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LINQ to Objects : How to Group Elements (part 6) - ow to Use Grouping Results in the Same Query

12/30/2013 8:48:19 PM
6. How to Use Grouping Results in the Same Query (Query Continuation) It is possible to use grouping results as a local variable within the same query by appending the into clause after the group when using the query expression syntax (when using extension method syntax, you just continue your query from the result of the GroupBy extension method). The into clause projects the grouping result into a local variable that you can reference throughout the rest of the query expression, giving you access to the key values and the grouped elements. The simplest form of query continuation using groups is for providing aggregation results within the query’s select projection clause. Listing 9 demonstrates how to calculate various subtotals for a set of grouped data. The Console results are shown in Output 6. Listing 9. Query continuation allows you to calculate various aggregation values for each group (think subtotaling)—see Output 6 `List<CallLog> calls = CallLog.SampleData();var q = from c in calls group c by c.Number into g select new { Number = g.Key, InCalls = g.Count(c => c.Incoming), OutCalls = g.Count(c => !c.Incoming), TotalTime = g.Sum(c => c.Duration), AvgTime = g.Average(c => c.Duration) };foreach (var number in q) Console.WriteLine( "{0} ({1} in, {2} out) Avg Time: {3} mins", number.Number, number.InCalls, number.OutCalls, Math.Round(number.AvgTime, 2));` Output 6. `885 983 8858 (4 in, 2 out) Avg Time: 9.33 mins165 737 1656 (2 in, 0 out) Avg Time: 13.5 mins364 202 3644 (2 in, 2 out) Avg Time: 13.75 mins603 303 6030 (0 in, 4 out) Avg Time: 12.5 mins546 607 5462 (2 in, 4 out) Avg Time: 4.33 mins848 553 8487 (2 in, 2 out) Avg Time: 13.75 mins278 918 2789 (2 in, 0 out) Avg Time: 11 mins` Query continuation allows very complex rollup analysis to be performed on grouped data with any level of nesting. Listing 10 demonstrates how to group two levels deep. This query groups phone calls by year and month for each customer record in the source collection. The groupings are captured in an anonymous type field (YearGroup and MonthGroup), and these are accessible when iterating the result sequence using nested foreach loops. The Console output of this code is shown in Output 7. Listing 10. The example demonstrates grouping data multiple levels deep—see Output 7 `List<Contact> contacts = Contact.SampleData();List<CallLog> callLog = CallLog.SampleData();var q = from contact in contacts select new { Name = contact.FirstName + " " + contact.LastName, YearGroup = from call in callLog where call.Number == contact.Phone group call by call.When.Year into groupYear select new { Year = groupYear.Key, MonthGroup = from c in groupYear group c by c.When.ToString("MMMM") } };foreach (var con in q){ Console.WriteLine("Customer: {0}", con.Name); foreach (var year in con.YearGroup) { Console.WriteLine(" Year:{0}", year.Year); foreach (var month in year.MonthGroup) { Console.WriteLine(" Month:{0}", month.Key); foreach (var call in month) { Console.WriteLine(" {0} - for {1} minutes", call.When, call.Duration); } } }} ` Output 7. `Customer: Barney Gottshall Year:2006 Month:August 8/7/2006 8:12:00 AM - for 2 minutes 8/7/2006 1:12:00 PM - for 15 minutes 8/7/2006 1:47:00 PM - for 3 minutes Month:July 7/12/2006 8:12:00 AM - for 5 minutes 7/7/2006 1:47:00 PM - for 21 minutes Month:June 6/7/2006 1:12:00 PM - for 10 minutesCustomer: Armando Valdes Year:2006 Month:August 8/8/2006 2:00:00 PM - for 3 minutes 8/8/2006 2:37:00 PM - for 7 minutes Month:July 7/8/2006 2:00:00 PM - for 32 minutes 7/8/2006 2:37:00 PM - for 13 minutes...Customer: Ariel Hazelgrove Year:2006 Month:August 8/7/2006 9:23:00 AM - for 15 minutes Month:June 6/14/2006 9:23:00 AM - for 12 minutes `
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