在C系列语言中，for循环扮演着重要的角色。很难想象，一百行C代码里面没有一个for循环（我有个朋友，写了个几千行的算法，没有用的for循环，我当时很惊讶），就好比，一百行中文里面，没有一个"的"。可见，for循环是代码的基本构造块。由于for循环，一般是用来，对一串类型相同的对象进行操作的，从侧面可以看出，它经常伴随着"数组"而来的。用比较通俗的话说，"for循环"与"数组"是黄金搭档。
在C#里面，引进了foreach循环，它与for循环本质是相同的，由于在foreach循环中，省去了指标i（常常，只用来取第i个项，别无他用），很多人欣然接受了foreach循环，毕竟没有夺走for循环，它还在！
编程语言一直在进化，先后经历了：...-汇编语言-...-过程式语言-...-面向对象语言-...。总体来说，越来越高级，越来越抽象。当代程序员可以不知道硬件是啥就可以编程；调用一个sort方法就排序了，不知道用的是"冒泡"还是"快速"排序算法（外国人都帮我们弄好了！每当认识到"差距超过20年"这个事实，我都...，好了，不想伤心事了！）。
在C# 3.0中，引进了Extension Methods，伴随而来的是一个新玩意儿Linq。用实用工具Reflector.exe打开System.Core.dll中的System.Linq命名空间，有个Enumerable静态类，其中有大量的对"数组"操作的扩展方法（你能想到的基本都有，不信就去看看！）。
对于用惯了for循环的朋友，如果要他/她停止使用，肯定会觉得日子没法过了。放心好了，我不会劝他/她停止使用的，就像戒烟一样，都是自己的事。（又一次跑题，言归正传！）
下面我用代码来演示，如何用"扩展方法/Linq"来干掉"for循环"：
[Test]
public void OldSum()
{
int sum0 = 0;
for (int i = 0; i < 10; i++)
{
sum0 += i;
}
Assert.AreEqual(45, sum0);
}
[Test]
public void NewSum()
{
int sum1 = Enumerable.Range(0, 10).Sum();
int sum2 = Enumerable.Range(0, 10).Aggregate((x, y) => x + y);
int sum3 = Enumerable.Range(0, 10).Aggregate(0, (x, y) => x + y);
Assert.AreEqual(45, sum1);
Assert.AreEqual(45, sum2);
Assert.AreEqual(45, sum3);
}
public void OldSum()
{
int sum0 = 0;
for (int i = 0; i < 10; i++)
{
sum0 += i;
}
Assert.AreEqual(45, sum0);
}
[Test]
public void NewSum()
{
int sum1 = Enumerable.Range(0, 10).Sum();
int sum2 = Enumerable.Range(0, 10).Aggregate((x, y) => x + y);
int sum3 = Enumerable.Range(0, 10).Aggregate(0, (x, y) => x + y);
Assert.AreEqual(45, sum1);
Assert.AreEqual(45, sum2);
Assert.AreEqual(45, sum3);
}
注：无论是对一串数字求和还是求积，归根到底，都是把一串东西变成一个东西，此时就用Aggregate！
[Test]
public void OldFilter()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
List<int> odd_list = new List<int>();
for (int i = 0; i < arr.Length; i++)
{
if (arr[i] % 2 == 1)
{
odd_list.Add(arr[i]);
}
}
int[] odd_arr = odd_list.ToArray();
Assert.That(odd_arr, Is.EquivalentTo(new int[] { 1, 3, 5, 7, 9 }));
}
[Test]
public void NewFilter()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
int[] odd_arr = arr.Where(x => x % 2 == 1).ToArray();
Assert.That(odd_arr, Is.EquivalentTo(new int[] { 1, 3, 5, 7, 9 }));
}
public void OldFilter()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
List<int> odd_list = new List<int>();
for (int i = 0; i < arr.Length; i++)
{
if (arr[i] % 2 == 1)
{
odd_list.Add(arr[i]);
}
}
int[] odd_arr = odd_list.ToArray();
Assert.That(odd_arr, Is.EquivalentTo(new int[] { 1, 3, 5, 7, 9 }));
}
[Test]
public void NewFilter()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
int[] odd_arr = arr.Where(x => x % 2 == 1).ToArray();
Assert.That(odd_arr, Is.EquivalentTo(new int[] { 1, 3, 5, 7, 9 }));
}
注：无论是取奇数还是偶数，归根到底，都是取一串东西中的某些东西，此时就用Where！
