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  • SQL Exception error??

    - by Kyle Sevenoaks
    I just came into work and found this where our site should be: SQLException ERROR: connect failed [Native Error: Host 'linux7.fastname.no' is blocked because of many connection errors; unblock with 'mysqladmin flush-hosts'] [User Info: Array] What does it mean? www.euroworker.no

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  • malformed url exception

    - by Bunny Rabbit
    https://twitter.com/sessions?authenticity_token=b1b43178e09c1e6ccec1b3183f1f139f39643aaf&session%5Busername_or_email%5D=ddddd&session%5Bpassword%5D=rrrrr&q=&site_action=https%3A%2F%2Ftwitter.com%2Fsessions&serialized_string=Hello why is this url throwing a java.net.MalformedURLException??

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  • About NullPointer Exception in Arraylist

    - by user2234456
    List<PrcSubList> listSl = new ArrayList<PrcSubList>(); if (listSl == null || listSl.size() == 0) { PrcSubList subListAdd=coreService.addSubListByAddAlert(childSub); System.out.println("sublist after insert db :" + subListAdd.getName()); listSl.add(subListAdd); System.out.println(listSl.size() +" sublist after insert list:" + listSl.get(0).getName()); } Ouput with first System.out.println("sublist after insert db :" + subListAdd.getName()); sublist after insert db :SYSTEM_ALERT_12312313 Problem But i have NullPointerException with 2nd System.out.println(listSl.size() +" sublist after insert list:" + listSl.get(0).getName()); Can you help me!

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  • Python raises a KeyError (for an out of dictionary key) even though the key IS in the dictionary

    - by ignorantslut
    I'm getting a KeyError for an out of dictionary key, even though I know the key IS in fact in the dictionary. Any ideas as to what might be causing this? print G.keys() returns the following: ['24', '25', '20', '21', '22', '23', '1', '3', '2', '5', '4', '7', '6', '9', '8', '11', '10', '13', '12', '15', '14', '17', '16', '19', '18'] but when I try to access a value in the dictionary on the next line of code... for w in G[v]: #note that in this example, v = 17 I get the following error message: KeyError: 17 Any help, tips, or advice are all appreciated. Thanks.

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  • jUnit same exception in different cases

    - by coubeatczech
    Hi, I'm writing a jUnit test for a constructor that parses a String and then check numerous things. When there's wrong data, for every thing, some IllegalArgumentException with different message is thrown. So I would like to write tests for it, but how can i recognize what error was thrown? This is how can I do it: @Test(expected=IllegalArgumentException.class) public void testRodneCisloRok(){ new RodneCislo("891415",dopocitej("891415")); } and this is how I would like to be, but I don't know if it is possible to write it somehow: @Test(expected=IllegalArgumentException.class("error1")) public void testRodneCisloRok(){ new RodneCislo("891415",dopocitej("891415")); }

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  • Which of the two exceptions was called?

    - by Rob
    If I have a routine that can throw an ArgumentException in two places, something like... if (Var1 == null) { throw new ArgumentException ("Var1 is null, this cannot be!"); } if (Val2 == null) { throw new ArgumentException ("Var2 is null, this cannot be either!"); } What’s the best way of determining in my calling procedure which of the two exceptions was thrown? Or Am I doing this in the wrong fashion?

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  • floating point exception in C++ code !

    - by mekasperasky
    #include<stdio.h> #include<math.h> int main () { FILE *fp; fp=fopen("output","w"); float t,y=0,x=0,e=5,f=1,w=1; for (t=0;t<10;t=t+0.01) { if( y==inf && y== nan) break; fprintf(fp,"%lf\t%lf\n",y,x); y = y + ((e*(1 - x*x)*y) - x + f*cos(w*t))*t; x = x + y*t; } return (0); } why is the ouput giving infinite and NAN values?

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  • Patterns to deal with with functions that can have different kinds of results.

    - by KaptajnKold
    Suppose you have an method on an object that given the some input alters the objects state if the input validates according to some complex logic. Now suppose that when the input doesn't validate, it can be due to several different things, each of which we would like to be able to deal with in different ways. I'm sure many of you are thinking: That's what exceptions are for! I've thought of this also. But my reservation against using exceptions is that in some cases there is nothing exceptional about the input not validating and I really would like to avoid using exceptions to control what is really just in the expected flow of the program. If there were only one interpretation possible, I could simply choose to return a boolean value indicating whether or not the operation resulted in a state change or not and the respond appropriately when it did not. There is of course also the option to return a status code which the client can then choose to interpret or not. I don't like this much either because there is nothing semantic about status codes. The solution I have so far is to always check for each possible situation which I am able to handle before I call the method which then returns a boolean to inform the client if the object changed state. This leaves me the flexibility to handle as few or as many as the possible situations as I wish depending on the context I am in. It also has the benefit of making the method I am calling simpler to write. The drawback is that there is quite a lot of duplication in the client code wherever I call the method. Which of these solutions do you prefer and why? What other patterns do people use for providing meaningful feedback from functions? I know that some languages support multiple return values, and I if I had that option I would surely prefer it.

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  • Why do I get a Null Pointer Exception?

    - by Roman
    I have this code: Manager manager = new Manager("Name"); MyWindowListener windowListener = new MyWindowListener(); manager.addWindowListener(windowListener); Eclipse writes that I have a NullPointerException in the last line. What can be the reason for that. I do have constructors in the Manager and MyWindowListener. If it's important MyWindowListener implements WindowListener.

