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• Achieving Thread-Safety

  - by Smasher

  Question How can I make sure my application is thread-safe? Are their any common practices, testing methods, things to avoid, things to look for? Background I'm currently developing a server application that performs a number of background tasks in different threads and communicates with clients using Indy (using another bunch of automatically generated threads for the communication). Since the application should be highly availabe, a program crash is a very bad thing and I want to make sure that the application is thread-safe. No matter what, from time to time I discover a piece of code that throws an exception that never occured before and in most cases I realize that it is some kind of synchronization bug, where I forgot to synchronize my objects properly. Hence my question concerning best practices, testing of thread-safety and things like that. mghie: Thanks for the answer! I should perhaps be a little bit more precise. Just to be clear, I know about the principles of multithreading, I use synchronization (monitors) throughout my program and I know how to differentiate threading problems from other implementation problems. But nevertheless, I keep forgetting to add proper synchronization from time to time. Just to give an example, I used the RTL sort function in my code. Looked something like FKeyList.Sort (CompareKeysFunc); Turns out, that I had to synchronize FKeyList while sorting. It just don't came to my mind when initially writing that simple line of code. It's these thins I wanna talk about. What are the places where one easily forgets to add synchronization code? How do YOU make sure that you added sync code in all important places?

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• Problem with Boost::Asio for C++

  - by Martin Lauridsen

  Hi there, For my bachelors thesis, I am implementing a distributed version of an algorithm for factoring large integers (finding the prime factorisation). This has applications in e.g. security of the RSA cryptosystem. My vision is, that clients (linux or windows) will download an application and compute some numbers (these are independant, thus suited for parallelization). The numbers (not found very often), will be sent to a master server, to collect these numbers. Once enough numbers have been collected by the master server, it will do the rest of the computation, which cannot be easily parallelized. Anyhow, to the technicalities. I was thinking to use Boost::Asio to do a socket client/server implementation, for the clients communication with the master server. Since I want to compile for both linux and windows, I thought windows would be as good a place to start as any. So I downloaded the Boost library and compiled it, as it said on the Boost Getting Started page: bootstrap .\bjam It all compiled just fine. Then I try to compile one of the tutorial examples, client.cpp, from Asio, found (here.. edit: cant post link because of restrictions). I am using the Visual C++ compiler from Microsoft Visual Studio 2008, like this: cl /EHsc /I D:\Downloads\boost_1_42_0 client.cpp But I get this error: /out:client.exe client.obj LINK : fatal error LNK1104: cannot open file 'libboost_system-vc90-mt-s-1_42.lib' Anyone have any idea what could be wrong, or how I could move forward? I have been trying pretty much all week, to get a simple client/server socket program for c++ working, but with no luck. Serious frustration kicking in. Thank you in advance.

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• Find all the child node of specific value in C#

  - by sara brown

  <main> <myself> <pid>1</pid> <name>abc</name> </myself> <myself> <pid>2</pid> <name>efg</name> </myself> </main> that is my XML file named simpan. I have two button. next and previous. What i want to do is, all the info will shows off on the TextBox when the user click the button. The searching node will be based on the pid. Next button will adding 1 value of pid (let's say pid=2) and it will search on the node that have the same value of pid=2. it also will show the name for the pid=2. (showing name=abc) Same goes to the previous button where it will reduce 1value of pid (pid=1). Does anybody knows how to do this? //-------------EDIT------------------ thanks to L.B, im trying to use his code. however i got an error. is my implementation of code correct? private void previousList_Click(object sender, EventArgs e) { pid = 14; XDocument xDoc = XDocument.Parse("C:\\Users\\HDAdmin\\Documents\\Fatty\\SliceEngine\\SliceEngine\\bin\\Debug\\simpan.xml"); var name = xDoc.Descendants("myself") .First(m => (int)m.Element("PatientID") == pid) .Value; textETA.Text = name; //////////////////// }

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• Problem with writing mobilesubstrate plugins for iOS

