Grouping Data

Grouping refers to the operation of putting data into groups so that the elements in each group share a common attribute.

The following illustration shows the results of grouping a sequence of characters. The key for each group is the character.

The standard query operator methods that group data elements are listed in the following section.

Methods

Query Expression Syntax Example

The following code example uses the group by clause in C# or the Group By clause in Visual Basic to group integers in a list according to whether they are even or odd.

Dim numbers As New System.Collections.Generic.List(Of Integer) _
    (New Integer() {35, 44, 200, 84, 3987, 4, 199, 329, 446, 208})

Dim query = From number In numbers _
            Group By Remainder = (number Mod 2) Into Group

Dim sb As New System.Text.StringBuilder()
For Each group In query
    sb.AppendLine(If(group.Remainder = 0, vbCrLf & "Even numbers:", vbCrLf & "Odd numbers:"))
    For Each num In group.Group
        sb.AppendLine(num)
    Next 
Next 

' Display the results.
MsgBox(sb.ToString())

' This code produces the following output: 

' Odd numbers: 
' 35 
' 3987 
' 199 
' 329 

' Even numbers: 
' 44 
' 200 
' 84 
' 4 
' 446 
' 208

List<int> numbers = new List<int>() { 35, 44, 200, 84, 3987, 4, 199, 329, 446, 208 };

IEnumerable<IGrouping<int, int>> query = from number in numbers
                                         group number by number % 2;

foreach (var group in query)
{
    Console.WriteLine(group.Key == 0 ? "\nEven numbers:" : "\nOdd numbers:");
    foreach (int i in group)
        Console.WriteLine(i);
}

/* This code produces the following output:

    Odd numbers:
    35
    3987
    199
    329

    Even numbers:
    44
    200
    84
    4
    446
    208
*/

See Also

Tasks

How to: Group a Group (C# Programming Guide)

How to: Group Files by Extension (LINQ)

How to: Group Results in Various Ways (C# Programming Guide)

How to: Perform a Subquery on a Grouping Operation (C# Programming Guide)

How to: Split a File Into Many Files by Using Groups (LINQ)

Concepts

Standard Query Operators Overview

Reference

group clause (C# Reference)

Group By Clause (Visual Basic)

System.Linq