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  • Java application return codes

    - by doele
    I have a Java program that processes one file at a time. This Java program is called from a wrapper script which logs the return code from the Java program. There are 2 types of errors. Expected errors and unexpected errors. In both cases I just need to log them. My wrapper knows about 3 different states. 0-OK, 1-PROCESSING_FAILED, 2- ERROR. Is this a valid approach? Here is my approach: enum ReturnCodes {OK,PROCESSING_FAILED,ERROR}; public static void main(String[] args) { ... proc.processMyFile(); ... System.exit(ReturnCodes.OK.ordinal()); } catch (Throwable t) { ... System.exit(ReturnCodes.ERROR.ordinal()); } private void processMyFile() { try { ... }catch( ExpectedException e) { ... System.exit(ReturnCodes.PROCESSING_FAILED.ordinal()); } }

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  • Qt : crash due to delete (trying to handle exceptions...)

    - by Seub
    I am writing a program with Qt, and I would like it to show a dialog box with a Exit | Restart choice whenever an error is thrown somewhere in the code. What I did causes a crash and I really can't figure out why it happens, I was hoping you could help me understanding what's going on. Here's my main.cpp: #include "my_application.hpp" int main(int argc, char *argv[]) { std::cout << std::endl; My_Application app(argc, argv); return app.exec(); } And here's my_application:hpp: #ifndef MY_APPLICATION_HPP #define MY_APPLICATION_HPP #include <QApplication> class Window; class My_Application : public QApplication { public: My_Application(int& argc, char ** argv); virtual ~My_Application(); virtual bool notify(QObject * receiver, QEvent * event); private: Window *window_; void exit(); void restart(); }; #endif // MY_APPLICATION_HPP Finally, here's my_application.cpp: #include "my_application.hpp" #include "window.hpp" #include <QMessageBox> My_Application::My_Application(int& argc, char ** argv) : QApplication(argc, argv) { window_ = new Window; window_->setAttribute(Qt::WA_DeleteOnClose, false); window_->show(); } My_Application::~My_Application() { delete window_; } bool My_Application::notify(QObject * receiver, QEvent * event) { try { return QApplication::notify(receiver, event); } catch(QString error_message) { window_->setEnabled(false); QMessageBox message_box; message_box.setWindowTitle("Error"); message_box.setIcon(QMessageBox::Critical); message_box.setText("The program caught an unexpected error:"); message_box.setInformativeText("What do you want to do? <br>"); QPushButton *restart_button = message_box.addButton(tr("Restart"), QMessageBox::RejectRole); QPushButton *exit_button = message_box.addButton(tr("Exit"), QMessageBox::RejectRole); message_box.setDefaultButton(restart_button); message_box.exec(); if ((QPushButton *) message_box.clickedButton() == exit_button) { exit(); } else if ((QPushButton *) message_box.clickedButton() == restart_button) { restart(); } } return false; } void My_Application::exit() { window_->close(); //delete window_; return; } void My_Application::restart() { window_->close(); //delete window_; window_ = new Window; window_->show(); return; } Note that the line window_->setAttribute(Qt::WA_DeleteOnClose, false); means that window_ (my main window) won't be deleted when it is closed. The code I've written above works, but as far as I understand, there's a memory leak: I should uncomment the line //delete window_; in My_Application::exit() and My_Application::restart(). But when I do that, the program crashes when I click restart (or exit but who cares). (I'm not sure this is useful, in fact it might be misleading, but here's what my debugger tells me: a segmentation fault occurs in QWidgetPrivate::PaintOnScreen() const which is called by a function called by a function... called by My_Application::notify()) When I do some std::couts, I notice that the program runs through the entire restart() function and in fact through the entire notify() function before it crashes. I have no idea why it crashes. Thanks in advance for your insights! Update: I've noticed that My_Application::notify() is called very often. For example, it is called a bunch of times while the error dialog box is open, also during the execution of the restart function. The crash actually occurs in the subfunction QApplication::notify(receiver, event). This is not too surprising in light of the previous remark (the receiver has probably been deleted) But even if I forbid the function My_Application::notify() to do anything while restart() is executed, it still crashes (after having called My_Application::notify() a bunch of times, like 15 times, isn't that weird)? How should I proceed? Maybe I should say (to make the question slightly more relevant) that my class My_Application also has a "restore" function, which I've not copied here to try to keep things short. If I just had that restart feature I wouldn't bother too much, but I do want to have that restore feature. I should also say that if I keep the code with the "delete window_" commented, the problem is not only a memory leak, it still crashes sometimes apparently. There must surely be a way to fix this! But I'm clueless, I'd really appreciate some help! Thanks in advance.