  - by overboming

  I am trying to hooking message sending for iOS 3.2, I implement my own hook on a working ExampleHook program I find on the web. But my hook apparently caused segmentation fault everytime it hooks and I don't know why. I want to hook to [NSURL initWithString:(NSString *)URLString relativeToURL:(NSURL *)baseURL; and here is my related implementation static id __$GFWInterceptor_NSURL_initWithString2(NSURL<GFWInterceptor> *_NSURL, NSString *URLString, NSURL* baseURL){ NSLog(@"We have intercepted this url: %@",URLString); [_NSURL __HelloNSURL_initWithString:URLString relativeToURL:baseURL]; establish hook Class _$NSURL = objc_getClass("NSURL"); MSHookMessage(_$NSURL, @selector(initWithString:relativeToURL:), (IMP) &__$GFWInterceptor_NSURL_initWithString2, "__HelloNSURL_"); original method declaration - (void)__HelloNSURL_initWithString:(NSString *)URLString relativeToURL:(NSURL *)baseURL; and here is my gdb backtrace Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_INVALID_ADDRESS at address: 0x74696e71 0x335625f8 in objc_msgSend () (gdb) bt 0 0x335625f8 in objc_msgSend () 1 0x32c05b1a in CFStringGetLength () 2 0x32c108a8 in _CFStringIsLegalURLString () 3 0x32b1c32a in -[NSURL initWithString:relativeToURL:] () 4 0x000877c0 in __$GFWInterceptor_NSURL_initWithString2 () 5 0x32b1c220 in +[NSURL URLWithString:relativeToURL:] () 6 0x32b1c1f4 in +[NSURL URLWithString:] () 7 0x3061c614 in createUniqueWebDataURL () 8 0x3061c212 in +[WebFrame(WebInternal) _createMainFrameWithSimpleHTMLDocumentWithPage:frameView:withStyle:editable:] () and apparently it hooks, but there is some memory issue there and I can't find anything to blame now

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• How to easily map c++ enums to strings

  - by Roddy

  I have a bunch of enum types in some library header files that I'm using, and I want to have a way of converting enum values to user strings - and vice-versa. RTTI won't do it for me, because the 'user strings' need to be a bit more readable than the enumerations. A brute force solution would be a bunch of functions like this, but I feel that's a bit too C-like. enum MyEnum {VAL1, VAL2,VAL3}; String getStringFromEnum(MyEnum e) { switch e { case VAL1: return "Value 1"; case VAL2: return "Value 2"; case VAL1: return "Value 3"; default: throw Exception("Bad MyEnum"); } } I have a gut feeling that there's an elegant solution using templates, but I can't quite get my head round it yet. UPDATE: Thanks for suggestions - I should have made clear that the enums are defined in a third-party library header, so I don't want to have to change the definition of them. My gut feeling now is to avoid templates and do something like this: char * MyGetValue(int v, char *tmp); // implementation is trivial #define ENUM_MAP(type, strings) char * getStringValue(const type &T) \ { \ return MyGetValue((int)T, strings); \ } ; enum eee {AA,BB,CC}; - exists in library header file ; enum fff {DD,GG,HH}; ENUM_MAP(eee,"AA|BB|CC") ENUM_MAP(fff,"DD|GG|HH") // To use... eee e; fff f; std::cout<< getStringValue(e); std::cout<< getStringValue(f);

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• NSTimer Reset Not Working

  - by user355900

  hi, i have a nstimer and it works perfectly counting down from 2:00 but when i hit the reset button it does not work it just stops the timer and when i press start again it will carry on with the timer as if it had never been stopped. Here is my code `@implementation TimerAppDelegate @synthesize window; (void)applicationDidFinishLaunching:(UIApplication *)application { timerLabel.text = @"2:00"; seconds = 120; // Override point for customization after application launch [window makeKeyAndVisible]; } (void)viewDidLoad { [timer invalidate]; } (void)countDownOneSecond { seconds--; int currentTime = [timerLabel.text intValue]; int newTime = currentTime - 1; int displaySeconds = !(seconds % 60) ? 0 : seconds < 60 ? seconds : seconds - 60; int displayMinutes = floor(seconds / 60); NSString *time = [NSString stringWithFormat:@"%d:%@%d", displayMinutes, [[NSString stringWithFormat:@"%d", displaySeconds] length] == 1 ? @"0" : @"", displaySeconds ]; timerLabel.text = time; if(seconds == 0) { [timer invalidate]; } } (void)startOrStopTimer { if(timerIsRunning){ [timer invalidate]; [startOrStopButton setTitle:@"Start" forState:UIControlStateNormal]; } else { timer = [[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(countDownOneSecond) userInfo:nil repeats:YES] retain]; [startOrStopButton setTitle:@"Stop" forState:UIControlStateNormal]; } timerIsRunning = !timerIsRunning; } (void)resetTimer { [timer invalidate]; [startOrStopButton setTitle:@"Start" forState:UIControlStateNormal]; [timer invalidate]; timerLabel.text = @"2:00"; } (void)dealloc { [window release]; [super dealloc]; } @end` thanks

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• Best approach for Java/Maven/JPA/Hibernate build with multiple database vendor support?

