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  • Use Shakespeare-text and external file

    - by Adam
    How can I convert the below example to use an external file instead of the embedded lazy text quasi quotes? {-# LANGUAGE QuasiQuotes, OverloadedStrings #-} import Text.Shakespeare.Text import qualified Data.Text.Lazy.IO as TLIO import Data.Text (Text) import Control.Monad (forM_) data Item = Item { itemName :: Text , itemQty :: Int } items :: [Item] items = [ Item "apples" 5 , Item "bananas" 10 ] main :: IO () main = forM_ items $ \item -> TLIO.putStrLn [lt|You have #{show $ itemQty item} #{itemName item}.|] This is from the yesod online book.

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  • Write a network simulator for fun

    - by Jono
    I want to write my own network simulator, for fun and for personal challenge. I hope to learn both new programming techniques, and a little bit more about networking. Previous object-oriented attempts ended very quickly, but I've recently downloaded and played with Microsoft's Axum (a new version was released today) and their Concurrency and Co-ordination Runtime. As I come from a very OO dominant background, I had never heard of Actor-oriented programming before; now it seems I've had my head in the sand until Scala and F# brought the paradigm to me. My questions are: a) is actor-oriented programming a better choice than object-oriented programming for this task, and if so b) where is a good place to start learning actor-oriented design?

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  • Is OOP based on any branch of mathematics?

    - by ektrules
    I know relational databases are based on set-theory, functional programming is based on lambda calculus, logic programming is based on logic (of course :)), and now that I think of it; I'm not sure if imperative and generic programming is based on any particular branch of mathematics either.

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  • How to avoid becoming a programmer while still beign closely involved with computer science/Industry

    - by WeShallOvercome
    I am studying computer science (A masters at an Ivy league), however most of the jobs i find involve way too much of programming. And frankly programming is not an issue, however programming without a meaning (read financial institution (non trading), other non mainstream jobs) bore me to death! I dont want to end up becoming a .NET,C#, Java kind of programmer. Can someone tell where i should look for jobs if i wish to do some real computer science work such as Machine Learning etc. I don't mind programming but becoming a Financial Software dev at Bloomeberg or an SDET at Microsoft isn't actually one of my goals. [note: I have interviewed for intern both positions listed above, and thankfully i got an intern for a data mining position in a top 750 Alexa rank web company] Sorry if angered anyone with a subjective question

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  • Yet another question about C++ books..

    - by suicideducky
    Intro rant Hey all, so after just over a year of browsing I decided it's time to ask a question for myself, it's sadly similar to many that have been asked before. I'm 18, am studying towards a BSc in Comp SCi and have been programming 'on and off' for about 6 years now, after wrestling with C++ for a bit then stumbling upon (and reading cover to cover) "Programming: Principles and Practice Using C++" by the great man himself I feel pretty comfortable with C++, I am almost finished writing my first open source program in C++ (a mediawiki parser, http://code.google.com/p/apertium-mediawiki/). I have decided I want to really get to know the power of C++, get familiar with some of its 'darker' corners and also delve into game programming, at this point I am rather keen on the book "Essential 3D Game Programming: with C++ and OpenGL" (released 24 may '10), I would also like to get 1 or 2 other books on 'general C++' that I can read cover to cover during my spare time. At this point I am pretty open to suggestions I would like something in the intermediate/advanced zone, some books I am rather keen on include: The C++ Standard Library: A tutorial and reference, C++ Template, the complete guide, and Modern C++ Design: Generic Programming and Design Patterns Applied Thanks in advance.

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  • Entities equals() - hashcode() - toString(). How to correctly implement them?

    - by spike07
    I'm implementing equals() - hashcode() - toString() of my Entities using all the available fields in the bean. I'm getting some Lazy init Exception on the frontend when I try to compare the equality or when I print the obj state. That's because some list in the entity can be lazy initialized. I'm wondering what's the correct way to for implementing equals() and toString() on an Entity Obj

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  • PC to Macbook Pro Transition - Getting (re)started?