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  • exception occurred for backend host '127.0.0.1/7001/7001': 'CONNECTION_REFUSED [os error=0, line 1715 of URL.cpp]: Error connecting to host 127.0.0.1:7001'

    - by Vijaya Moderator -Oracle
    When you hit the WLS Server via Proxy server, we may get the issue like below... [09/Jun/2014:06:05:50] failure ( 3284): for host 127.0.0.1 trying to GET /.../soundlink_wireless_​speaker/index.jsp, wl-proxy reports: exception occurred for backend host '127.0.0.1/7001/0': 'PROTOCOL_ERROR [line 835 of URL.cpp]: Backend Server not responding' [09/Jun/2014:06:05:50] failure ( 3284): for host 127.0.0.1 trying to GET /.../soundlink_wireless_​speaker/index.jsp, wl-proxy reports: exception occurred for backend host '127.0.0.1/7001/0': 'PROTOCOL_ERROR [line 835 of URL.cpp]: Backend Server not responding' [09/Jun/2014:06:05:51] failure ( 3284): for host 127.0.0.1 trying to GET /.../soundlink_wireless_​speaker/index.jsp, wl-proxy reports: exception occurred for backend host '127.0.0.1/7001/7001': 'CONNECTION_REFUSED [os error=0, line 1715 of URL.cpp]: Error connecting to host 127.0.0.1:7001' To solve the issue 1.  Check if there is any issue at the firewall by executing the below command     telnet 127.0.0.1 7001 2.  Also access  using the hostname instead of IP Address  If it errors out like below   Microsoft Telnet> o 127.0.0.1 7001 connecting to 127.0.0.1  Warning thrown at weblogic console: <BEA-000449> <Closing socket as no data read from it on 127.0.0.1:54,356 during the configured idle timeout of 5 secs>  Test the same again by disabling the firewall. Below are the steps... 1)Go to Start ----> Run  2)Type services.msc  3)In services.msc search for "Windows Firewall" and Right Click on "Windows Firewall"---->Select Stop. 4)Access the Weblogic URL  If the issue is resolved after disabling firewall, Request your  OS System Admin to open the port 7001 at the firewall end to access the application

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  • The type 'XXX' is defined in an assembly that is not referenced exception after upgrade to ASP.NET 4

    - by imran_ku07
       Introduction :          I found two posts in ASP.NET MVC forums complaining that they are getting exception, The type XXX is defined in an assembly that is not referenced, after upgrading thier application into Visual Studio 2010 and .NET Framework 4.0 at here and here .   Description :           The reason why they are getting the above exception is the use of new clean web.config without referencing the assemblies which were presents in ASP.NET 3.5 web.config. The quick solution for this problem is to add the old assemblies in new web.config.          <assemblies>             <add assembly="System.Web.Abstractions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>             <add assembly="System.Web.Routing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>             <add assembly="System.Web.Mvc, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>              <add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />              <add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, publicKeyToken=b77a5c561934e089" />          </assemblies>    How It works :            Currently i have not tested the above scenario in ASP.NET 4.0 because i have not yet get it. But the above scenario can easily be tested and verified in VS 2008. Just Open the root web.config and remove           <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>             Even you add the reference of System.Core in your project, you will still get the above exception because aspx pages are compiled in separate assembly. You can check this yourself by checking Show Detailed Compiler Output: below in the yellow screen of death, you will find something,/out:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\e907aee4\5fa0acc8\App_Web_y5rd6bdg.dll"             This shows that aspx pages are compiled in separate assembly in Temporary ASP.NET Files.Summary :             After getting the above exception make sure to add the assemblies in web.config or add the Assembly directive at Page level. Hopefully this will helps to solve your problem.       

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  • Is it safe to use any USB charger for charging any USB device?

    - by Vineet Menon
    I have an apple USB AC charger which is rated to provide 5V, 1A output and a Sony mp3 player NWZ B143F. I don't know the spec of Sony audio player. But I have another mp3 player from Transcend which is rated to take 5V, 1A input voltage. So, is it safe to assume that Sony or for that matter any mainstream mp3 player would take 5V, 1A as input? Is it safe to use the apple charger with Sony mp3 player?

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  • How can I tell if Windows is running in Safe Mode?

    - by cwd
    I have a Windows server that will sometimes reboot into safe mode after updates. I'm working on that issue but what I'd really like to know is how can I check to see if Windows is running in safe mode or not. Ideally I would like to incorporate it into a script that would send a passive check to our Nagios box with the status. Is there some environmental variable I can use or some way to get this information via the command line?

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  • CoreData update problems