  - by HDave

  I have an enterprise application that uses a single database, but the application needs to support mysql, oracle, and sql*server as installation options. To try to remain portable we are using JPA annotations with Hibernate as the implementation. We also have a test-bed instance of each database running for development. The app is building nicely in Maven, and I've played around with the hibernate3-maven-plugin and can auto-generate DDL for a given database dialect. What is the best way to approach this so that individual developers can easily test against all three databases and our Hudson based CI server can build things propertly. More specifically: 1) I thought the hbm2ddl goal in the hibernate3-maven-plugin would just generate a schema file, but apparently it connects to a live database and attempts to create the schema. Is there a way to have this just create the schema file for each database dialect without connecting to a database? 2) If the hibernate3-maven-plug insists on actually creating the database schema, is there a way to have it drop the database and recreate it before creating the schema? 3) I am thinking that each developer (and the hudson build machine) should have their own separate database on each database server. Is this typical? 4) Will developers have to run Maven three times...once for each database vendor? If so, how do I merge the results on the build machine? 5) There is a hbm2doc goal within hibernate3-maven-plugin. It seems overkill to run this three times...I gotta believe it'd be nearly identical for each database.

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• Endianness and C API's: Specifically OpenSSL.

  - by Hassan Syed

  I have an algorithm that uses the following OpenSSL calls: HMAC_update() / HMAC_final() // ripe160 EVP_CipherUpdate() / EVP_CipherFinal() // cbc_blowfish These algorithm take a unsigned char * into the "plain text". My input data is comes from a C++ std::string::c_str() which originate from a protocol buffer object as a encoded UTF-8 string. UTF-8 strings are meant to be endian neutrial. However I'm a bit paranoid about how OpenSSL may perform operations on the data. My understanding is that encryption algorithms work on 8-bit blocks of data, and if a unsigned char * is used for pointer arithmetic when the operations are performed the algorithms should be endian neutral and I do not need to worry about anything. My uncertainty is compounded by the fact that I am working on a little-endian machine and have never done any real cross-architecture programming. My beliefs/reasoning are/is based on the following two properties std::string (not wstring) internally uses a 8-bit ptr and a the resulting c_str() ptr will itterate the same way regardless of the CPU architecture. Encryption algorithms are either by design, or by implementation, endian neutral. I know the best way to get a definitive answer is to use QEMU and do some cross-platform unit tests (which I plan to do). My question is a request for comments on my reasoning, and perhaps will assist other programmers when faced with similar problems.

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• Delaying LINQ to SQL Select Query Execution

  - by Maxim Z.

  I'm building an ASP.NET MVC site that uses LINQ to SQL. In my search method that has some required and some optional parameters, I want to build a LINQ query while testing for the existence of those optional parameters. Here's what I'm currently thinking: using(var db = new DBDataContext()) { IQueryable<Listing> query = null; //Handle required parameter query = db.Listings.Where(l => l.Lat >= form.bounds.extent1.latitude && l.Lat <= form.bounds.extent2.latitude); //Handle optional parameter if (numStars != null) query = query.Where(l => l.Stars == (int)numStars); //Other parameters... //Execute query (does this happen here?) var result = query.ToList(); //Process query... Will this implementation "bundle" the where clauses and then execute the bundled query? If not, how should I implement this feature? Also, is there anything else that I can improve? Thanks in advance.

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• List of Big-O for PHP functions?