    - by Torus Linvald
    I'm in my second computer science course right now. I've enjoyed programming so far, but really have just scraped my way by. I've not done much programming outside of required class work. For similar reasons, I never really invested in downloading/learning software to help me program (IDE's, editors, compilers, etc). I know it sounds tedious, but my current setup is: notepad++ for coding; Filezilla to transfer .cpp & .h files to school's aludra/unix and compiling; unix tells me where my bugs are and I go back to notepad++ to debug; repeat until done. This isn't fun - and I know it could be easier. But I put it off knowing that I was soon going to switch to a Mac. And, tomorrow, I'm switching. So... How should I set up my Macbook for the best programming experience? What IDEs and editors and debuggers and so on should I download? How will Mac programming differ from PC? I'm open to all ideas and comments, even the most basic. (Background - I'm learning/programming in C++ right now. Next semester, my classes switch to Java. I'm also going to take a class in web development, with HTML/CSS/Javascript/PHP. My new laptop will be a late 2009 Macbook Pro with Leopard, or maybe Snow Leopard. Free would be preferrable for all programs.) Thank you all.

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  • Haskell "Source reduction"

    - by Martin
    I'm revising for an upcoming Haskell exam and I don't understand one of the questions on a past paper. Google turns up nothing useful fst(x, y) = x square i = i * i i) Source reduce, using Haskells lazy evaluation, the expression: fst(square(3+4), square 8) ii) Source reduce, using strict evaluation, the same expression iii) State one advantage of lazy evaluation and one advantage of strict evaluation

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  • Must JsUnit Cases Reside Under the Same Directory as JsUnit?

    - by chernevik
    I have installed JsUnit and a test case as follows: /home/chernevik/Programming/JavaScript/jsunit /home/chernevik/Programming/JavaScript/jsunit/testRunner.html /home/chernevik/Programming/JavaScript/jsunit/myTests/lineTestAbs.html /home/chernevik/Programming/JavaScript/lineTestAbs.html When I open the test runner in a browser as a file, and test lineTestAbs.html from the jsunit/myTests directory, it passes. When I test the same file from the JavaScript directory, the test runner times out, asking if the file exists or is a test page. Questions: Am I doing something wrong here, or is this the expected behavior? Is it possible to put test cases in a different directory structure, and if so what is the proper path reference to to JsUnitCore.js? Would JsUnit behave differently if the files were retrieved from an HTTP server? <html> <head> <title>Test Page line(m, x, b)</title> <script language="JavaScript" src="/home/chernevik/Programming/JavaScript/jsunit/app/jsUnitCore.js"></script> <script language="JavaScript"> function line(m, x, b) { return m*x + b; } function testCalculationIsValid() { assertEquals("zero intercept", 10, line(5, 2, 0)); assertEquals("zero slope", 5, line(0, 2, 5)); assertEquals("at x = 10", 25, line(2, 10, 5)); } </script> </head> <body> This pages tests line(m, x, b). </body> </html>

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  • Where is Prolog used for traffic control systems?

    - by Masi
    The user Laurent had an interesting reply to the question [Why hasn’t logic programming caught on?]: If you look at the influence logic-programming has had in the field of -- air traffic control -- I don't think it can be said logic-programming has not caught on. A question arises: Where is prolog used for traffic control systems on the roads? Why is it used instead of languages, such as C or Python, in such environments?

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  • Adding object to child collection causes entire collection to load in Fluent NHibernate.

    - by Mike C.
    Hello, I have my Parent object, which contains an ICollection of Children objects. The Children are lazy loaded and I do not need them in the context of my scenario. However, when I try to add a new child object to my Children collection, it kicks off the lazy load and loads all 7000 child records. I assume I am making a newbie mistake. Anybody out there know how I can fix this? Thanks!

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  • Oracle pl\sql question for my homework in oracle 11G class [migrated]