[Test]
public void OldMap()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
List<int> new_list = new List<int>();
for (int i = 0; i < arr.Length; i++)
{
new_list.Add(arr[i] * 10);
}
int[] new_arr = new_list.ToArray();
Assert.That(new_arr, Is.EquivalentTo(new int[] { 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 }));
}
[Test]
public void NewMap()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
int[] new_arr = arr.Select(x => x * 10).ToArray();
Assert.That(new_arr, Is.EquivalentTo(new int[] { 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 }));
}
public void OldMap()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
List<int> new_list = new List<int>();
for (int i = 0; i < arr.Length; i++)
{
new_list.Add(arr[i] * 10);
}
int[] new_arr = new_list.ToArray();
Assert.That(new_arr, Is.EquivalentTo(new int[] { 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 }));
}
[Test]
public void NewMap()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
int[] new_arr = arr.Select(x => x * 10).ToArray();
Assert.That(new_arr, Is.EquivalentTo(new int[] { 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 }));
}
注：无论是x10还是+99，归根到底，都是把一串东西变成一串新东西，此时就用Select！
[Test]
public void PrintMultiplicationFact()
{
Console.Write(
" 1 x 1= 1 "
+ " 1 x 2= 2 2 x 2= 4 "
+ " 1 x 3= 3 2 x 3= 6 3 x 3= 9 "
+ " 1 x 4= 4 2 x 4= 8 3 x 4=12 4 x 4=16 "
+ " 1 x 5= 5 2 x 5=10 3 x 5=15 4 x 5=20 5 x 5=25 "
+ " 1 x 6= 6 2 x 6=12 3 x 6=18 4 x 6=24 5 x 6=30 6 x 6=36 "
+ " 1 x 7= 7 2 x 7=14 3 x 7=21 4 x 7=28 5 x 7=35 6 x 7=42 7 x 7=49 "
+ " 1 x 8= 8 2 x 8=16 3 x 8=24 4 x 8=32 5 x 8=40 6 x 8=48 7 x 8=56 8 x 8=64 "
+ " 1 x 9= 9 2 x 9=18 3 x 9=27 4 x 9=36 5 x 9=45 6 x 9=54 7 x 9=63 8 x 9=72 9 x 9=81 "
);
/*********************方法一: 嵌套循环*************************/
for (int j = 1; j < 10; j++)
{
for (int i = 1; i < 10; i++)
{
if (i <= j)
{
Console.Write("{0, 2} x{1, 2}={2, 2} ", i, j, i * j);
}
}
Console.Write(" ");
}
/*********************方法二: 扩展方法*************************/
Enumerable.Range(1, 9)
.SelectMany(j => Enumerable.Range(1, 9), (j, i) => new { i, j })
.Where(x => x.i <= x.j)
.GroupBy(x => x.j)
.Select(g => g.Aggregate("", (a, x) => a + string.Format("{0, 2} x{1, 2}={2, 2} ", x.i, x.j, x.i * x.j)))
.ToList().ForEach(x => Console.WriteLine(x));
/*********************方法三: Linq表达式************************/
(
from j in Enumerable.Range(1, 9)
from i in Enumerable.Range(1, 9)
where i <= j
group new { i, j } by j into g
select new
{
LineNo = g.Key,
Line = g.Aggregate("", (a, x) => a + string.Format("{0, 2} x{1, 2}={2, 2} ", x.i, x.j, x.i * x.j))
}
).ToList().ForEach(g => Console.WriteLine(g.Line));
}
public void PrintMultiplicationFact()
{
Console.Write(