  - by Kendall Hopkins

  After using PHP for a while now, I've noticed that not all PHP built in functions as fast as expected. Consider the below two possible implementations of a function that finds if a number is prime using a cached array of primes. //very slow for large $prime_array $prime_array = array( 2, 3, 5, 7, 11, 13, .... 104729, ... ); $result_array = array(); foreach( $array_of_number => $number ) { $result_array[$number] = in_array( $number, $large_prime_array ); } //still decent performance for large $prime_array $prime_array => array( 2 => NULL, 3 => NULL, 5 => NULL, 7 => NULL, 11 => NULL, 13 => NULL, .... 104729 => NULL, ... ); foreach( $array_of_number => $number ) { $result_array[$number] = array_key_exists( $number, $large_prime_array ); } This is because in_array is implemented with a linear search O(n) which will linearly slow down as $prime_array grows. Where the array_key_exists function is implemented with a hash lookup O(1) which will not slow down unless the hash table gets extremely populated (in which case it's only O(logn)). So far I've had to discover the big-O's via trial and error, and occasionally looking at the source code. Now for the question... I was wondering if there was a list of the theoretical (or practical) big O times for all* the PHP built in functions. *or at least the interesting ones For example find it very hard to predict what the big O of functions listed because the possible implementation depends on unknown core data structures of PHP: array_merge, array_merge_recursive, array_reverse, array_intersect, array_combine, str_replace (with array inputs), etc.

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• 'WebException' error on back button even when calling 'void' async method

  - by BlazingFrog

  I have a windows phone app that allows the user to interact with it. Each interaction will always result in an async WCF call. In addition to that, some interactions will result in opening the browser, maps, email, etc... The problem is that, when hitting the back button, I sometime get the following error "An error (WebException) occurred while transmitting data over the HTTP channel." with the following stack trace: at System.ServiceModel.Channels.HttpChannelUtilities.ProcessGetResponseWebException(WebException webException, HttpWebRequest request, HttpAbortReason abortReason) at System.ServiceModel.Channels.HttpChannelFactory.HttpRequestChannel.HttpChannelAsyncRequest.CompleteGetResponse(IAsyncResult result) at System.ServiceModel.Channels.HttpChannelFactory.HttpRequestChannel.HttpChannelAsyncRequest.OnGetResponse(IAsyncResult result) at System.Net.Browser.ClientHttpWebRequest.<>c__DisplayClassa.<InvokeGetResponseCallback>b__8(Object state2) at System.Threading.ThreadPool.WorkItem.WaitCallback_Context(Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadPool.WorkItem.doWork(Object o) at System.Threading.Timer.ring() My understanding is that it's happening because my app opened another app (browser, maps, etc) before it had the time to execute the EndMyAsyncMethod(System.IAsyncResult result). Fair enough... What's really annoying is that it seems it should get fixed by cloning the server-side method, only making it void with the following operation contract [OperationContract(IsOneWay = true)] but I'm still getting the error. What's worse is that the exception is thrown in a system-generated part of the code and, thus, cannot be manually caught causing the app to just crash. I simply don't understand the need to execute an Endxxx method when it's explicitely marked as OneWay and void. EDIT I did find a similar issue here. It does seem that it is related to the message getting to the service (not the client callback). My next question is: if I'm now calling a method marked AsyncPattern and OneWay, what exactly should I be waiting for on the client to be sure the message was transmitted successfully? This is new service definition: [OperationContract(IsOneWay = true, AsyncPattern = true)] IAsyncResult BeginCacheQueryWithoutCallback(string param1, QueryInfoDataContract queryInfo, AsyncCallback cb, Object s); void EndCacheQueryWithoutCallback(IAsyncResult r); And the implementation: public IAsyncResult BeginCacheQueryWithoutCallback(string param1, QueryInfoDataContract queryInfo, AsyncCallback cb, Object s) { // do some stuff return new CompletedAsyncResult<string>(""); } public void EndCacheQueryWithoutCallback(IAsyncResult r) { }

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• How to create a snowstorm on your Windows desktop?