    - by kpower
    My app makes updates in background thread then saves context changes. And in main context there is a table view that works with NSFetchedResultsController. For some time updates work correctly, but then exception is thrown. To check this I've added NSLog(@"%@", [self.controller fetchedObjects]); to -controllerDidChangeContent:. Here is what I got: "<PRBattle: 0x6d30530> (entity: PRBattle; id: 0x6d319d0 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p2> ; data: {\n battleId = \"-1\";\n finishedAt = \"2012-11-06 11:37:36 +0000\";\n opponent = \"0x6d2f730 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PROpponent/p1>\";\n opponentScore = nil;\n score = nil;\n status = 4;\n})", "<PRBattle: 0x6d306f0> (entity: PRBattle; id: 0x6d319f0 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p1> ; data: {\n battleId = \"-1\";\n finishedAt = \"2012-11-06 11:37:36 +0000\";\n opponent = \"0x6d2ddb0 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PROpponent/p3>\";\n opponentScore = nil;\n score = nil;\n status = 4;\n})", "<PRBattle: 0x6d30830> (entity: PRBattle; id: 0x6d31650 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p11> ; data: <fault>)", "<PRBattle: 0x6d306b0> (entity: PRBattle; id: 0x6d319e0 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p5> ; data: {\n battleId = 325;\n finishedAt = nil;\n opponent = \"0x6d2f730 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PROpponent/p1>\";\n opponentScore = 91;\n score = 59;\n status = 3;\n})", "<PRBattle: 0x6d30730> (entity: PRBattle; id: 0x6d31a00 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p6> ; data: {\n battleId = 323;\n finishedAt = nil;\n opponent = \"0x6d2ddb0 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PROpponent/p3>\";\n opponentScore = 0;\n score = 0;\n status = 3;\n})", "<PRBattle: 0x6d307b0> (entity: PRBattle; id: 0x6d31630 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p9> ; data: {\n battleId = 370;\n finishedAt = \"2012-11-06 14:24:14 +0000\";\n opponent = \"0x79a8e90 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PROpponent/p2>\";\n opponentScore = 180;\n score = 180;\n status = 4;\n})", "<PRBattle: 0x6d307f0> (entity: PRBattle; id: 0x6d31640 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p10> ; data: {\n battleId = 309;\n finishedAt = \"2012-11-02 01:19:27 +0000\";\n opponent = \"0x79a8e90 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PROpponent/p2>\";\n opponentScore = 120;\n score = 240;\n status = 4;\n})", "<PRBattle: 0x6d30770> (entity: PRBattle; id: 0x6d31620 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p7> ; data: {\n battleId = 315;\n finishedAt = \"2012-11-02 02:26:24 +0000\";\n opponent = \"0x79a8e90 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PROpponent/p2>\";\n opponentScore = 119;\n score = 179;\n status = 4;\n})" ) Faulted object (0xe972610) here causes crash. I've logged data during update & before saving. This object is in updatedObjects only. Why can this method return "bad" object? (Moreover, during updates this object is affected almost each update. And only after some passes becomes "bad" one). P.S.: I use RestKit to manage CoreData. UPDATED: The exception was got, when I did smth. like this: for (PRBattle *battle in [self.controller fetchedObjects) { switch (battle.statusScalar) { case ... default: [battle willAccessValueForKey:nil]; NSAssert1(NO, @"Unexpected battle status found: %@", battle); } } The exception is on line with -willAccessValueForKey:. Scalar status for battle is enum, that is bind to integer values 1..4. I've mentioned all possible values in switch's cases (above default:). And the last one has break;. So this one is possible only when battle.statusScalar returns non-enum value. Status scalar implementation in PRBattle: - (PRBattleStatuses)statusScalar { [self willAccessValueForKey:@"statusScalar"]; PRBattleStatuses result = (PRBattleStatuses)[self.status integerValue]; [self didAccessValueForKey:@"statusScalar"]; return result; } And battle.status has validation rules: - min-value: 1 - max-value: 4 - default: no value And the last thing - debug log: objc[4664]: EXCEPTIONS: throwing 0x7d33f80 (object 0xe67d2a0, a _NSCoreDataException) objc[4664]: EXCEPTIONS: searching through frame [ip=0x97b401 sp=0xbfffd9b0] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: catch(id) objc[4664]: EXCEPTIONS: unwinding through frame [ip=0x97b401 sp=0xbfffd9b0] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: handling exception 0x7d33f60 at 0x97b79f objc[4664]: EXCEPTIONS: rethrowing current exception objc[4664]: EXCEPTIONS: searching through frame [ip=0x97b911 sp=0xbfffd9b0] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: searching through frame [ip=0x9ac8b7 sp=0xbfffdc20] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: searching through frame [ip=0x97ee80 sp=0xbfffdc40] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: searching through frame [ip=0x361d0 sp=0xbfffdc70] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: searching through frame [ip=0xa701d8 sp=0xbfffde10] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: catch(id) objc[4664]: EXCEPTIONS: unwinding through frame [ip=0x97b911 sp=0xbfffd9b0] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: finishing handler objc[4664]: EXCEPTIONS: searching through frame [ip=0x97b963 sp=0xbfffd9b0] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: searching through frame [ip=0x9ac8b7 sp=0xbfffdc20] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: searching through frame [ip=0x97ee80 sp=0xbfffdc40] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: searching through frame [ip=0x361d0 sp=0xbfffdc70] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: searching through frame [ip=0xa701d8 sp=0xbfffde10] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: catch(id) objc[4664]: EXCEPTIONS: unwinding through frame [ip=0x97b963 sp=0xbfffd9b0] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: unwinding through frame [ip=0x9ac8b7 sp=0xbfffdc20] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: unwinding through frame [ip=0x97ee80 sp=0xbfffdc40] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: unwinding through frame [ip=0x361d0 sp=0xbfffdc70] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: unwinding through frame [ip=0x3656f sp=0xbfffdc70] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: unwinding through frame [ip=0xa701d8 sp=0xbfffde10] for exception 0x7d33f60 objc[4664]: EXCEPTIONS: handling exception 0x7d33f60 at 0xa701f5 2012-11-07 13:37:55.463 TestApp[4664:fb03] CoreData: error: Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. CoreData could not fulfill a fault for '0x6d31650 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p10>' with userInfo { NSAffectedObjectsErrorKey = ( "<PRBattle: 0x6d30830> (entity: PRBattle; id: 0x6d31650 <x-coredata://882BD521-90CD-4682-B19A-000A4976E471/PRBattle/p10> ; data: <fault>)" ); }