" 1 x 1= 1 "
+ " 1 x 2= 2 2 x 2= 4 "
+ " 1 x 3= 3 2 x 3= 6 3 x 3= 9 "
+ " 1 x 4= 4 2 x 4= 8 3 x 4=12 4 x 4=16 "
+ " 1 x 5= 5 2 x 5=10 3 x 5=15 4 x 5=20 5 x 5=25 "
+ " 1 x 6= 6 2 x 6=12 3 x 6=18 4 x 6=24 5 x 6=30 6 x 6=36 "
+ " 1 x 7= 7 2 x 7=14 3 x 7=21 4 x 7=28 5 x 7=35 6 x 7=42 7 x 7=49 "
+ " 1 x 8= 8 2 x 8=16 3 x 8=24 4 x 8=32 5 x 8=40 6 x 8=48 7 x 8=56 8 x 8=64 "
+ " 1 x 9= 9 2 x 9=18 3 x 9=27 4 x 9=36 5 x 9=45 6 x 9=54 7 x 9=63 8 x 9=72 9 x 9=81 "
);
/*********************方法一: 嵌套循环*************************/
for (int j = 1; j < 10; j++)
{
for (int i = 1; i < 10; i++)
{
if (i <= j)
{
Console.Write("{0, 2} x{1, 2}={2, 2} ", i, j, i * j);
}
}
Console.Write(" ");
}
/*********************方法二: 扩展方法*************************/
Enumerable.Range(1, 9)
.SelectMany(j => Enumerable.Range(1, 9), (j, i) => new { i, j })
.Where(x => x.i <= x.j)
.GroupBy(x => x.j)
.Select(g => g.Aggregate("", (a, x) => a + string.Format("{0, 2} x{1, 2}={2, 2} ", x.i, x.j, x.i * x.j)))
.ToList().ForEach(x => Console.WriteLine(x));
/*********************方法三: Linq表达式************************/
(
from j in Enumerable.Range(1, 9)
from i in Enumerable.Range(1, 9)
where i <= j
group new { i, j } by j into g
select new
{
LineNo = g.Key,
Line = g.Aggregate("", (a, x) => a + string.Format("{0, 2} x{1, 2}={2, 2} ", x.i, x.j, x.i * x.j))
}
).ToList().ForEach(g => Console.WriteLine(g.Line));
}
注：对于嵌套的for循环，就用SelectMany！
声明：for循环很好，你可以继续用，如果你想用的话。如果你喜欢尝试新东西，我想告诉你:"也许可以试试！"
附录1：乘法口诀
1 x 1= 1
1 x 2= 2 2 x 2= 4
1 x 3= 3 2 x 3= 6 3 x 3= 9
1 x 4= 4 2 x 4= 8 3 x 4=12 4 x 4=16
1 x 5= 5 2 x 5=10 3 x 5=15 4 x 5=20 5 x 5=25
1 x 6= 6 2 x 6=12 3 x 6=18 4 x 6=24 5 x 6=30 6 x 6=36
1 x 7= 7 2 x 7=14 3 x 7=21 4 x 7=28 5 x 7=35 6 x 7=42 7 x 7=49
1 x 8= 8 2 x 8=16 3 x 8=24 4 x 8=32 5 x 8=40 6 x 8=48 7 x 8=56 8 x 8=64
1 x 9= 9 2 x 9=18 3 x 9=27 4 x 9=36 5 x 9=45 6 x 9=54 7 x 9=63 8 x 9=72 9 x 9=81
1 x 2= 2 2 x 2= 4
1 x 3= 3 2 x 3= 6 3 x 3= 9
1 x 4= 4 2 x 4= 8 3 x 4=12 4 x 4=16
1 x 5= 5 2 x 5=10 3 x 5=15 4 x 5=20 5 x 5=25
1 x 6= 6 2 x 6=12 3 x 6=18 4 x 6=24 5 x 6=30 6 x 6=36
1 x 7= 7 2 x 7=14 3 x 7=21 4 x 7=28 5 x 7=35 6 x 7=42 7 x 7=49
1 x 8= 8 2 x 8=16 3 x 8=24 4 x 8=32 5 x 8=40 6 x 8=48 7 x 8=56 8 x 8=64
1 x 9= 9 2 x 9=18 3 x 9=27 4 x 9=36 5 x 9=45 6 x 9=54 7 x 9=63 8 x 9=72 9 x 9=81
附录2：完整代码
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using NUnit.Framework;
namespace KSharp
{
[TestFixture]
public class TestForLoop
{
[Test]
public void OldSum()
{
int sum0 = 0;
for (int i = 0; i < 10; i++)
{
sum0 += i;
}
Assert.AreEqual(45, sum0);
}
[Test]
public void NewSum()
{
int sum1 = Enumerable.Range(0, 10).Sum();
int sum2 = Enumerable.Range(0, 10).Aggregate((x, y) => x + y);