    - by Bjolds
    I am new to oracle 11G programming and i have run into a tough situation with pl\sql funtions and automation. I ame unsure how to create the function for the automation of Registration system for a College registration system. Here is what i want to do. I want to automate the registrations system so that it automaticly registers students. Then I want a procedure to automate the grading system. I have included the code that i am written to make most of this assignment work which it does but unsure how to incorporate Pl\SQL automated fuctions for the registrations system, and the grading system. So Any help or Ideas I would greatly appreciate please. set Linesize 250 set pagesize 150 drop table student; drop table faculty; drop table Course; drop table Section; drop table location; DROP TABLE courseInstructor; DROP TABLE Registration; DROP TABLE grade; create table student( studentid number(10), Lastname varchar2(20), Firstname Varchar2(20), MI Char(1), address Varchar2(20), city Varchar2(20), state Char(2), zip Varchar2(10), HomePhone Varchar2(10), Workphone Varchar2(10), DOB Date, Pin VARCHAR2(10), Status Char(1)); ALTER TABLE Student Add Constraint Student_StudentID_pk Primary Key (studentID); Insert into student values (1,'xxxxxxxx','xxxxxxxxxx','x','xxxxxxxxxxxxxxx','Columbus','oh','44159','xxx-xxx-xxxx','xxx-xxx-xxxx','06-Mar-1957','1211','c'); create table faculty( FacultyID Number(10), FirstName Varchar2(20), Lastname Varchar2(20), MI Char(1), workphone Varchar2(10), CellPhone Varchar2(10), Rank Varchar2(20), Experience Varchar2(10), Status Char(1)); ALTER TABLE Faculty ADD Constraint Faculty_facultyId_PK PRIMARY KEY (FacultyID); insert into faculty values (1,'xxx','xxxxxxxxxxxx',xxx-xxx-xxxx','xxx-xxx-xxxx','professor','20','f'); create table Course( CourseId number(10), CourseNumber Varchar2(20), CourseName Varchar(20), Description Varchar(20), CreditHours Number(4), Status Char(1)); ALTER TABLE Course ADD Constraint Course_CourseID_pk PRIMARY KEY(CourseID); insert into course values (1,'cit 100','computer concepts','introduction to PCs','3.0','o'); insert into course values (2,'cit 101','Database Program','Database Programming','4.0','o'); insert into course values (3,'Math 101','Algebra I','Algebra I Concepts','5.0','o'); insert into course values (4,'cit 102a','Pc applications','Aplications 1','3.0','o'); insert into course values (5,'cit 102b','pc applications','applications 2','3.0','o'); insert into course values (6,'cit 102c','pc applications','applications 3','3.0','o'); insert into course values (7,'cit 103','computer concepts','introduction systems','3.0','c'); insert into course values (8,'cit 110','Unified language','UML design','3.0','o'); insert into course values (9,'cit 165','cobol','cobol programming','3.0','o'); insert into course values (10,'cit 167','C++ Programming 1','c++ programming','4.0','o'); insert into course values (11,'cit 231','Expert Excel','spreadsheet apps','3.0','o'); insert into course values (12,'cit 233','expert Access','database devel.','3.0','o'); insert into course values (13,'cit 169','Java Programming I','Java Programming I','3.0','o'); insert into course values (14,'cit 263','Visual Basic','Visual Basic Prog','3.0','o'); insert into course values (15,'cit 275','system analysis 2','System Analysis 2','3.0','o'); create table Section( SectionID Number(10), CourseId Number(10), SectionNumber VarChar2(10), Days Varchar2(10), StartTime Date, EndTime Date, LocationID Number(10), SeatAvailable Number(3), Status Char(1)); ALTER TABLE