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  • C#/.NET Little Wonders: The Concurrent Collections (1 of 3)

    - by James Michael Hare
    Once again we consider some of the lesser known classes and keywords of C#.  In the next few weeks, we will discuss the concurrent collections and how they have changed the face of concurrent programming. This week’s post will begin with a general introduction and discuss the ConcurrentStack<T> and ConcurrentQueue<T>.  Then in the following post we’ll discuss the ConcurrentDictionary<T> and ConcurrentBag<T>.  Finally, we shall close on the third post with a discussion of the BlockingCollection<T>. For more of the "Little Wonders" posts, see the index here. A brief history of collections In the beginning was the .NET 1.0 Framework.  And out of this framework emerged the System.Collections namespace, and it was good.  It contained all the basic things a growing programming language needs like the ArrayList and Hashtable collections.  The main problem, of course, with these original collections is that they held items of type object which means you had to be disciplined enough to use them correctly or you could end up with runtime errors if you got an object of a type you weren't expecting. Then came .NET 2.0 and generics and our world changed forever!  With generics the C# language finally got an equivalent of the very powerful C++ templates.  As such, the System.Collections.Generic was born and we got type-safe versions of all are favorite collections.  The List<T> succeeded the ArrayList and the Dictionary<TKey,TValue> succeeded the Hashtable and so on.  The new versions of the library were not only safer because they checked types at compile-time, in many cases they were more performant as well.  So much so that it's Microsoft's recommendation that the System.Collections original collections only be used for backwards compatibility. So we as developers came to know and love the generic collections and took them into our hearts and embraced them.  The problem is, thread safety in both the original collections and the generic collections can be problematic, for very different reasons. Now, if you are only doing single-threaded development you may not care – after all, no locking is required.  Even if you do have multiple threads, if a collection is “load-once, read-many” you don’t need to do anything to protect that container from multi-threaded access, as illustrated below: 1: public static class OrderTypeTranslator 2: { 3: // because this dictionary is loaded once before it is ever accessed, we don't need to synchronize 4: // multi-threaded read access 5: private static readonly Dictionary<string, char> _translator = new Dictionary<string, char> 6: { 7: {"New", 'N'}, 8: {"Update", 'U'}, 9: {"Cancel", 'X'} 10: }; 11:  12: // the only public interface into the dictionary is for reading, so inherently thread-safe 13: public static char? Translate(string orderType) 14: { 15: char charValue; 16: if (_translator.TryGetValue(orderType, out charValue)) 17: { 18: return charValue; 19: } 20:  21: return null; 22: } 23: } Unfortunately, most of our computer science problems cannot get by with just single-threaded applications or with multi-threading in a load-once manner.  Looking at  today's trends, it's clear to see that computers are not so much getting faster because of faster processor speeds -- we've nearly reached the limits we can push through with today's technologies -- but more because we're adding more cores to the boxes.  With this new hardware paradigm, it is even more important to use multi-threaded applications to take full advantage of parallel processing to achieve higher application speeds. So let's look at how to use collections in a thread-safe manner. Using historical collections in a concurrent fashion The early .NET collections (System.Collections) had a Synchronized() static method that could be used to wrap the early collections to make them completely thread-safe.  This paradigm was dropped in the generic collections (System.Collections.Generic) because having a synchronized wrapper resulted in atomic locks for all operations, which could prove overkill in many multithreading situations.  Thus the paradigm shifted to having the user of the collection specify their own locking, usually with an external object: 1: public class OrderAggregator 2: { 3: private static readonly Dictionary<string, List<Order>> _orders = new Dictionary<string, List<Order>>(); 4: private static readonly _orderLock = new object(); 5:  6: public void Add(string accountNumber, Order newOrder) 7: { 8: List<Order> ordersForAccount; 9:  10: // a complex operation like this should all be protected 11: lock (_orderLock) 12: { 13: if (!_orders.TryGetValue(accountNumber, out ordersForAccount)) 14: { 15: _orders.Add(accountNumber, ordersForAccount = new List<Order>()); 16: } 17:  18: ordersForAccount.Add(newOrder); 19: } 20: } 21: } Notice how we’re performing several operations on the dictionary under one lock.  With the Synchronized() static methods of the early collections, you wouldn’t be able to specify this level of locking (a more macro-level).  So in the generic collections, it was decided that if a user needed synchronization, they could implement their own locking scheme instead so that they could provide synchronization as needed. The need for better concurrent access to collections Here’s the problem: it’s relatively easy to write a collection that locks itself down completely for access, but anything more complex than that can be difficult and error-prone to write, and much less to make it perform efficiently!  For example, what if you have a Dictionary that has frequent reads but in-frequent updates?  Do you want to lock down the entire Dictionary for every access?  This would be overkill and would prevent concurrent reads.  