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.
Method Name | Description | C# Query Expression Syntax | Visual Basic Query Expression Syntax | More Information |
|---|---|---|---|---|
GroupBy | Groups elements that share a common attribute. Each group is represented by an IGrouping<TKey, TElement> object. | group … by -or- group … by … into … | Group … By … Into … | |
ToLookup | Inserts elements into a Lookup<TKey, TElement> (a one-to-many dictionary) based on a key selector function. | Not applicable. | Not applicable. |
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.
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 */
