1. CSCI 3328 Object Oriented Programming in C# Chapter 8: LINQ and Generic Collections 1 Xiang Lian The University of Texas Rio Grande Valley Edinburg, TX 78539 xiang.lian@utrgv.edu

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 2

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 In C#, you must include namespace System.Linq –using System.Linq Data sources –Arrays –Collections –Files Query expression is similar to SQL 4

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 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

6. 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

7. 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;

8. Display the Results Method call –Display (sorted, "values greater than 4, descending:"); Method declaration public static void Display(IEnumerable results, string header) { Console.Write("{0}", header); foreach (var element in results) Console.Write(" {0}", element); } 8

9. 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

10. 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 10

11. 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 11

12. 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 numbers = new List (); –numbers.Add(3); 12

13. Example of List Coding string[] grades = txtEnterGrade.Text.Split(' '); List scores = new List (); int numScores = grades.Length; foreach (string grade in grades) { scores.Add(Convert.ToInt32(grade)); ListBoxGrades.Items.Add(grade); } 13

14. Example of List Coding (cont'd) public double summation(List scores, int numScores) //declaration { int sum=0; foreach (int score in scores) { sum = sum + score; } return sum; } 14

15. 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 15

16. Stack Operation (built-in) Stack students = new Stack (); students.Push("Abbit"); students.Push("Aguero"); students.Push("Castro"); students.Push("Chen"); students.Push("Cruz"); while (students.Count > 0) MessageBox.Show("Popped: " + students.Pop()); 16

17. Struct – Example public struct Info { public string fName; public string lName; public string Address1; public string Address2; public int zip; public string Tele; } 17

18. List of Struct public List friendsList = new List (); 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); 18

19. 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; 19

20. 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:"); 20

21. Select Those with GPA >+ var foundGPA = from person in friendsList where person.GPA>=Convert.ToSingle(txtSearchGPA.Text) select person; 21

22. 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:") 22

23. More About File Input/Output … 23

24. Obtaining File Name From Chooser OpenFileDialog fDialog = new OpenFileDialog(); if (fDialog.ShowDialog() == DialogResult.OK) { fileName = (fDialog.FileName.ToString()); MessageBox.Show(fileName); } 24

25. Reading From a File TextReader 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

26. 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(); 26

27. Example of Employees public class Employee { public string FirstName; public string LastName; public decimal monthlySalaryValue; } 27

28. 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"); } 28