Section ADD Constraint Section_SectionID_PK PRIMARY KEY(SectionID); insert into section values (1,1,'18977','r','21-Sep-2011','10-Dec-2011','1','89','o'); create table Location( LocationId Number(10), Building Varchar2(20), Room Varchar2(5), Capacity Number(5), Satus Char(1)); ALTER TABLE Location ADD Constraint Location_LocationID_pk PRIMARY KEY (LocationID); insert into Location values (1,'Clevleand Hall','cl209','35','o'); insert into Location values (2,'Toledo Circle','tc211','45','o'); insert into Location values (3,'Akron Square','as154','65','o'); insert into Location values (4,'Cincy Hall','ch100','45','o'); insert into Location values (5,'Springfield Dome','SD','35','o'); insert into Location values (6,'Dayton Dorm','dd225','25','o'); insert into Location values (7,'Columbus Hall','CB354','15','o'); insert into Location values (8,'Cleveland Hall','cl204','85','o'); insert into Location values (9,'Toledo Circle','tc103','75','o'); insert into Location values (10,'Akron Square','as201','46','o'); insert into Location values (11,'Cincy Hall','ch301','73','o'); insert into Location values (12,'Dayton Dorm','dd245','57','o'); insert into Location values (13,'Springfield Dome','SD','65','o'); insert into Location values (14,'Cleveland Hall','cl241','10','o'); insert into Location values (15,'Toledo Circle','tc211','27','o'); insert into Location values (16,'Akron Square','as311','28','o'); insert into Location values (17,'Cincy Hall','ch415','73','o'); insert into Location values (18,'Toledo Circle','tc111','67','o'); insert into Location values (19,'Springfield Dome','SD','69','o'); insert into Location values (20,'Dayton Dorm','dd211','45','o'); Alter Table Student Add Constraint student_Zip_CK Check(Rtrim (Zip,'1234567890-') is null); Alter Table Student ADD Constraint Student_Status_CK Check(Status In('c','t')); Alter Table Student ADD Constraint Student_MI_CK2 Check(RTRIM(MI,'abcdefghijklmnopqrstuvwxyz')is Null); Alter Table Student Modify pin not Null; Alter table Faculty Add Constraint Faculty_Status_CK Check(Status In('f','a','i')); Alter table Faculty ADD Constraint Faculty_Rank_CK Check(Rank In ('professor','doctor','instructor','assistant','tenure')); Alter table Faculty ADD Constraint Faculty_MI_CK2 Check(RTRIM(MI,'abcdefghijklmnopqrstuvwxyz')is Null); Update Section Set Starttime = To_date('09-21-2011 6:00 PM', 'mm-dd-yyyy hh:mi pm'); Update Section Set Endtime = To_date('12-10-2011 9:50 PM', 'mm-dd-yyyy hh:mi pm'); alter table Section Add Constraint StartTime_Status_CK Check (starttime < Endtime); Alter Table Section Add Constraint Section_StartTime_ck check (StartTime < EndTime); Alter Table Section ADD Constraint Section_CourseId_FK FOREIGN KEY (CourseID) References Course(CourseId); Alter Table Section ADD Constraint Section_LocationID_FK FOREIGN KEY (LocationID) References Location (LocationId); Alter Table Section ADD Constraint Section_Days_CK Check(RTRIM(Days,'mtwrfsu')IS Null); update section set seatavailable = '99'; Alter Table Section ADD Constraint Section_SeatsAvailable_CK Check (SeatAvailable < 100); Alter Table Course Add Constraint Course_CreditHours_ck check(CreditHours < = 6.0); update location set capacity = '99'; Alter Table Location Add Constraint Location_Capacity_CK Check(Capacity < 100); Create Table Registration ( StudentID Number(10), SectionID Number(10), Constraint Registration_pk Primary key (studentId, Sectionid)); Insert into registration values (1, 2); Insert into Registration values (2, 3); Insert into registration values (3, 4); Insert into registration