  - by Vilx-

  Practical uses aside, how (if it is possible at all) could you create a "snowing" effect on your desktop PC running Windows? Preferably with nothing but raw C/C++ and WinAPI. The requirements for the snow are: Appears over everything else shown; Snowflakes are small, possibly simple dots or clusters of a few white pixels; Does not bother working with the computer (clicking a snowflake sends the click through to the underlying window); Plays nicely with users dragging windows; Multimonitor capable. Bonus points for any of the following features: Snow accumulates on the lower edge of the window or the taskbar (if it's at the bottom of the screen); Snow accumulates also on top-level windows. Or perhaps some snow accumulates, some continues down, accumulating on every window with a title bar; Snow accumulated on windows gets "shaken off" when windows are dragged; Snow accumulated on taskbar is aware of the extended "Start" button under Vista/7. Snowflakes have shadows/outlines, so they are visible on white backgrounds; Snowflakes have complex snowflike-alike shapes (they must still be tiny). Most of these effects are straightforward enough, except the part where snow is click-through and plays nicely with dragging of windows. In my early days I've made an implementation that draws on the HDC you get from GetDesktopWindow(), which was click-through, but had problems with users dragging windows (snowflakes rendered on them got "dragged along"). The solution may use Vista/7 Aero features, but, of course, a universal solution is preferred. Any ideas? :)

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• Circular dependency with generics

  - by devoured elysium

  I have defined the following interface: public interface IStateSpace<State, Action> where State : IState where Action : IAction<State, Action> // <-- this is the line that bothers me { void SetValueAt(State state, Action action); Action GetValueAt(State state); } Basically, an IStateSpace interface should be something like a chess board, and in each position of the chess board you have a set of possible movements to do. Those movements here are called IActions. I have defined this interface this way so I can accommodate for different implementations: I can then define concrete classes that implement 2D matrix, 3D matrix, graphs, etc. public interface IAction<State, Action> { IStateSpace<State, Action> StateSpace { get; } } An IAction, would be to move up(this is, if in (2, 2) move to (2, 1)), move down, etc. Now, I'll want that each action has access to a StateSpace so it can do some checking logic. Is this implementation correct? Or is this a bad case of a circular dependence? If yes, how to accomplish "the same" in a different way? Thanks

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• Solr/Lucene Scorer

  - by TFor

  We are currently working on a proof-of-concept for a client using Solr and have been able to configure all the features they want except the scoring. Problem is that they want scores that make results fall in buckets: Bucket 1: exact match on category (score = 4) Bucket 2: exact match on name (score = 3) Bucket 3: partial match on category (score = 2) Bucket 4: partial match on name (score = 1) First thing we did was develop a custom similarity class that would return the correct score depending on the field and an exact or partial match. The only problem now is that when a document matches on both the category and name the scores are added together. Example: searching for "restaurant" returns documents in the category restaurant that also have the word restaurant in their name and thus get a score of 5 (4+1) but they should only get 4. I assume for this to work we would need to develop a custom Scorer class but we have no clue on how to incorporate this in Solr. Another option is to create a custom SortField implementation similar to the RandomSortField already present in Solr. Maybe there is even a simpler solution that we don't know about. All suggestions welcome!

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• Plotting an Arc in Discrete Steps

  - by phobos51594

  Good afternoon, Background My question relates to the plotting of an arbitrary arc in space using discrete steps. It is unique, however, in that I am not drawing to a canvas in the typical sense. The firmware I am designing is for a gcode interpreter for a CNC mill that will translate commands into stepper motor movements. Now, I have already found a similar question on this very site, but the methodology suggested (Bresenham's Algorithm) appears to be incompatable for moving an object in space, as it only relies on the calculation of one octant of a circle which is then mirrored about the remaining axes of symmetry. Furthermore, the prescribed method of calculation an arc between two arbitrary angles relies on trigonometry (I am implementing on a microcontroller and would like to avoid costly trig functions, if possible) and simply not taking the steps that are out of the range. Finally, the algorithm only is designed to work in one rotational direction (e.g. counterclockwise). Question So, on to the actual question: Does anyone know of a general-purpose algorithm that can be used to "draw" an arbitrary arc in discrete steps while still giving respect to angular direction (CW / CCW)? The final implementation will be done in C, but the language for the purpose of the question is irrelevant. Thank you in advance. References S.O post on drawing a simple circle using Bresenham's Algorithm: "Drawing" an arc in discrete x-y steps Wiki page describing Bresenham's Algorithm for a circle http://en.wikipedia.org/wiki/Midpoint_circle_algorithm Gcode instructions to be implemented (see. G2 and G3) http://linuxcnc.org/docs/html/gcode.html

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• Encapsulating user input of data for a class (C++)

