C# LINQ干掉for循环

public void OldSum()
{
    int sum0 = 0;
    for (int i = 0; i < 10; i++)
    {
        sum0 += i;
    }
    Assert.AreEqual(45, sum0);
}

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！

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 }));
}

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！

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 }));
}

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！

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！