values (4, 5); Insert into registration values (5, 6); Insert into registration values (6, 7); Insert into registration values (7, 8); Insert into registration values (8, 9); insert into registration values (9, 10); insert into registration values (10, 11); insert into registration values (9, 12); insert into registration values (8, 13); insert into registration values (7, 14); insert into registration values (6, 15); insert into registration values (5, 17); insert into registration values (4, 18); insert into registration values (3, 19); insert into registration values (2, 20); insert into registration values (1, 21); insert into registration values (2, 22); insert into registration values (3, 23); insert into registration values (4, 24); insert into registration values (5, 25); Insert into registration values (6, 24); insert into registration values (7, 23); insert into registration values (8, 22); insert into registration values (9, 21); insert into registration values (10, 20); insert into registration values (9, 19); insert into registration values (8, 17); Create Table courseInstructor( FacultyID Number(10), SectionID Number(10), Constraint CourseInstructor_pk Primary key (FacultyId, SectionID)); insert into courseInstructor values (1, 1); insert into courseInstructor values (2, 2); insert into courseInstructor values (3, 3); insert into courseInstructor values (4, 4); insert into courseInstructor values (5, 5); insert into courseInstructor values (5, 6); insert into courseInstructor values (4, 7); insert into courseInstructor values (3, 8); insert into courseInstructor values (2, 9); insert into courseInstructor values (1, 10); insert into courseInstructor values (5, 11); insert into courseInstructor values (4, 12); insert into courseInstructor values (3, 13); insert into courseInstructor values (2, 14); insert into courseInstructor values (1, 15); Create table grade( StudentID Number(10), SectionID Number(10), Grade Varchar2(1), Constraint grade_pk Primary key (StudentID, SectionID)); CREATE OR REPLACE TRIGGER TR_CreateGrade AFTER INSERT ON Registration FOR EACH ROW BEGIN INSERT INTO grade (SectionID,StudentID,Grade) VALUES(:New.SectionID,:New.StudentID,NULL); END TR_createGrade; / CREATE OR REPLACE FORCE VIEW V_reg_student_course AS SELECT Registration.StudentID, student.LastName, student.FirstName, course.CourseName, Registration.SectionID, course.CreditHours, section.Days, TO_CHAR(StartTime, 'MM/DD/YYYY') AS StartDate, TO_CHAR(StartTime, 'HH:MI PM') AS StartTime, TO_CHAR(EndTime, 'MM/DD/YYYY') AS EndDate, TO_CHAR(EndTime, 'HH:MI PM') AS EndTime, location.Building, location.Room FROM registration, student, section, course, location WHERE registration.StudentID = student.StudentID AND registration.SectionID = section.SectionID AND section.LocationID = location.LocationID AND section.CourseID = course.CourseID; CREATE OR REPLACE FORCE VIEW V_teacher_to_course AS SELECT courseInstructor.FacultyID, faculty.FirstName, faculty.LastName, courseInstructor.SectionID, section.Days, TO_CHAR(StartTime, 'MM/DD/YYYY') AS StartDate, TO_CHAR(StartTime, 'HH:MI PM') AS StartTime, TO_CHAR(EndTime, 'MM/DD/YYYY') AS EndDate, TO_CHAR(EndTime, 'HH:MI PM') AS EndTime, location.Building, location.Room FROM courseInstructor, faculty, section, course, location WHERE courseInstructor.FacultyID = faculty.FacultyID AND courseInstructor.SectionID = section.SectionID AND section.LocationID = location.LocationID AND section.CourseID = course.CourseID; SELECT * FROM V_reg_student_course; SELECT * FROM V_teacher_to_course;

