LINQ表達式總結筆記

 Lambda表達式的寫法
ArgumentsToProcess => StatementsToProcessThem
小試牛刀一把，看看LAMDA表達式的威力.
List<int> list = new List<int>() { 2, 3, 4, 5, 8 };
List<int> evenNumbers = list.FindAll(i => (i % 2) == 0);
foreach (int even in evenNumbers)
{
 Console.WriteLine(even);
}

1. Where子句條件過濾結果集型（集合數據使用這種，譬如數組、列表數據，同樣適用于Datatable等多列數據集）
類SQL語句的寫法，對集合進行運算；
int[] factorsOf300 = { 2, 2, 3, 5, 5 };
var filters = from p in factorsOf300 where p<5 select p;
foreach (var filter in filters)
{
 Console.WriteLine("Value: " + Convert.ToString(filter));
}

2. 對數據集進行函數運算型(特別注意select方法里面需要用new關鍵字來確定記錄子集合）
string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };
var upperLowerWords = from w in words
  select new { Upper = w.ToUpper(), Lower = w.ToLower() };
foreach (var ul in upperLowerWords)
{
 Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);
}

3. 多個數據子集進行復合計算：
int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
int[] numbersB = { 1, 3, 5, 7, 8 };

var pairs = from a in numbersA
        from b in numbersB
        where a < b
        select new { a, b };

Console.WriteLine("Pairs where a < b:");
foreach (var pair in pairs)
{
 Console.WriteLine("{0} is less than {1}", pair.a, pair.b);
}

4. 子集運算，其中常見的算符有 Take, Skip, TakeWhile, SkipWhile.
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
var first3Numbers = numbers.Take(3);

Console.WriteLine("First 3 numbers:");
foreach (var n in first3Numbers)
{
 Console.WriteLine(n);
}

5. 排序運算，
double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 };
var sortedDoubles = from d in doubles
        orderby d descending select d;

Console.WriteLine("The doubles from highest to lowest:");
foreach (var d in sortedDoubles)
{
 Console.WriteLine(d);
}

6. 分組運算
string[] words = { "blueberry", "chimpanzee", "abacus", "banana", "apple", "cheese" };
var wordGroups = from w in words
 group w by w[0] into g
 select new { FirstLetter = g.Key, Words = g };
foreach (var g in wordGroups)
{
 Console.WriteLine("Words that start with the letter '{0}':", g.FirstLetter);
 foreach (var w in g.Words)
 {
     Console.WriteLine(w);
 }
}

7. 集合運算，算符包括Distinct, Union, Intersect, Except。
int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
int[] numbersB = { 1, 3, 5, 7, 8 };
var uniqueNumbers = numbersA.Union(numbersB);

Console.WriteLine("Unique numbers from both arrays:");
foreach (var n in uniqueNumbers)
{
   Console.WriteLine(n);
}
此集合輸出為進行整合過后的一個數據集；

8. 轉換操作，主要是包含 ToArray, ToList, ToDictionary, OfType這幾個操作指令。
double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 };
var sortedDoubles = from d in doubles
 orderby d descending
 select d;
var doublesArray = sortedDoubles.ToArray();

Console.WriteLine("Every other double from highest to lowest:");
for (int d = 0; d < doublesArray.Length; d += 2)
{
 Console.WriteLine(doublesArray[d]);
}

9. 元素操作：
 -- First - Simple
 -- First - Condition
 -- FirstOrDefault - Simple
 -- FirstOrDefault - Condition
 -- ElementAt
string[] strings = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };
string startsWithO = strings.First(s => s[0] == 'o');

10. 子序列生成功能：
Range功能:
var numbers = from n in Enumerable.Range(100, 50)
 select new { Number = n, OddEven = n % 2 == 1 ? "odd" : "even" };
foreach (var n in numbers)
{
Console.WriteLine("The number {0} is {1}.", n.Number, n.OddEven);
}
Repeat功能:
var numbers = Enumerable.Repeat(7, 10);
foreach (var n in numbers)
{ 
   Console.WriteLine(n);
}

11.集合篩選判斷功能：
 -- Any - Simple
 -- Any - Grouped
 -- All - Simple
 -- All - Grouped
- Any - Simple
string[] words = { "believe", "relief", "receipt", "field" };
bool iAfterE = words.Any(w => w.Contains("ei"));

-- Any - Grouped功能：
List<Product> products = GetProductList();
var productGroups =
 from p in products
 group p by p.Category into g
 where g.Any(p => p.UnitsInStock == 0)
select new { Category = g.Key, Products = g };

12.數據集的聚合功能： Count, Sum, Min, Max, Average, Aggregate函數
    int[] factorsOf300 = { 2, 2, 3, 5, 5 };
    int uniqueFactors = factorsOf300.Distinct().Count();

13.集合疊加以及順序比較功能： Concat，EqualAll函數
int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
int[] numbersB = { 1, 3, 5, 7, 8 };
var allNumbers = numbersA.Concat(numbersB);

14.自定義函數功能：
int[] vectorA = { 0, 2, 4, 5, 6 };
int[] vectorB = { 1, 3, 5, 7, 8 };
int dotProduct = vectorA.Combine(vectorB, (a, b) => a * b).Sum();
Console.WriteLine("Dot product: {0}", dotProduct);

15. Query的使用：Deferred Execution、Immediate Execution、Query Reuse。
int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
int i = 0;
var q = from n in numbers select ++n;
foreach (var v in q)
{
 Console.WriteLine("v = {0}, i = {1}", v, ++i);
}

16. 集合連接運算： Cross Join、Group Join、Cross Join with Group Join、Left Outer Join。
string[] productsPrefix = new string[]{
 "B",
 "D",
 "V",
 "D",
 "R"
};

string[] categories = new string[]{ 
 "Beverages",  
 "Condiments",  
 "Vegetables",  
 "Dairy Products",  
 "Seafood" };

var q =
 from p in productsPrefix
 join c in categories on p equals c.Substring(0, 1)
 select new { p, Category = c};

foreach (var v in q)
{
 Console.WriteLine(v.p + ": " + v.Category);
}
有了LINQ（LAMDA表達式）技術的支撐，我們常規的集合計算不需要自己再動手開發代碼了，利用此表達式直接計算，真是方便又簡潔。