  - by Dr. Monkey

  For an assignment I've made a simple C++ program that uses a superclass (Student) and two subclasses (CourseStudent and ResearchStudent) to store a list of students and print out their details, with different details shown for the two different types of students (using overriding of the display() method from Student). My question is about how the program collects input from the user of things like the student name, ID number, unit and fee information (for a course student) and research information (for research students): My implementation has the prompting for user input and the collecting of that input handled within the classes themselves. The reasoning behind this was that each class knows what kind of input it needs, so it makes sense to me to have it know how to ask for it (given an ostream through which to ask and an istream to collect the input from). My lecturer says that the prompting and input should all be handled in the main program, which seems to me somewhat messier, and would make it trickier to extend the program to handle different types of students. I am considering, as a compromise, to make a helper class that handles the prompting and collection of user input for each type of Student, which could then be called on by the main program. The advantage of this would be that the student classes don't have as much in them (so they're cleaner), but also they can be bundled with the helper classes if the input functionality is required. This also means more classes of Student could be added without having to make major changes to the main program, as long as helper classes are provided for these new classes. Also the helper class could be swapped for an alternative language version without having to make any changes to the class itself. What are the major advantages and disadvantages of the three different options for user input (fully encapsulated, helper class or in the main program)?

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• Bulletin board - Database optimisation

  - by andrew

  This question is a follow on from this Question The project and problem The project I am currently working on is a bulletin board for a large non-profit organisation. The bulletin board will be used to allow inter-office communication within the organisation. I am building the application and have been having trouble extracting the results that I need from my database because I don't think it is properly normalized and because of limitations in my knowledge of relational database theory and mysql. I would appreciate input into the design of the board in general and in particular, ways that the database structure can be improved to facilitate efficient queries and help me develop this application and future application faster Business Logic The bulletin board will be used in the following way Posting bulletins and responses to bulletins Employees or 'users' in offices around the country will be able to post messages to the bulletin board.Bulletins must be posted to a location and categorised- i'll call these "bulletins". Users will be able to post any number of replies to any one bulletin and users will be able to reply to their own bulletin - i'll call these 'replies'. Rating bulletins and replies Users will be able to either 'like' or 'dislike' a bulletin or a reply and the total number of likes or dislikes will be shown for each bulletin or reply. Viewing the bulletin board and responses Bulletins can be displayed chronologically. Users can sort bulletins chronologically or chronologically by the latest reply to that bulletin(let me know if you need more explanation) When a particular bulletin is selected, replies to that bulletin will be displayed chronologically @PerformanceDBA - edited 10:34 est 28/12/10I have begun implementing the data model. I assume that the 6th data model is the physical model because it contains the associative tables. I am going to post any questions that I have below. I will put up a database dump once I am done. I will then put up a list of all the queries that I need to run on the database and begin writing them. I hope you had a good Christmas. I'm in Canada and there's snow! Implementation of Physical model

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• How can I simply change a class variable from another class in ObjectiveC?

  - by Daniel

  I simply want to change a variable of an object from another class. I can compile without a problem, but my variable always is set to 'null'. I used the following code: Object.h: @interface Object : NSObject { //... NSString *color; //... } @property(nonatomic, retain) NSString* color; + (id)Object; - (void)setColor:(NSString*)col; - (NSString*)getColor; @end Object.m: +(id)Object{ return [[[Object alloc] init] autorelease]; } - (void)setColor:(NSString*)col { self.color = col; } - (NSString*)getColor { return self.color; } MyViewController.h #import "Object.h" @interface ClassesTestViewController : UIViewController { Object *myObject; UILabel *label1; } @property UILabel *label1; @property (assign) Object *myObject; @end MyViewController.m: #import "Object.h" @implementation MyViewController @synthesize myObject; - (void)viewDidLoad { [myObject setColor:@"red"]; NSLog(@"Color = %@", [myObject getColor]); [super viewDidLoad]; } The NSLog message is always Color = (null) I tried many different ways to solve this problem, but no success. Any help would be appreciated.

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• Help me write my LISP :) LISP environments, Ruby Hashes...