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  • C#/.NET Little Wonders: Interlocked CompareExchange()

    - by James Michael Hare
    Once again, in this series of posts I look at the parts of the .NET Framework that may seem trivial, but can help improve your code by making it easier to write and maintain. The index of all my past little wonders posts can be found here. Two posts ago, I discussed the Interlocked Add(), Increment(), and Decrement() methods (here) for adding and subtracting values in a thread-safe, lightweight manner.  Then, last post I talked about the Interlocked Read() and Exchange() methods (here) for safely and efficiently reading and setting 32 or 64 bit values (or references).  This week, we’ll round out the discussion by talking about the Interlocked CompareExchange() method and how it can be put to use to exchange a value if the current value is what you expected it to be. Dirty reads can lead to bad results Many of the uses of Interlocked that we’ve explored so far have centered around either reading, setting, or adding values.  But what happens if you want to do something more complex such as setting a value based on the previous value in some manner? Perhaps you were creating an application that reads a current balance, applies a deposit, and then saves the new modified balance, where of course you’d want that to happen atomically.  If you read the balance, then go to save the new balance and between that time the previous balance has already changed, you’ll have an issue!  Think about it, if we read the current balance as $400, and we are applying a new deposit of $50.75, but meanwhile someone else deposits $200 and sets the total to $600, but then we write a total of $450.75 we’ve lost $200! Now, certainly for int and long values we can use Interlocked.Add() to handles these cases, and it works well for that.  But what if we want to work with doubles, for example?  Let’s say we wanted to add the numbers from 0 to 99,999 in parallel.  We could do this by spawning several parallel tasks to continuously add to a total: 1: double total = 0; 2:  3: Parallel.For(0, 10000, next => 4: { 5: total += next; 6: }); Were this run on one thread using a standard for loop, we’d expect an answer of 4,999,950,000 (the sum of all numbers from 0 to 99,999).  But when we run this in parallel as written above, we’ll likely get something far off.  The result of one of my runs, for example, was 1,281,880,740.  That is way off!  If this were banking software we’d be in big trouble with our clients.  So what happened?  The += operator is not atomic, it will read in the current value, add the result, then store it back into the total.  At any point in all of this another thread could read a “dirty” current total and accidentally “skip” our add.   So, to clean this up, we could use a lock to guarantee concurrency: 1: double total = 0.0; 2: object locker = new object(); 3:  4: Parallel.For(0, count, next => 5: { 6: lock (locker) 7: { 8: total += next; 9: } 10: }); Which will give us the correct result of 4,999,950,000.  One thing to note is that locking can be heavy, especially if the operation being locked over is trivial, or the life of the lock is a high percentage of the work being performed concurrently.  In the case above, the lock consumes pretty much all of the time of each parallel task – and the task being locked on is relatively trivial. Now, let me put in a disclaimer here before we go further: For most uses, lock is more than sufficient for your needs, and is often the simplest solution!    So, if lock is sufficient for most needs, why would we ever consider another solution?  The problem with locking is that it can suspend execution of your thread while it waits for the signal that the lock is free.  Moreover, if the operation being locked over is trivial, the lock can add a very high level of overhead.  This is why things like Interlocked.Increment() perform so well, instead of locking just to perform an increment, we perform the increment with an atomic, lockless method. As with all things performance related, it’s important to profile before jumping to the conclusion that you should optimize everything in your path.  If your profiling shows that locking is causing a high level of waiting in your application, then it’s time to consider lighter alternatives such as Interlocked. CompareExchange() – Exchange existing value if equal some value So let’s look at how we could use CompareExchange() to solve our problem above.  The general syntax of CompareExchange() is: T CompareExchange<T>(ref T location, T newValue, T expectedValue) If the value in location == expectedValue, then newValue is exchanged.  Either way, the value in location (before exchange) is returned. Actually, CompareExchange() is not one method, but a family of overloaded methods that can take int, long, float, double, pointers, or references.  It cannot take other value types (that is, can’t CompareExchange() two DateTime instances directly).  Also keep in mind that the version that takes any reference type (the generic overload) only checks for reference equality, it does not call any overridden Equals(). So how does this help us?  Well, we can grab the current total, and exchange the new value if total hasn’t changed.  This would look like this: 1: // grab the snapshot 2: double current = total; 3:  4: // if the total hasn’t changed since I grabbed the snapshot, then 5: // set it to the new total 6: Interlocked.CompareExchange(ref total, current + next, current); So what the code above says is: if the amount in total (1st arg) is the same as the amount in current (3rd arg), then set total to current + next (2nd arg).  This check and exchange pair is atomic (and thus thread-safe). This works if total is the same as our snapshot in current, but the problem, is what happens if they aren’t the same?  Well, we know that in either case we will get the previous value of total (before the exchange), back as a result.  Thus, we can test this against our snapshot to see if it was the value we expected: 1: // if the value returned is != current, then our snapshot must be out of date 2: // which means we didn't (and shouldn't) apply current + next 3: if (Interlocked.CompareExchange(ref total, current + next, current) != current) 4: { 5: // ooops, total was not equal to our snapshot in current, what should we do??? 6: } So what do we do if we fail?  That’s up to you and the problem you are trying to solve.  It’s possible you would decide to abort the whole transaction, or perhaps do a lightweight spin and try again.  