int sum3 = Enumerable.Range(0, 10).Aggregate(0, (x, y) => x + y);
Assert.AreEqual(45, sum1);
Assert.AreEqual(45, sum2);
Assert.AreEqual(45, sum3);
}
[Test]
public void OldFilter()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
List<int> odd_list = new List<int>();
for (int i = 0; i < arr.Length; i++)
{
if (arr[i] % 2 == 1)
{
odd_list.Add(arr[i]);
}
}
int[] odd_arr = odd_list.ToArray();
Assert.That(odd_arr, Is.EquivalentTo(new int[] { 1, 3, 5, 7, 9 }));
}
[Test]
public void NewFilter()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
int[] odd_arr = arr.Where(x => x % 2 == 1).ToArray();
Assert.That(odd_arr, Is.EquivalentTo(new int[] { 1, 3, 5, 7, 9 }));
}
[Test]
public void OldMap()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
List<int> new_list = new List<int>();
for (int i = 0; i < arr.Length; i++)
{
new_list.Add(arr[i] * 10);
}
int[] new_arr = new_list.ToArray();
Assert.That(new_arr, Is.EquivalentTo(new int[] { 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 }));
}
[Test]
public void NewMap()
{
int[] arr = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
int[] new_arr = arr.Select(x => x * 10).ToArray();
Assert.That(new_arr, Is.EquivalentTo(new int[] { 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 }));
}
[Test]
public void PrintMultiplicationFact()
{
Console.Write(
" 1 x 1= 1
"
+ " 1 x 2= 2 2 x 2= 4
"
+ " 1 x 3= 3 2 x 3= 6 3 x 3= 9
"
+ " 1 x 4= 4 2 x 4= 8 3 x 4=12 4 x 4=16
"
+ " 1 x 5= 5 2 x 5=10 3 x 5=15 4 x 5=20 5 x 5=25
"
+ " 1 x 6= 6 2 x 6=12 3 x 6=18 4 x 6=24 5 x 6=30 6 x 6=36
"
+ " 1 x 7= 7 2 x 7=14 3 x 7=21 4 x 7=28 5 x 7=35 6 x 7=42 7 x 7=49
"
+ " 1 x 8= 8 2 x 8=16 3 x 8=24 4 x 8=32 5 x 8=40 6 x 8=48 7 x 8=56 8 x 8=64
"
+ " 1 x 9= 9 2 x 9=18 3 x 9=27 4 x 9=36 5 x 9=45 6 x 9=54 7 x 9=63 8 x 9=72 9 x 9=81
"
);
/*********************方法一: 嵌套循环*************************/
for (int j = 1; j < 10; j++)
{
for (int i = 1; i < 10; i++)
{
if (i <= j)
{
Console.Write("{0, 2} x{1, 2}={2, 2} ", i, j, i * j);
}
}
Console.Write("
");
}
/*********************方法二: 扩展方法*************************/
Enumerable.Range(1, 9)
.SelectMany(j => Enumerable.Range(1, 9), (j, i) => new { i, j })
.Where(x => x.i <= x.j)
.GroupBy(x => x.j)
.Select(g => g.Aggregate("", (a, x) => a + string.Format("{0, 2} x{1, 2}={2, 2} ", x.i, x.j, x.i * x.j)))
.ToList().ForEach(x => Console.WriteLine(x));
/*********************方法三: Linq表达式************************/
(
from j in Enumerable.Range(1, 9)
from i in Enumerable.Range(1, 9)
where i <= j
group new { i, j } by j into g
select new
{
LineNo = g.Key,
Line = g.Aggregate("", (a, x) => a + string.Format("{0, 2} x{1, 2}={2, 2} ", x.i, x.j, x.i * x.j))
}
).ToList().ForEach(g => Console.WriteLine(g.Line));
}
}