MSDN Magazine - September 2008 - (Page 40)

Figure 10 Using Concat, Union, Intersect, Except ' From SimpleSetDemo in the sample: Concatenates two sequences, taking ' all members of each. Union removes duplicates. Dim db As New SimpleDataContext ' Create two sequences: Dim group1 = db.Customers.Select(Function(cust) cust.Country).Take(5) Dim group2 = db.Customers.Select(Function(cust) cust.Country) _ .Skip(30).Take(5) ' Show both groups: sw.WriteLine("group1:" & GetCommaList(group1)) sw.WriteLine("group2:" & GetCommaList(group2)) ' Return the complete list of countries: Dim group3 = group1.Concat(group2) ' Return a unique list of countries in the two groups: group3 = group1.Union(group2) ' Find all elements in group1 that are also in group2: Can supply custom ' IEqualityComparer instance to compare items if they're not simple ' types: group3 = group1.Intersect(group2) ' Find the differences between group1 and group2. That is, return the set ' of all items that are in the first group but not the second. This is ' equivalent to calculating the union of the two, and removing the ' contents of the intersection of the two. Can supply custom ' IEqualityComparer instance to compare items if they're not simple ' types: group3 = group1.Except(group2) Results group1: Germany, Mexico, Mexico, UK, Sweden group2: France, France, France, France, Germany group1.Concat(group2): Germany, Mexico, Mexico, UK, Sweden, France, France, France, France, Germany group1.Union(group2): Germany, Mexico, UK, Sweden, France group1.Intersect(group2): Germany group1.Except(group2): Mexico, UK, Sweden To call Aggregate, supply a “seed” value (that is, the initial value of the calculated result) and a calculating function that accepts two parameters: the first contains the current total value, and the second contains the specific item to be aggregated. Within the function, perform the calculation. For example, to sum the squares of the difference between the UnitPrice and the mean price, you could create an aggregating function like this: Function(current As Decimal, item As Product) _ current + CDec((item.UnitPrice - averagePrice) ^ 2)) bilities. The first example, Figure 10, shows the Enumerable.Concat, Enumerable.Union, Enumerable.Intersect, and Enumerable.Except methods. Each method operates on a pair of sequences. The Concat and Union methods seem similar, combining the results of two sequences. However, the Concat method adds the output of one sequence to another, while the Union method removes duplicates from the result. The Intersect method returns a sequence with all the items common to both input sequences, and the Except method returns all items that are in the first sequence but not in the second (in other words, the difference between the two sequences). Running the code in Figure 10 produces the output at the bottom of the figure (the output first displays the two sequences, then the results of the calculations). Note that all the samples you see here use simple, default comparers. If you want to perform set operations on sequences of more complex objects, you can call the overloaded versions of the methods that accept instances of custom comparers, as you’ve seen in previous examples. The Enumerate class also supports several more complex set operations, using the Enumerable.Join, Enumerable.GroupBy, and Enumerable.GroupJoin methods. I will describe and demonstrate each of these methods in a moment. Imagine that some method hands you two sequences that are related, and you want to join them based on the correlation of a key value in both sequences. Obviously, this task is more typically thought of as being something for a relational database to handle. However, you can use the Enumerable.Join method to do this same kind of work. Figure 11 joins a sequence containing categories with a sequence containing products. To call the Join method, you must supply a function that returns the primary key in the parent sequence, a function that returns the foreign key in the child sequence, and a function that projects the data from the two sequences into the output sequence. (If the correlation key isn’t a simple type, you must also supply a custom comparer, as you’ve seen previously, to compare the key values.) Figure 11 Joining Two Sequences ' From ComplexSetOperationDemo in the sample: Dim categories = _ db.Categories.Where(Function(cat) cat.CategoryID = 3) ' If the correlation key isn't a simple type, you could ' provide a comparer to compare the two key values: Dim result = categories.Join(db.Products, _ Function(cat) cat.CategoryID, _ Function(prod) prod.CategoryID, _ Function(cat, prod) _ New With {.CategoryName = cat.CategoryName, _ .ProductName = prod.ProductName}) sw = New StringWriter For Each item In result sw.WriteLine("{0} -- {1}", item.CategoryName, item.ProductName) Next The code in Figure 9 calculates the standard deviation. You can use the Aggregate method on non-numeric values as well. You previously saw an example that cast a list as an array of String values, so that you could use the String.Join method. You can accomplish the same goal using the Aggregate method: ' From AggregateDemo in the sample: Dim db As New SimpleDataContext Dim customers = _ From cust In db.Customers _ Where cust.Country = "France" _ Select cust.ContactName ' Note that the seed value is an empty string: Dim customerNames = customers.Aggregate(String.Empty, _ Function(current, name) _ If(String.IsNullOrEmpty(current), name, current & ", " & name)) Results Confections -- Pavlova Confections -- Teatime Chocolate Biscuits Confections -- Sir Rodney’s Marmalade ‘ More items here. . . Confections -- Tarte au sucre Confections -- Scottish Longbreads Performing Set Operations The Enumerable class provides methods that perform set operations, such as calculations of unions and intersections. This final section investigates the methods of the class that provide these capa40 msdn magazine Advanced Basics

Table of Contents Feed for the Digital Edition of MSDN Magazine - September 2008

MSDN Magazine - September 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Office Space Cutting Edge Hierarchy ID New Features for Microsoft SQL Server 2008 Prism Data Services Advanced WPF Test Run Security Briefs Foundations { End Bracket }

MSDN Magazine - September 2008

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