Let’s try that: 1: double current = total; 2:  3: // make first attempt... 4: if (Interlocked.CompareExchange(ref total, current + i, current) != current) 5: { 6: // if we fail, go into a spin wait, spin, and try again until succeed 7: var spinner = new SpinWait(); 8:  9: do 10: { 11: spinner.SpinOnce(); 12: current = total; 13: } 14: while (Interlocked.CompareExchange(ref total, current + i, current) != current); 15: } 16:  This is not trivial code, but it illustrates a possible use of CompareExchange().  What we are doing is first checking to see if we succeed on the first try, and if so great!  If not, we create a SpinWait and then repeat the process of SpinOnce(), grab a fresh snapshot, and repeat until CompareExchnage() succeeds.  You may wonder why not a simple do-while here, and the reason it’s more efficient to only create the SpinWait until we absolutely know we need one, for optimal efficiency. Though not as simple (or maintainable) as a simple lock, this will perform better in many situations.  Comparing an unlocked (and wrong) version, a version using lock, and the Interlocked of the code, we get the following average times for multiple iterations of adding the sum of 100,000 numbers: 1: Unlocked money average time: 2.1 ms 2: Locked money average time: 5.1 ms 3: Interlocked money average time: 3 ms So the Interlocked.CompareExchange(), while heavier to code, came in lighter than the lock, offering a good compromise of safety and performance when we need to reduce contention. CompareExchange() - it’s not just for adding stuff… So that was one simple use of CompareExchange() in the context of adding double values -- which meant we couldn’t have used the simpler Interlocked.Add() -- but it has other uses as well. If you think about it, this really works anytime you want to create something new based on a current value without using a full lock.  For example, you could use it to create a simple lazy instantiation implementation.  In this case, we want to set the lazy instance only if the previous value was null: 1: public static class Lazy<T> where T : class, new() 2: { 3: private static T _instance; 4:  5: public static T Instance 6: { 7: get 8: { 9: // if current is null, we need to create new instance 10: if (_instance == null) 11: { 12: // attempt create, it will only set if previous was null 13: Interlocked.CompareExchange(ref _instance, new T(), (T)null); 14: } 15:  16: return _instance; 17: } 18: } 19: } So, if _instance == null, this will create a new T() and attempt to exchange it with _instance.  If _instance is not null, then it does nothing and we discard the new T() we created. This is a way to create lazy instances of a type where we are more concerned about locking overhead than creating an accidental duplicate which is not used.  In fact, the BCL implementation of Lazy<T> offers a similar thread-safety choice for Publication thread safety, where it will not guarantee only one instance was created, but it will guarantee that all readers get the same instance.  Another possible use would be in concurrent collections.  Let’s say, for example, that you are creating your own brand new super stack that uses a linked list paradigm and is “lock free”.  We could use Interlocked.CompareExchange() to be able to do a lockless Push() which could be more efficient in multi-threaded applications where several threads are pushing and popping on the stack concurrently. Yes, there are already concurrent collections in the BCL (in .NET 4.0 as part of the TPL), but it’s a fun exercise!  So let’s assume we have a node like this: 1: public sealed class Node<T> 2: { 3: // the data for this node 4: public T Data { get; set; } 5:  6: // the link to the next instance 7: internal Node<T> Next { get; set; } 8: } Then, perhaps, our stack’s Push() operation might look something like: 1: public sealed class SuperStack<T> 2: { 3: private volatile T _head; 4:  5: public void Push(T value) 6: { 7: var newNode = new Node<int> { Data = value, Next = _head }; 8:  9: if (Interlocked.CompareExchange(ref _head, newNode, newNode.Next) != newNode.Next) 10: { 11: var spinner = new SpinWait(); 12:  13: do 14: { 15: spinner.SpinOnce(); 16: newNode.Next = _head; 17: } 18: while (Interlocked.CompareExchange(ref _head, newNode, newNode.Next) != newNode.Next); 19: } 20: } 21:  22: // ... 23: } Notice a similar paradigm here as with adding our doubles before.  What we are doing is creating the new Node with the data to push, and with a Next value being the original node referenced by _head.  This will create our stack behavior (LIFO – Last In, First Out).  Now, we have to set _head to now refer to the newNode, but we must first make sure it hasn’t changed! So we check to see if _head has the same value we saved in our snapshot as newNode.Next, and if so, we set _head to newNode.  This is all done atomically, and the result is _head’s original value, as long as the original value was what we assumed it was with newNode.Next, then we are good and we set it without a lock!  If not, we SpinWait and try again. Once again, this is much lighter than locking in highly parallelized code with lots of contention.  If I compare the method above with a similar class using lock, I get the following results for pushing 100,000 items: 1: Locked SuperStack average time: 6 ms 2: Interlocked SuperStack average time: 4.5 ms So, once again, we can get more efficient than a lock, though there is the cost of added code complexity.  Fortunately for you, most of the concurrent collection you’d ever need are already created for you in the System.Collections.Concurrent (here) namespace – for more information, see my Little Wonders – The Concurent Collections Part 1 (here), Part 2 (here), and Part 3 (here). Summary We’ve seen before how the Interlocked class can be used to safely and efficiently add, increment, decrement, read, and exchange values in a multi-threaded environment.  In addition to these, Interlocked CompareExchange() can be used to perform more complex logic without the need of a lock when lock contention is a concern. The added efficiency, though, comes at the cost of more complex code.  As such, the standard lock is often sufficient for most thread-safety needs.  But if profiling indicates you spend a lot of time waiting for locks, or if you just need a lock for something simple such as an increment, decrement, read, exchange, etc., then consider using the Interlocked class’s methods to reduce wait. Technorati Tags: C#,CSharp,.NET,Little Wonders,Interlocked,CompareExchange,threading,concurrency