In such cases you could use something like a ReaderWriterLockSlim which allows for multiple readers in a lock, and then once a writer grabs the lock it blocks all further readers until the writer is done (in a nutshell).  This is all very complex stuff to consider. Fortunately, this is where the Concurrent Collections come in.  The Parallel Computing Platform team at Microsoft went through great pains to determine how to make a set of concurrent collections that would have the best performance characteristics for general case multi-threaded use. Now, as in all things involving threading, you should always make sure you evaluate all your container options based on the particular usage scenario and the degree of parallelism you wish to acheive. This article should not be taken to understand that these collections are always supperior to the generic collections. Each fills a particular need for a particular situation. Understanding what each container is optimized for is key to the success of your application whether it be single-threaded or multi-threaded. General points to consider with the concurrent collections The MSDN points out that the concurrent collections all support the ICollection interface. However, since the collections are already synchronized, the IsSynchronized property always returns false, and SyncRoot always returns null.  Thus you should not attempt to use these properties for synchronization purposes. Note that since the concurrent collections also may have different operations than the traditional data structures you may be used to.  Now you may ask why they did this, but it was done out of necessity to keep operations safe and atomic.  For example, in order to do a Pop() on a stack you have to know the stack is non-empty, but between the time you check the stack’s IsEmpty property and then do the Pop() another thread may have come in and made the stack empty!  This is why some of the traditional operations have been changed to make them safe for concurrent use. In addition, some properties and methods in the concurrent collections achieve concurrency by creating a snapshot of the collection, which means that some operations that were traditionally O(1) may now be O(n) in the concurrent models.  I’ll try to point these out as we talk about each collection so you can be aware of any potential performance impacts.  Finally, all the concurrent containers are safe for enumeration even while being modified, but some of the containers support this in different ways (snapshot vs. dirty iteration).  Once again I’ll highlight how thread-safe enumeration works for each collection. ConcurrentStack<T>: The thread-safe LIFO container The ConcurrentStack<T> is the thread-safe counterpart to the System.Collections.Generic.Stack<T>, which as you may remember is your standard last-in-first-out container.  If you think of algorithms that favor stack usage (for example, depth-first searches of graphs and trees) then you can see how using a thread-safe stack would be of benefit. The ConcurrentStack<T> achieves thread-safe access by using System.Threading.Interlocked operations.  This means that the multi-threaded access to the stack requires no traditional locking and is very, very fast! For the most part, the ConcurrentStack<T> behaves like it’s Stack<T> counterpart with a few differences: Pop() was removed in favor of TryPop() Returns true if an item existed and was popped and false if empty. PushRange() and TryPopRange() were added Allows you to push multiple items and pop multiple items atomically. Count takes a snapshot of the stack and then counts the items. This means it is a O(n) operation, if you just want to check for an empty stack, call IsEmpty instead which is O(1). ToArray() and GetEnumerator() both also take snapshots. This means that iteration over a stack will give you a static view at the time of the call and will not reflect updates. Pushing on a ConcurrentStack<T> works just like you’d expect except for the aforementioned PushRange() method that was added to allow you to push a range of items concurrently. 1: var stack = new ConcurrentStack<string>(); 2:  3: // adding to stack is much the same as before 4: stack.Push("First"); 5:  6: // but you can also push multiple items in one atomic operation (no interleaves) 7: stack.PushRange(new [] { "Second", "Third", "Fourth" }); For looking at the top item of the stack (without removing it) the Peek() method has been removed in favor of a TryPeek().  This is because in order to do a peek the stack must be non-empty, but between the time you check for empty and the time you execute the peek the stack contents may have changed.  Thus the TryPeek() was created to be an atomic check for empty, and then peek if not empty: 1: // to look at top item of stack without removing it, can use TryPeek. 2: // Note that there is no Peek(), this is because you need to check for empty first. TryPeek does. 3: string item; 4: if (stack.TryPeek(out item)) 5: { 6: Console.WriteLine("Top item was " + item); 7: } 8: else 9: { 10: Console.WriteLine("Stack was empty."); 11: } Finally, to remove items from the stack, we have the TryPop() for single, and TryPopRange() for multiple items.  Just like the TryPeek(), these operations replace Pop() since we need to ensure atomically that the stack is non-empty before we pop from it: 1: // to remove items, use TryPop or TryPopRange to get multiple items atomically (no interleaves) 2: if (stack.TryPop(out item)) 3: { 4: Console.WriteLine("Popped " + item); 5: } 6:  7: // TryPopRange will only pop up to the number of spaces in the array, the actual number popped is returned. 8: var poppedItems = new string[2]; 9: int numPopped = stack.TryPopRange(poppedItems); 10:  11: foreach (var theItem in poppedItems.Take(numPopped)) 12: { 13: Console.WriteLine("Popped " + theItem); 14: } Finally, note that as stated before, GetEnumerator() and ToArray() gets a snapshot of the data at the time of the call.  That means if you are enumerating the stack you will get a snapshot of the stack at the time of the call.  