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C# List<T>排序总结
转 http://blog.csdn.net/jimo_lonely/article/details/51711821
这里有很多种方法对List进行排序,本文总结了三种方法,但多种实现。
1.对基础类型排序
方法一:
调用sort方法,如果需要降序,进行反转:
List<int> list = new List<int>();list.Sort();// 升序排序list.Reverse();// 反转顺序
方法二:
使用lambda表达式,在前面加个负号就是降序了
List<int> list= new List<int>(){5,1,22,11,4};list.Sort((x, y) => x.CompareTo(y));//升序list.Sort((x, y) => -x.CompareTo(y));//降序
接下来是对非基本类型排序,以一个类为例。
2.准备
首先写一个类用于排序,里面有两个属性,一个构造方法,重写了ToString方法:
class People { private int _id; private string _name; public People(int id,string name) { this._id = id; this.Name = name; } public int Id { get { return _id; } set { _id = value; } } public string Name { get { return _name; } set { _name = value; } } //重写ToString public override string ToString() { return "ID:"+_id+" Name:"+_name; } }
然后添加一些随机数据,仍希望用Sort排序
List<People> list = new List<People>(); Random r = new Random(); //添加数据 for(int i = 0; i < 10; i++) { int j = r.Next(0, 10); list.Add(new People(j, "name" + j)); } Console.WriteLine("排序前:"); foreach(var p in list) { Console.WriteLine(p); } list.Sort();//排序 Console.WriteLine("排序后:"); foreach (var p in list) { Console.WriteLine(p); }
很不幸,前面输出正常,后面抛异常了:
查看Sort源码可知它有如下几个重载:
第三和第四个差不多。
3.实现IComparable接口
可以看到它只有一个方法,我们只需要修改类本身
class People: IComparable<People> { private int _id; private string _name; public People(int id,string name) { this._id = id; this.Name = name; } public int Id { get { return _id; } set { _id = value; } } public string Name { get { return _name; } set { _name = value; } } //重写的CompareTo方法,根据Id排序 public int CompareTo(People other) { if (null == other) { return 1;//空值比较大,返回1 } //return this.Id.CompareTo(other.Id);//升序 return other.Id.CompareTo(this.Id);//降序 } //重写ToString public override string ToString() { return "ID:"+_id+" Name:"+_name; } }
4.实现IComparer接口
我们首先来看看这个接口:
public interface IComparer<in T> { // Parameters: // x: // The first object to compare. // // y: // The second object to compare. // // Returns: // A signed integer that indicates the relative values of x and y, as shown in the // following table.Value Meaning Less than zerox is less than y.Zerox equals y.Greater // than zerox is greater than y. int Compare(T x, T y); }
重点就看返回值,小于0代表x < y,等于0代表x=y,大于0代表x > y.
下面看一下类的实现,非常简单,一句代码:
class People:IComparer<People> { private int _id; private string _name; public People() { } public People(int id,string name) { this._id = id; this.Name = name; } public int Id { get { return _id; } set { _id = value; } } public string Name { get { return _name; } set { _name = value; } } //Compare函数 public int Compare(People x, People y) { return x.Id.CompareTo(y.Id);//升序 } //重写ToString public override string ToString() { return "ID:"+_id+" Name:"+_name; } }
但是还没完,我们其实是用了第2点说的第一个重载方法,所以List还需要参数:
IComparer<People> comparer = new People(); list.Sort(comparer);
5.更简单的
虽然想实现排序上面的接口代码也不多,但有时候只是偶尔排序,并不像修改类,怎么办呢?当然有更简单的方法,委托和lambda表达式:
所以就有了下面的代码,不需要修改类,只需要用委托构造重载而已:
list.Sort( delegate(People p1,People p2) { return p1.Id.CompareTo(p2.Id);//升序 } );
当然,lambda表达式实现更简单:
list.Sort((x,y)=> { return x.Id.CompareTo(y.Id); })
6.OrderBy方法
此方法将排序好的list再赋给原来的list,也可以给其他的。
list = list.OrderBy(o => o.Id).ToList();//升序list = list.OrderByDescending(o => o.Id).ToList();//降序
- 1
- 2
- 1
- 2
7.多权重排序
排序的方法我就知道这么多了(其实有更多),接下来还有一个问题,如果希望当ID相同时比较Name,上面的代码就需要改改了。
其中,接口IComparable这样写:
//重写的CompareTo方法,根据Id排序 public int CompareTo(People other) { if (null == other) { return 1;//空值比较大,返回1 } //等于返回0 int re = this.Id.CompareTo(other.Id); if (0 == re) { //id相同再比较Name return this.Name.CompareTo(other.Name); } return re; }
IComparer和delegate还有lambda里可以这样:
public int Compare(People x, People y) { int re = x.Id.CompareTo(y.Id); if (0 == re) { return x.Name.CompareTo(y.Name); } return re; }
OrderBy方法有点不同:
list = list.OrderBy(o => o.Id).ThenBy(o=>o.Name).ToList();list = list.OrderByDescending(o => o.Id).ThenByDescending(o=>o.Name).ToList();//降序
8.总结
虽然说了那么多,其实说到底也就三种方法,两个接口和OrderBy方法,lambda表达式只是让形式更简单。
C# List<T>排序总结