
//
// resultSelector:
// A function that specifies how to merge the elements from the two sequences.
//
// Type parameters:
// TFirst:
// The type of the elements of the first input sequence.
//
// TSecond:
// The type of the elements of the second input sequence.
//
// TResult:
// The type of the elements of the result sequence.
//
// Returns:
// An System.Collections.Generic.IEnumerable<T> that contains merged elements
// of two input sequences.
public static IEnumerable<TResult> Zip<TFirst, TSecond, TResult>(this IEnumerable<TFirst> first,
IEnumerable<TSecond> second,
Func<TFirst, TSecond, TResult> resultSelector);
它可以用来合并列表，并且提供了自定义的组合规则:Func<TFirst, TSecond, TResult> resultSelector。
2.示例
下面是一段和代码1功能一样的C#4.0程序：
List<String> firstName = new List<String> { "Freesc", "Joshua", "Ken" };
List<String> lastName = new List<String> { "Huang", "Guan", "Wang" };
foreach (var name in firstName.Zip(lastName, (fname, lname) => fname + " " + lname))
{
Console.WriteLine(name);
}
List<String> lastName = new List<String> { "Huang", "Guan", "Wang" };
foreach (var name in firstName.Zip(lastName, (fname, lname) => fname + " " + lname))
{
Console.WriteLine(name);
}
3.Zip()的实现
在python中要实现一个zip，很简单(这里省去了异常处理)，只需要用到三个内建函数，iter,map和next:
def zip(*iterables):
# zip('ABCD', 'xy') --> Ax By
iterables = map(iter, iterables)
while iterables:
yield tuple(map(next, iterables))
# zip('ABCD', 'xy') --> Ax By
iterables = map(iter, iterables)
while iterables:
yield tuple(map(next, iterables))
类似的，如果不考虑异常处理，C#的Zip扩展方法可以是如下实现[2]：
代码54.总结
Zip作为LINQ系统的新成员，提供了一种自由组合两个集合的方式，要注意的是，这个Zip使用时要求两个序列的长度一致，如果不一致，它会yield较短的长度。这一点和python中的zip是一样的。另外，您不妨可以试着写一个组合多个集合的MultiZip方法，也许它对您更加有用;-)
5.引用
[1] http://msdn.microsoft.com/en-us/library/dd267698(VS.100).aspx