This is illustrated below: 1: var stack = new ConcurrentStack<string>(); 2:  3: // adding to stack is much the same as before 4: stack.Push("First"); 5:  6: var results = stack.GetEnumerator(); 7:  8: // but you can also push multiple items in one atomic operation (no interleaves) 9: stack.PushRange(new [] { "Second", "Third", "Fourth" }); 10:  11: while(results.MoveNext()) 12: { 13: Console.WriteLine("Stack only has: " + results.Current); 14: } The only item that will be printed out in the above code is "First" because the snapshot was taken before the other items were added. This may sound like an issue, but it’s really for safety and is more correct.  You don’t want to enumerate a stack and have half a view of the stack before an update and half a view of the stack after an update, after all.  In addition, note that this is still thread-safe, whereas iterating through a non-concurrent collection while updating it in the old collections would cause an exception. ConcurrentQueue<T>: The thread-safe FIFO container The ConcurrentQueue<T> is the thread-safe counterpart of the System.Collections.Generic.Queue<T> class.  The concurrent queue uses an underlying list of small arrays and lock-free System.Threading.Interlocked operations on the head and tail arrays.  Once again, this allows us to do thread-safe operations without the need for heavy locks! The ConcurrentQueue<T> (like the ConcurrentStack<T>) has some departures from the non-concurrent counterpart.  Most notably: Dequeue() was removed in favor of TryDequeue(). Returns true if an item existed and was dequeued and false if empty. Count does not take a snapshot It subtracts the head and tail index to get the count.  This results overall in a O(1) complexity which is quite good.  It’s still recommended, however, that for empty checks you call IsEmpty instead of comparing Count to zero. ToArray() and GetEnumerator() both take snapshots. This means that iteration over a queue will give you a static view at the time of the call and will not reflect updates. The Enqueue() method on the ConcurrentQueue<T> works much the same as the generic Queue<T>: 1: var queue = new ConcurrentQueue<string>(); 2:  3: // adding to queue is much the same as before 4: queue.Enqueue("First"); 5: queue.Enqueue("Second"); 6: queue.Enqueue("Third"); For front item access, the TryPeek() method must be used to attempt to see the first item if the queue.  There is no Peek() method since, as you’ll remember, we can only peek on a non-empty queue, so we must have an atomic TryPeek() that checks for empty and then returns the first item if the queue is non-empty. 1: // to look at first item in queue without removing it, can use TryPeek. 2: // Note that there is no Peek(), this is because you need to check for empty first. TryPeek does. 3: string item; 4: if (queue.TryPeek(out item)) 5: { 6: Console.WriteLine("First item was " + item); 7: } 8: else 9: { 10: Console.WriteLine("Queue was empty."); 11: } Then, to remove items you use TryDequeue().  Once again this is for the same reason we have TryPeek() and not Peek(): 1: // to remove items, use TryDequeue. If queue is empty returns false. 2: if (queue.TryDequeue(out item)) 3: { 4: Console.WriteLine("Dequeued first item " + item); 5: } Just like the concurrent stack, the ConcurrentQueue<T> takes a snapshot when you call ToArray() or GetEnumerator() which means that subsequent updates to the queue will not be seen when you iterate over the results.  Thus once again the code below will only show the first item, since the other items were added after the snapshot. 1: var queue = new ConcurrentQueue<string>(); 2:  3: // adding to queue is much the same as before 4: queue.Enqueue("First"); 5:  6: var iterator = queue.GetEnumerator(); 7:  8: queue.Enqueue("Second"); 9: queue.Enqueue("Third"); 10:  11: // only shows First 12: while (iterator.MoveNext()) 13: { 14: Console.WriteLine("Dequeued item " + iterator.Current); 15: } Using collections concurrently You’ll notice in the examples above I stuck to using single-threaded examples so as to make them deterministic and the results obvious.  Of course, if we used these collections in a truly multi-threaded way the results would be less deterministic, but would still be thread-safe and with no locking on your part required! For example, say you have an order processor that takes an IEnumerable<Order> and handles each other in a multi-threaded fashion, then groups the responses together in a concurrent collection for aggregation.  This can be done easily with the TPL’s Parallel.ForEach(): 1: public static IEnumerable<OrderResult> ProcessOrders(IEnumerable<Order> orderList) 2: { 3: var proxy = new OrderProxy(); 4: var results = new ConcurrentQueue<OrderResult>(); 5:  6: // notice that we can process all these in parallel and put the results 7: // into our concurrent collection without needing any external locking! 8: Parallel.ForEach(orderList, 9: order => 10: { 11: var result = proxy.PlaceOrder(order); 12:  13: results.Enqueue(result); 14: }); 15:  16: return results; 17: } Summary Obviously, if you do not need multi-threaded safety, you don’t need to use these collections, but when you do need multi-threaded collections these are just the ticket! The plethora of features (I always think of the movie The Three Amigos when I say plethora) built into these containers and the amazing way they acheive thread-safe access in an efficient manner is wonderful to behold. Stay tuned next week where we’ll continue our discussion with the ConcurrentBag<T> and the ConcurrentDictionary<TKey,TValue>. For some excellent information on the performance of the concurrent collections and how they perform compared to a traditional brute-force locking strategy, see this wonderful whitepaper by the Microsoft Parallel Computing Platform team here.   Tweet Technorati Tags: C#,.NET,Concurrent Collections,Collections,Multi-Threading,Little Wonders,BlackRabbitCoder,James Michael Hare

