CSCI 3328 Object Oriented CSCI 3328 Object Oriented Programming in C# Programming in C#

Chapter 8: LINQ and Generic Chapter 8: LINQ and Generic CollectionsCollections

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Xiang Lian

The University of Texas Rio Grande Valley

Edinburg, TX 78539

xiang.lian@utrgv.edu

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

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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

• In C#, you must include namespace System.Linq– using System.Linq

• Data sources– Arrays– Collections– Files

• Query expression is similar to SQL

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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 foreach)– E.g., passed = from score in scores

• The where clause gives a condition for the search and evaluates to true or false – If true, the value is selected

• The select clause determines what value appears in the results

• Do not forget semicolon (;) at the end

Querying an Array Using LINQ

• var filtered = from value in values

where value > 4

select value;

• var sorted = from value in filtered

orderby value

select value; default: ascending

Querying an Array Using LINQ (cont'd)

• var sorted = from value in filtered

orderby value descending

select value;

• var sorted = from value in values

where (value > 4)

orderby value descending

select value;

Display the Results

• Method call– Display (sorted, "values greater than 4, descending:");

• Method declarationpublic static void Display(IEnumerable <int> results, string header){

Console.Write("{0}", header);foreach (var element in results)

Console.Write(" {0}", element);}

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Another Example: Querying an Array Using LINQ

• To search for scores >=60

var passed = from score in scores

where score >= 60

select score;

foreach (var score in passed)

{

ListLINQresult.Items.Add(score);

}9

Compare Arrays with Collections

• You must specify an arrays size• You can resize it in some languages at runtime• .Net framework generic collections gives greater

flexibility– Reusable– Reliable– Powerful– Efficient

• We are going to use the List class from this collection

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Lists

• Dynamic resizing• LINQ can be used with different types of data

sources, including lists and arrays– SQL is only used with databases

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

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Collections

• Collections do not have a fixed size• Size of a collection is increased automatically when elements

are added to it• Collections: lists, sorted lists, queues, stacks and array lists• UnTyped and typed collections• Using System.Collections

– ArrayList numbers = new ArrayList(); – numbers.Add(3);

• Using System.Collections.Generic– List<int> numbers = new List<int>();– numbers.Add(3);

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Example of List Coding

string[] grades = txtEnterGrade.Text.Split(' ');

List<int> scores = new List<int>();

int numScores = grades.Length;

foreach (string grade in grades)

{

scores.Add(Convert.ToInt32(grade));

ListBoxGrades.Items.Add(grade);

}

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Example of List Coding (cont'd)

public double summation(List<int> scores, int numScores) //declaration

{int sum=0;foreach (int score in scores)

{ sum = sum + score;

}return sum;

}14

Some Methods/Properties of List

• .Add – Adds to the end of the list

• .Insert – Inserts an element at the specified index

• .Clear – Removes all elements from the list

• .Remove – Removes the first occurrence of the specified value

• .Contains – Returns true if the List contains value

• .Sort

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Stack Operation (built-in)

Stack<string> students = new Stack<string>(); students.Push("Abbit");

students.Push("Aguero");students.Push("Castro");students.Push("Chen");students.Push("Cruz");

while (students.Count > 0) MessageBox.Show("Popped: " + students.Pop());

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Struct – Example

public struct Info { public string fName;

public string lName; public string Address1; public string Address2; public int zip; public string Tele; }

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List of Struct

public List <Info> friendsList = new List<Info>();

Info onePerson;

onePerson.fName=txt_fName.Text;onePerson.lName=txt_lName.Text;onePerson.Address1=txtAddr1.Text;onePerson.Address2=txtAddr2.Text;onePerson.zip=Convert.ToInt32 (txtZip.Text);onePerson.Tele=txtTele.Text;

friendsList.Add(onePerson);

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LINQ Search for a Last Name in a List

//will select every person with that last name

var foundTele = from person in friendsList

where person.lName==txtSearchName.Text

select person;

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Display the Selected Names and Telephone Numbers

foreach (var person in foundTele)

{

display +=person.fName+" "+person.lName+"\t";

display += person.Tele + "\n";

}

MessageBox.Show(display, "Names and Telephone Nos. Persons searched:");

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Select Those with GPA >+

var foundGPA =

from person in friendsList

where person.GPA>=Convert.ToSingle(txtSearchGPA.Text)

select person;

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Display

foreach (var person in foundGPA){ display += person.fName + " " + person.lName + "\t"; display += person.Tele + "\t"; display += Convert.ToString(person.GPA)+"\n";}MessageBox.Show(display, "Names and Telephone Nos. Persons

searched:")

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More About File Input/Output

• …

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Obtaining File Name From Chooser

OpenFileDialog fDialog = new OpenFileDialog();

if (fDialog.ShowDialog() == DialogResult.OK)

{

fileName = (fDialog.FileName.ToString());

MessageBox.Show(fileName);

}

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Reading From a FileTextReader ofile = new StreamReader(fileName);while (ofile.Peek()!=-1){ string oneline = ofile.ReadLine(); MessageBox.Show(oneline,"Reading From File.."); string[] items = oneline.Split(','); onePerson.fName = items[0]; onePerson.lName = items[1]; onePerson.GPA = Convert.ToSingle(items[3]); onePerson.Tele = items[2]; friendsList.Add(onePerson);}ofile.Close(); 25

Writing to the File

StreamWriter outfile = new StreamWriter(fileName);

foreach (Info person in friendsList)

outfile.WriteLine(person.fName+","+person.lName+","+person.Tele+","+Convert.ToString(person.GPA));

outfile.Close();

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Example of Employees

public class Employee

{

public string FirstName;

public string LastName;

public decimal monthlySalaryValue;

}

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Example of Employees (cont'd)public class LINQwithArray{

Employee [] employee_array; LINQwithArray(){

// … initializing arrays}public void LINQ_query(){

var nameSorted = (from e in employee_array where e.MonthlySalary >= 4000 orderby e.LastName, e.FirstName select e).Distinct();

if (!nameSorted.Any())Console.WriteLine("not found\n");

}}

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