  - by MikeC8

  I'm implementing a rudimentary version of LISP in Ruby just in order to familiarize myself with some concepts. I'm basing my implementation off of Peter Norvig's Lispy (http://norvig.com/lispy.html). There's something I'm missing here though, and I'd appreciate some help... He subclasses Python's dict as follows: class Env(dict): "An environment: a dict of {'var':val} pairs, with an outer Env." def __init__(self, parms=(), args=(), outer=None): self.update(zip(parms,args)) self.outer = outer def find(self, var): "Find the innermost Env where var appears." return self if var in self else self.outer.find(var) He then goes on to explain why he does this rather than just using a dict. However, for some reason, his explanation keeps passing in through my eyes and out through the back of my head. Why not use a dict, and then inside the eval function, when a new "sub-environment" needs to be created, just take the existing dict and update the key/value pairs that need to be updated, and pass that new dict into the next eval? Won't the Python interpreter keep track of the previous "outer" envs? And won't the nature of the recursion ensure that the values are pulled out from "inner" to "outer"? I'm using Ruby, and I tried to implement things this way. Something's not working though, and it might be because of this, or perhaps not. Here's my eval function, env being a regular Hash: def eval(x, env = $global_env) ........ elsif x[0] == "lambda" then ->(*args) { eval(x[2], env.merge(Hash[*x[1].zip(args).flatten(1)])) } ........ end The line that matters of course is the "lambda" one. If there is a difference, what's importantly different between what I'm doing here and what Norvig did with his Env class? If there's no difference, then perhaps someone can enlighten me as to why Norvig uses the Env class. Thanks :)

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• ASP.NET MVC editor template for property

  - by Idsa

  Usually I render my forms by @Html.RenderModel, but this time I have a complex rendering logic and I render it manually. I decided to create a editor template for one property. Here is the code (copy pasted from default object editor template implementation): <%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl" %> <% var modelMetadata = ViewData.ModelMetadata; %> <% if (modelMetadata.HideSurroundingHtml) { %> <%= Html.Editor(modelMetadata.PropertyName) %> <% } else { %> <% if (!String.IsNullOrEmpty(Html.Label(modelMetadata.PropertyName).ToHtmlString())) { %> <div class="editor-label"><%= Html.Label(modelMetadata.PropertyName) %></div> <% } %> <div class="editor-field"> <%= Html.Editor(modelMetadata.PropertyName) %> <%= Html.ValidationMessage(modelMetadata.PropertyName) %> </div> <% } %> And here is how I use it: @Html.EditorFor(x => x.SomeProperty, "Property") //"Property" is template above But it didn't work: labels are rendered regardless of DisplayName and editors are not rendered at all (in Watches Html.Editor(modelMetadata.PropertyName shows empty string). What am I doing wrong?

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• Why does this program take up so much memory?

  - by Adrian

  I am learning Objective-C. I am trying to release all of the memory that I use. So, I wrote a program to test if I am doing it right: #import <Foundation/Foundation.h> #define DEFAULT_NAME @"Unknown" @interface Person : NSObject { NSString *name; } @property (copy) NSString * name; @end @implementation Person @synthesize name; - (void) dealloc { [name release]; [super dealloc]; } - (id) init { if (self = [super init]) { name = DEFAULT_NAME; } return self; } @end int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; Person *person = [[Person alloc] init]; NSString *str; int i; for (i = 0; i < 1e9; i++) { str = [NSString stringWithCString: "Name" encoding: NSUTF8StringEncoding]; person.name = str; [str release]; } [person release]; [pool drain]; return 0; } I am using a mac with snow leopard. To test how much memory this is using, I open Activity Monitor at the same time that it is running. After a couple of seconds, it is using gigabytes of memory. What can I do to make it not use so much?

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• Hashtable is that fast

  - by Costa

  Hi s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]. Is the hash function of the java string, I assume the rest of languages is similar or close to this implementation. If we have hash-Table and a list of 50 elements. each element is 7 chars ABCDEF1, ABCDEF2, ABCDEF3..... ABCDEFn If each bucket of hashtable contains 5 strings (I think this function will make it one string per bucket, but let us assume it is 5). If we call col.Contains("ABCDEFn"); // will do 6 comparisons and discover the difference on the 7th. The hash-table will take around 70 operations (multiplication and additions) to get the hashcode and to compare with 5 strings in bucket. and BANG it found. For list it will take around 300 comparisons to find it. for the case that there is only 10 elements, the list will take around 70 operations but the Hashtable will take around 50 operations. and note that hashtable operations are more time consuming (it is multiplications). I conclude that HybirdDictionary in .Net probably is the best choice for that most cases that require Hashtable with unknown size, because it will let me use a list till the list becomes more than 10 elements. still need something like HashSet rather than a Dictionary of keys and values, I wonder why there is no HybirdSet!! So what do u think? Thanks