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  • Giving a Bomberman AI intelligent bomb placement

    - by Paul Manta
    I'm trying to implement an AI algorithm for Bomberman. Currently I have a working but not very smart rudimentary implementation (the current AI is overzealous in placing bombs). This is the first AI I've ever tried implementing and I'm a bit stuck. The more sophisticated algorithms I have in mind (the ones that I expect to make better decisions) are too convoluted to be good solutions. What general tips do you have for implementing a Bomberman AI? Are there radically different approaches for making the bot either more defensive or offensive? Edit: Current algorithm My current algorithm goes something like this (pseudo-code): 1) Try to place a bomb and then find a cell that is safe from all the bombs, including the one that you just placed. To find that cell, iterate over the four directions; if you can find any safe divergent cell and reach it in time (eg. if the direction is up or down, look for a cell that is found to the left or right of this path), then it's safe to place a bomb and move in that direction. 2) If you can't find and safe divergent cells, try NOT placing a bomb and look again. This time you'll only need to look for a safe cell in only one direction (you don't have to diverge from it). 3) If you still can't find a safe cell, don't do anything. for $(direction) in (up, down, left, right): place bomb at current location if (can find and reach divergent safe cell in current $(direction)): bomb = true move = $(direction) return for $(direction) in (up, down, left, right): do not place bomb at current location if (any safe cell in the current $(direction)): bomb = false move = $(direction) return else: bomb = false move = stay_put This algorithm makes the bot very trigger-happy (it'll place bombs very frequently). It doesn't kill itself, but it does have a habit of making itself vulnerable by going into dead ends where it can be blocked and killed by the other players. Do you have any suggestions on how I might improve this algorithm? Or maybe I should try something completely different? One of the problems with this algorithm is that it tends to leave the bot with very few (frequently just one) safe cells on which it can stand. This is because the bot leaves a trail of bombs behind it, as long as it doesn't kill itself. However, leaving a trail of bombs behind leaves few places where you can hide. If one of the other players or bots decide to place a bomb somewhere near you, it often happens that you have no place to hide and you die. I need a better way to decide when to place bombs.

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  • WebDAV "PROPFIND" exception in IIS due to network share?

    - by jacko
    Hi all, We're finding continuous exceptions in our event viewer on our live box to the following exception: [snippet] Process information: Process ID: 3916 Process name: w3wp.exe Account name: NT AUTHORITY\NETWORK SERVICE Exception information: Exception type: HttpException Exception message: Path 'PROPFIND' is forbidden. Thread information: Thread ID: 14 Thread account name: OURDOMAIN\Account Is impersonating: True Stack trace: at System.Web.HttpMethodNotAllowedHandler.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) Other Specs: Windows Server 2003 R2 & IIS 6.0 We've narrowed it down to occuring when people try to access shares on the box from within the network, and have discovered (we think) that its due to the WebDAV web services extension being previously disabled by past staff. The exceptions are being thrown when trying to access directories that are virtual dirs in IIS, and plain old UNC network shares What the implications for enabling the WebDAV extensions on our live web server? And will this solve our problems with the exceptions in our event log?

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  • Steam freezes at login screen

    - by Snail284069
    I have just installed Steam on Xubuntu, and after it finished installing it went to the login screen, but the screen is frozen, and I cannot press the buttons. When running Steam though the terminal it says: alex@Craptop:~$ steam Running Steam on ubuntu 14.04 32-bit STEAM_RUNTIME is enabled automatically Installing breakpad exception handler for appid(steam)/version(1400690891_client) Installing breakpad exception handler for appid(steam)/version(1400690891_client) Installing breakpad exception handler for appid(steam)/version(1400690891_client) Installing breakpad exception handler for appid(steam)/version(1400690891_client) Gtk-Message: Failed to load module "overlay-scrollbar" Gtk-Message: Failed to load module "unity-gtk-module" Installing breakpad exception handler for appid(steam)/version(1400690891_client) [0522/174755:WARNING:proxy_service.cc(958)] PAC support disabled because there is no system implementation Fontconfig error: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 70: non-double matrix element Fontconfig error: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 70: non-double matrix element Fontconfig warning: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 78: saw unknown, expected number Installing breakpad exception handler for appid(steam)/version(1400690891_client) Installing breakpad exception handler for appid(steam)/version(1400690891_client) [HTTP Remote Control] HTTP server listening on port 35849. Installing breakpad exception handler for appid(steam)/version(1400690891_client) Installing breakpad exception handler for appid(steam)/version(1400690891_client) Process 2764 created /alex-ValveIPCSharedObjects5 Installing breakpad exception handler for appid(steam)/version(1400690891_client) Generating new string page texture 12: 48x256, total string texture memory is 49.15 KB Generating new string page texture 13: 256x256, total string texture memory is 311.30 KB Generating new string page texture 14: 128x256, total string texture memory is 442.37 KB Generating new string page texture 15: 384x256, total string texture memory is 835.58 KB and then the terminal gets stuck too, letting me type into it but not doing anything. I tried reinstalling and restarting the computer, but it still keeps happening.

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  • WebDAV "PROPFIND" exception in IIS due to network share?

    - by jacko
    We're finding continuous exceptions in our event viewer on our live box to the following exception: [snippet] Process information: Process ID: 3916 Process name: w3wp.exe Account name: NT AUTHORITY\NETWORK SERVICE Exception information: Exception type: HttpException Exception message: Path 'PROPFIND' is forbidden. Thread information: Thread ID: 14 Thread account name: OURDOMAIN\Account Is impersonating: True Stack trace: at System.Web.HttpMethodNotAllowedHandler.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) Other Specs: Windows Server 2003 R2 & IIS 6.0 We've narrowed it down to occuring when people try to access shares on the box from within the network, and have discovered (we think) that its due to the WebDAV web services extension being previously disabled by past staff. The exceptions are being thrown when trying to access directories that are virtual dirs in IIS, and plain old UNC network shares What the implications for enabling the WebDAV extensions on our live web server? And will this solve our problems with the exceptions in our event log?

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