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  • The importance of Design Patterns with Javascript, NodeJs et al

    - by Lewis
    With Javascript appearing to be the ubiquitous programming language of the web over the next few years, new frameworks popping up every five minutes and event driven programming taking a lead both server and client side: Do you as a Javascript developer consider the traditional Design Patterns as important or less important than they have been with other languages / environments?. Please name the top three design patterns you, as a Javascript developer use regularly and give an example of how they have helped in your Javascript development.

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  • prerequisites of learnig hadoop, can php developer learn hadoop without java experience [closed]

    - by Rishabh Mathur
    i am willing to learn hadoop as a Developer , but i am confused over the prerequisite of learning it.? is having a good experience in java programming very essential to learn hadoop? I have 4 years of experience in application development in LAMP. But i am not in touch with java programming as a part of my regular work.My objective to get into hadoop is to increase my knowledge in bigdata analysis as well as to get an oppurtunity in this domain. Any suggestions?

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  • Ruby freelancing: realistic expectations?

    - by Sophie
    Hi guys, I'm in a situation where I only need $100 to live at a place. How is this relevant to programming? Well, I would like to hear the opinions of those on this site if it is at all realistic to expect a Ruby noobie to be able to make $100 freelancing by a month from now, assuming a great deal of effort and enthusiasm o_O I'm a complete noob to programming ;d, learning Ruby before Rails. (I also asked this on StackOverflow, but want lots of answers so .<)

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  • @CodeStock 2012 Review: Rob Gillen ( @argodev ) - Anatomy of a Buffer Overflow Attack

    Anatomy of a Buffer Overflow AttackSpeaker: Rob GillenTwitter: @argodevBlog: rob.gillenfamily.net Honestly, this talk was over my head due to my lack of knowledge of low level programming, and I think that most of the other attendees would agree. However I did get the basic concepts that we was trying to get across. Fortunately most high level programming languages handle most of the low level concerns regarding preventing buffer overflow attacks. What I got from this talk was to validate all input data from external sources.

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  • Slow Chat with Industry Experts: Developing Multithreaded Applications

    Sponsored by Intel Join the experts who created The Intel Guide for Developing Multithreaded Applications for a slow chat about multithreaded application development. Bring your questions about application threading, memory management, synchronization, programming tools and more and get answers from the parallel programming experts. Post your questions here

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  • Slow Chat with Industry Experts: Developing Multithreaded Applications

    Sponsored by Intel Join the experts who created The Intel Guide for Developing Multithreaded Applications for a slow chat about multithreaded application development. Bring your questions about application threading, memory management, synchronization, programming tools and more and get answers from the parallel programming experts. Post your questions here

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  • How to Write an E-Book

    A few days ago my attention was drawn to a tweet spat between Karl Seguin and Scott Hanselman around the relaunch of ASP.NET and the title element in HTML. Tempest in a teapot of course, but worthwhile as I did some googling on Karl and found his blog at codebetter.com. From there it was a short jump to his free e-book, The Foundations of Programming. This short book is distinguished by its orientation, opinionated, its tone, mentoring and its honesty, which is refreshing. In Foundations, Karl covers what he considers the basics of programming and good design, including test driven development, dependency injection and domain driven design. Karl is opinionated, as the topics suggest, and doesnt bother to pretend that he doesnt think what hes suggesting is the better way, not just another way. He is aligned with ALT.NET, and gives an excellent overview of what that means; an overview more enlightening than the ALT.NET site. ALT.NET has its critics, but presenting a strong opinion grabbed my attention as a reader. It is a short walk from opinionated to hectoring,  but Karl held my attention without insulting me. He takes the time to explain, with examples, from the ground up, the problems that test driven development and dependency injection solve. So for dependency injection he builds it up from no DI, to a hand crafted approach, to a full fledged DI framework. This approach is more persuasive than just proscriptive and engaged me as the reader to follow along with his train of thought. Foundations is not as pedantic as I am making it sound. The final ingredient in Karls mix is honesty. He acknowledges that sometimes unit testing does cost more up front and take more time. He admits that sometimes he designs something a certain way just to be testable. He also warns that focusing too much on DI and loose coupling can lead to the poor design you are trying to avoid. These points add depth to his argument as I could tell hes speaking from experience, with some hard won lessons. I enjoyed The Foundations of Programming. When I was done with it, I was amazed how much I got a lot out of its 80 some pages. It is a rarity to come across something worthwhile that is longer then a tweet, but shorter than a tome these days. Well done Karl.   -- Relevant Links -- The now titled and newly relaunched page in question: http://www.asp.net/ The pleasantly confusing ALT.NET homepage: http://altdotnet.org/ A longer review, with details, chapter listings and all that important stuff: http://accidentaltechnologist.com/book-reviews/book-review-foundations-of-programming-by-karl-seguin/Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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