Csharp/C Sharp/LINQ/GroupBy

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GroupBy partitions a list of numbers by their remainder when divided by 5.

<source lang="csharp"> using System; using System.Collections.Generic; using System.Linq; using System.Text; public class MainClass {

   public static void Main() {
       int[] numbers = { 5, 4, 1, 3, 9 };
       var numberGroups =
           from n in numbers
           group n by n % 5 into g
           select new { Remainder = g.Key, Numbers = g };
       foreach (var g in numberGroups) {
           Console.WriteLine("Numbers with a remainder of {0} when divided by 5:", g.Remainder);
           foreach (var n in g.Numbers) {
               Console.WriteLine(n);
           }
       }
   }

}

</source>


Use Group By

<source lang="csharp"> using System; using System.Collections; using System.Collections.Generic; using System.Text; using System.Linq; using System.Reflection; public class MainClass{

  public static void Main(){
           var query = from m in typeof(double).GetMethods()
                       select m.Name;
           var q = from m in typeof(double).GetMethods()
                   group m by m.Name into gb
                   select new {Name = gb.Key, Overloads = gb.Count()};
  }

}

</source>


uses group by to partition a list of each customer"s orders, first by year, and then by month.

<source lang="csharp"> using System; using System.Collections.Generic; using System.Linq; using System.Text; public class MainClass {

   public static void Main() {
       List<Customer> customers = GetCustomerList();
       var customerOrderGroups =
           from c in customers
           select
               new {
                   c.rupanyName,
                   YearGroups =
                       from o in c.Orders
                       group o by o.OrderDate.Year into yg
                       select
                           new {
                               Year = yg.Key,
                               MonthGroups =
                                   from o in yg
                                   group o by o.OrderDate.Month into mg
                                   select new { Month = mg.Key, Orders = mg }
                           }
               };
       Console.Write(customerOrderGroups);
   }
   static List<Customer> GetCustomerList() {
       List<Product> empTree = new List<Product>();
       empTree.Add(new Product { ProductName = "A", Category = "O", UnitPrice = 12, UnitsInStock = 5, Total = 36, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "B", Category = "O", UnitPrice = 2, UnitsInStock = 4, Total = 35, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "C", Category = "O", UnitPrice = 112, UnitsInStock = 3, Total = 34, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "D", Category = "O", UnitPrice = 112, UnitsInStock = 0, Total = 33, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "E", Category = "O", UnitPrice = 1112, UnitsInStock = 2, Total = 32, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "F", Category = "O", UnitPrice = 11112, UnitsInStock = 0, Total = 31, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       List<Customer> l = new List<Customer>();
       l.Add(new Customer { CompanyName = "A", Region = "R1", UnitsInStock = 1, Orders = empTree, CustomerId = 0 });
       l.Add(new Customer { CompanyName = "B", Region = "R2", UnitsInStock = 2, Orders = empTree, CustomerId = 1 });
       l.Add(new Customer { CompanyName = "C", Region = "R3", UnitsInStock = 3, Orders = empTree, CustomerId = 2 });
       l.Add(new Customer { CompanyName = "D", Region = "R4", UnitsInStock = 4, Orders = empTree, CustomerId = 3 });
       l.Add(new Customer { CompanyName = "E", Region = "R5", UnitsInStock = 5, Orders = empTree, CustomerId = 4 });
       return l;
   }

} class Customer : IComparable<Customer> {

   public string CompanyName { get; set; }
   public string Region { get; set; }
   public List<Product> Orders { get; set; }
   public int UnitsInStock { get; set; }
   public int CustomerId { get; set; }
   public override string ToString() {
       return String.Format("Id: {0}, Name: {1}, Region: {3}", this.CustomerId, this.rupanyName, this.Region);
   }
   int IComparable<Customer>.rupareTo(Customer other) {
       if (other == null)
           return 1;
       if (this.CustomerId > other.CustomerId)
           return 1;
       if (this.CustomerId < other.CustomerId)
           return -1;
       return 0;
   }

} class Product : IComparable<Product> {

   public string ProductName { get; set; }
   public string Category { get; set; }
   public int UnitPrice { get; set; }
   public int UnitsInStock { get; set; }
   public int Total { get; set; }
   public DateTime OrderDate { get; set; }
   public int Id { get; set; }
   public override string ToString() {
       return String.Format("Id: {0}, Name: {1} , Category: {3}", this.Id, this.ProductName, this.Category);
   }
   int IComparable<Product>.rupareTo(Product other) {
       if (other == null)
           return 1;
       if (this.Id > other.Id)
           return 1;
       if (this.Id < other.Id)
           return -1;
       return 0;
   }

}

</source>


uses group by to partition a list of products by category.

<source lang="csharp"> using System; using System.Collections.Generic; using System.Linq; using System.Text; public class MainClass {

   public static void Main() {
       List<Product> products = GetProductList();
       var orderGroups =
           from p in products
           group p by p.Category into g
           select new { Category = g.Key, Products = g };
       foreach (var s in orderGroups) {
           Console.Write(s.Category);
       }
   }
   static List<Product> GetProductList() {
       List<Product> empTree = new List<Product>();
       empTree.Add(new Product { ProductName = "A", Category = "O", UnitPrice = 12, UnitsInStock = 5, Total = 36, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "B", Category = "O", UnitPrice = 2, UnitsInStock = 4, Total = 35, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "C", Category = "O", UnitPrice = 112, UnitsInStock = 3, Total = 34, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "D", Category = "O", UnitPrice = 112, UnitsInStock = 0, Total = 33, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "E", Category = "O", UnitPrice = 1112, UnitsInStock = 2, Total = 32, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       empTree.Add(new Product { ProductName = "F", Category = "O", UnitPrice = 11112, UnitsInStock = 0, Total = 31, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
       return empTree;
   }

} class Product : IComparable<Product> {

   public string ProductName { get; set; }
   public string Category { get; set; }
   public int UnitPrice { get; set; }
   public int UnitsInStock { get; set; }
   public int Total { get; set; }
   public DateTime OrderDate { get; set; }
   public int Id { get; set; }
   public override string ToString() {
       return String.Format("Id: {0}, Name: {1} , Category: {3}", this.Id, this.ProductName, this.Category);
   }
   int IComparable<Product>.rupareTo(Product other) {
       if (other == null)
           return 1;
       if (this.Id > other.Id)
           return 1;
       if (this.Id < other.Id)
           return -1;
       return 0;
   }

}

</source>


uses GroupBy to partition trimmed elements of an array using a custom comparer

<source lang="csharp"> using System; using System.Collections.Generic; using System.Linq; using System.Text; public class MainClass {

   public static void Main() {
       string[] anagrams = { "from ", " salt", " earn ", " last ", " near ", " form " };
       var orderGroups = anagrams.GroupBy(
                   w => w.Trim(),
                   a => a.ToUpper(),
                   new AnagramEqualityComparer()
                   );
       foreach (var v in orderGroups) {
           Console.WriteLine(v);
       }
   }
   public class AnagramEqualityComparer : IEqualityComparer<string> {
       public bool Equals(string x, string y) {
           return getCanonicalString(x) == getCanonicalString(y);
       }
       public int GetHashCode(string obj) {
           return getCanonicalString(obj).GetHashCode();
       }
       private string getCanonicalString(string word) {
           char[] wordChars = word.ToCharArray();
           Array.Sort<char>(wordChars);
           return new string(wordChars);
       }
   }

}

</source>