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• Most Elegant Way to write isPrime in java

  - by Anantha Kumaran

  public class Prime { public static boolean isPrime1(int n) { if (n <= 1) { return false; } if (n == 2) { return true; } for (int i = 2; i <= Math.sqrt(n) + 1; i++) { if (n % i == 0) { return false; } } return true; } public static boolean isPrime2(int n) { if (n <= 1) { return false; } if (n == 2) { return true; } if (n % 2 == 0) { return false; } for (int i = 3; i <= Math.sqrt(n) + 1; i = i + 2) { if (n % i == 0) { return false; } } return true; } } public class PrimeTest { public PrimeTest() { } @Test public void testIsPrime() throws IllegalArgumentException, IllegalAccessException, InvocationTargetException { Prime prime = new Prime(); TreeMap<Long, String> methodMap = new TreeMap<Long, String>(); for (Method method : Prime.class.getDeclaredMethods()) { long startTime = System.currentTimeMillis(); int primeCount = 0; for (int i = 0; i < 1000000; i++) { if ((Boolean) method.invoke(prime, i)) { primeCount++; } } long endTime = System.currentTimeMillis(); Assert.assertEquals(method.getName() + " failed ", 78498, primeCount); methodMap.put(endTime - startTime, method.getName()); } for (Entry<Long, String> entry : methodMap.entrySet()) { System.out.println(entry.getValue() + " " + entry.getKey() + " Milli seconds "); } } } I am trying to find the fastest way to check whether the given number is prime or not. This is what is finally came up with. Is there any better way than the second implementation(isPrime2).

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• How do I optimize this postfix expression tree for speed?

  - by Peter Stewart

  Thanks to the help I received in this post: I have a nice, concise recursive function to traverse a tree in postfix order: deque <char*> d; void Node::postfix() { if (left != __nullptr) { left->postfix(); } if (right != __nullptr) { right->postfix(); } d.push_front(cargo); return; }; This is an expression tree. The branch nodes are operators randomly selected from an array, and the leaf nodes are values or the variable 'x', also randomly selected from an array. char *values[10]={"1.0","2.0","3.0","4.0","5.0","6.0","7.0","8.0","9.0","x"}; char *ops[4]={"+","-","*","/"}; As this will be called billions of times during a run of the genetic algorithm of which it is a part, I'd like to optimize it for speed. I have a number of questions on this topic which I will ask in separate postings. The first is: how can I get access to each 'cargo' as it is found. That is: instead of pushing 'cargo' onto a deque, and then processing the deque to get the value, I'd like to start processing it right away. I don't yet know about parallel processing in c++, but this would ideally be done concurrently on two different processors. In python, I'd make the function a generator and access succeeding 'cargo's using .next(). But I'm using c++ to speed up the python implementation. I'm thinking that this kind of tree has been around for a long time, and somebody has probably optimized it already. Any Ideas? Thanks

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• Recursive QuickSort suffering a StackOverflowException -- Need fresh eyes

  - by jon

  I am working on a Recursive QuickSort method implementation in a GenericList Class. I will have a second method that accepts a compareDelegate to compare different types, but for development purposes I'm sorting a GenericList<int I am recieving stackoverflow areas in different places depending on the list size. I've been staring at and tracing through this code for hours and probably just need a fresh pair of (more experienced)eyes. Definitely wanting to learn why it is broken, not just how to fix it. public void QuickSort() { int i, j, lowPos, highPos, pivot; GenericList<T> leftList = new GenericList<T>(); GenericList<T> rightList = new GenericList<T>(); GenericList<T> tempList = new GenericList<T>(); lowPos = 1; highPos = this.Count; if (lowPos < highPos) { pivot = (lowPos + highPos) / 2; for (i = 1; i <= highPos; i++) { if (this[i].CompareTo(this[pivot]) <= 0) leftList.Add(this[i]); else rightList.Add(this[i]); } leftList.QuickSort(); rightList.QuickSort(); for(i=1;i<=leftList.Count;i++) tempList.Add(leftList[i]); for(i=1;i<=rightList.Count;i++) tempList.Add(rightList[i]); this.items = tempList.items; this.count = tempList.count; } }

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