Introduction to LINQ and Collections

1. CSCI 3327 Visual Basic Chapter 8: Introduction to LINQ and Collections UTPA – Fall 2011

2. Objectives • In this chapter, you will: • Become familiar with the basic concept of LINQ • Learn how to use LINQ to query an array • Learn how to sort an array using LINQ • Learn how to manipulate collections (e.g., List) by LINQ

3. LINQ • Language Integrated Query (LINQ) • Old way: • Structured Query Language (SQL) • Queries are written in string and passed to database which interprets the string, processes the request, and returns the results • LINQ allows a programming language to submit queries directly to a wide range of data sources (not just databases!)

4. LINQ (cont'd) • LINQ is a logical extension of querying and manipulating data in databases • Data sources • Arrays • Collections • Items collection of a ListBox • Files • Query expression is similar to SQL

5. Writing an LINQ Query • A LINQ query begins with a From clause • After From clause, specify a range variable and the data source to query • The range variable represents each item in the data source (much like For…EachNext) • The Where clause evaluates to True or False • If True, the value is selected • The Select clause determines what value appears in the results

6. Querying an Array of Primitive-Type Elements Using LINQ • Dim filtered = From value In values Where value > 4 Select value • Dim sorted = From value In filtered OrderBy value Select value default: Ascending

7. Querying an Array of Primitive-Type Elements Using LINQ (cont'd) • Dim sorted = From value In filtered OrderBy value Descending Select value • Dim sorted = From value In values Where (value > 4) OrderBy value Descending Select value

8. Example 11.2: LINQWithArrayOfIntegers.vb • URL: • http://media.pearsoncmg.com/ph/esm/deitel/vb_htp_2010/codeexamples.html • Keywords in LINQ • From … In • Where • Select • Order By … Descending • Chained vs. one query

9. LINQ Providers • LINQ provider is a layer between the LINQ engine and the data source • It consists of a set of classes that implement operation needed by LINQ • Generic methods allow to display variables of various types with a single method • E.g., counting, accessing, comparing, and removing elements

10. Example 11.3-11.4: LINQWithEmployeeArray.vb • URL: • http://media.pearsoncmg.com/ph/esm/deitel/vb_htp_2010/codeexamples.html • nameSorted.Count() • nameSorted.First() • Keyword: Distinct • Anonymous types • The new class that does not have a name and cannot be used to create new objects

11. Other IEnumerable Extension Methods • Any • Average • Cast • Contains • Count • Distinct • ElementAt • First • Last • Max • Min • Reverse • Sum

12. Example 11.7: DeferredExecution.vb • URL: • http://media.pearsoncmg.com/ph/esm/deitel/vb_htp_2010/codeexamples.html • String • Color.StartsWith("r") • Color.ToUpper() • The changes to the data source will be included when we re-execute the query

13. Introduction to Collections • .NET framework class library provides several classes called collections • The collection class List (OFT) comes from namespace System.Collections.Generic • The T is the placeholder change as appropriate as: • Dim list1 As List (OFInteger)

14. Problems With Arrays • Must specify size of array • Only some languages allow redimesioning it at runtime • .NET framework’s collection class overcomes this limitation • We are going to use the List class from this collection

15. List Class • Lists are similar to arrays but provide additional functionality: • Dynamic resizing • Provides many built-in methods • Can use LINQ queries

16. Property or Method of List • Add (to the end) • Capacity (how many) • Clear (remove all) • Contains(returns true or false) • Count (how many) • Indexof (first occurrence of specified value in list) • Insert (add at specified index) • trimExcess

17. Example • Dim items as new List(OF String) • Items.Add(“red”) • Items.Insert(0,”yellow”)

18. Querying a Generic Collection • DimstartswithR = Fromitem Initems Whereitem.StartsWith(“r”) OrderByitem Selectitem.ToUpper() ForEachitem InstartswithR Console.Write(item) Next • Displays all items starts with r, but all in uppercase

19. STD DEV Program in Your Assignment • Reading the grades from a string Dim grades() AsString = Split(txtEnterGrade.Text, " ") • Now let us create our own array instead of using grades() which is an array of string Dim scores(80) AsInteger For i = 0 To grades.GetUpperBound(0) scores(i) = Val(grades(i)) total += scores(i) lstBoxGrades.Items.Add(scores(i)) Next