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

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 2

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!)

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 4

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…Each Next) The Where clause evaluates to True or False –If True, the value is selected The Select clause determines what value appears in the results

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 Order By value Select value default: Ascending

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

Example 11.2: LINQWithArrayOfIntegers.vb URL: – p_2010/codeexamples.htmlhttp://media.pearsoncmg.com/ph/esm/deitel/vb_ht p_2010/codeexamples.html Keywords in LINQ –From … In –Where –Select –Order By … Descending Chained vs. one query 8

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

Example : LINQWithEmployeeArray.vb URL: – p_2010/codeexamples.htmlhttp://media.pearsoncmg.com/ph/esm/deitel/vb_ht p_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

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

Example 11.7: DeferredExecution.vb URL: – p_2010/codeexamples.htmlhttp://media.pearsoncmg.com/ph/esm/deitel/vb_ht p_2010/codeexamples.html String –Color.StartsWith("r") –Color.ToUpper() The changes to the data source will be included when we re-execute the query 12

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

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

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

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

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

Querying a Generic Collection Dim startswithR = From item In items Where item.StartsWith(“r”) Order By item Select item.ToUpper() For Each item In startswithR Console.Write(item) Next Displays all items starts with r, but all in uppercase

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

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