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• python / sqlite - database locked despite large timeouts

  - by Chris Phillips

  Hi, I'm sure I'm missing something pretty obvious, but I can't for the life of me stop my pysqlite scripts crashing out with a database is locked error. I have two scripts, one to load data into the database, and one to read data out, but both will frequently, and instantly, crash depending on what the other is doing with the database at any given time. I've got the timeout on both scripts set to 30 seconds: cx = sqlite.connect("database.sql", timeout=30.0) and think I can see some evidence of the timeouts in that i get what appears to be a timing stamp (e.g 0.12343827e-06 0.1 - and how do I stop that being printed?) dumped occasionally in the middle of my curses formatted output screen, but no delay that ever gets remotely near the 30 second timeout, but still one of the other keeps crashing again and again from this. I'm running RHEL5.4 on a 64 bit 4 cpu HS21 IBM blade, and have heard some mention about issues about multi-threading and am not sure if this might be relevant. Packages in use are sqlite-3.3.6-5 and python-sqlite-1.1.7-1.2.1, and upgrading to newer versions outside of RedHat's official provisions is not a great option for me. Possible, but not desirable due to the environment in general. I have had autocommit=1 on previously on both scripts, but have since disabled on both, and am now cx.commit()ing on the inserting script and not committing on the select script. Ultimately as I only ever have one script actually making any modifications, I don't really see why this locking should ever ever happen. I have noticed that this is significantly worse over time when the database has gotten larger. It was recently at 13mb with 3 equal sized tables, which was about 1 day's worth of data. creating a new file has significantly improved this, which seems understandable, but the timeout ultimately just doesn't seem to be being obeyed. Any pointers very much appreciated. Thanks Chris

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• Ternary (and n-ary) relationships in Hibernate

  - by Bytecode Ninja

  Q 1) How can we model a ternary relationship using Hibernate? For example, how can we model the ternary relationship presented here using Hibernate (or JPA)? Ideally I prefer my model to be like this: class SaleAssistant { Long id; //... } class Customer { Long id; //... } class Product { Long id; //... } class Sale { SalesAssistant soldBy; Customer buyer; Product product; //... } Q 1.1) How can we model this variation, in which each Sale item might have many Products? class SaleAssistant { Long id; //... } class Customer { Long id; //... } class Product { Long id; //... } class Sale { SalesAssistant soldBy; Customer buyer; Set<Product> products; //... } Q 2) In general, how can we model n-ary, n = 3 relationships with Hibernate? Thanks in advance.

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• Access denied error on select into outfile using Zend

  - by Peter

  Hi, I'm trying to make a dump of a MySQL table on the server and I'm trying to do this in Zend. I have a model/mapper/dbtable structure for all my connections to my tables and I'm adding the following code to the mappers: public function dumpTable() { $db = $this->getDbTable()->getAdapter(); $name = $this->getDbTable()->info('name'); $backupFile = APPLICATION_PATH . '/backup/' . date('U') . '_' . $name . '.sql'; $query = "SELECT * INTO OUTFILE '$backupFile' FROM $name"; $db->query( $query ); } This should work peachy, I thought, but Message: Mysqli prepare error: Access denied for user 'someUser'@'localhost' (using password: YES) is what this results in. I checked the user rights for someUser and he has all the rights to the database and table in question. I've been looking around here and on the net in general and usually turning on "all" the rights for the user seems to be the solution, but not in my case (unless I'm overlooking something right now with my tired eyes + I don't want to turn on "all" on my production server). What am I doing wrong here? Or, does anybody know a more elegant way to get this done in Zend?

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• MonoRail: Testing, Route Extensions, Folder Structures

  - by Kezzer

  I've got a few questions related to the use of MonoRail Testing Does everyone tend to use NUnit for their testing? I haven't worked enough with testing to know if this is a good testing framework to use. I'm just looking to get more into testing my applications a lot more than before and wanted to know if there's any general guidelines. Are you supposed to copy the controller over to a test area and just rename it with test in the name and re-run it? How do you ensure your test project and main project coincide with one another? Is it just a case of copying everything over again or are there tools available to do it for you? Route Extensions MonoRail tends to use <action>.rails, can you omit the .rails part if you configure your routing correctly? Why does this seem to be the standard? Folder Structures I haven't found anywhere which really points out your standard folder structure. Sure, you have Controllers, Models, and Views. But your Models folder should contain your data access objects as well. I've seen some have something like -> Models -> DaoClasses -> Entities But what about custom structures used to get data out of views? And if you're using NHibernate, where's a good place to stick the mappings? I know it's entirely dependent on the developer, but I haven't really seen any standard approach. Cheers

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• Why is Available Physical Memory (dwAvailPhys) > Available Virtual Memory (dwAvailVirtual) in call G

  - by Andrew

  I am playing with an MSDN sample to do memory stress testing (see: http://msdn.microsoft.com/en-us/magazine/cc163613.aspx) and an extension of that tool that specifically eats physical memory (see http://www.donationcoder.com/Forums/bb/index.php?topic=14895.0;prev_next=next). I am obviously confused though on the differences between Virtual and Physical Memory. I thought each process has 2 GB of virtual memory (although I also read 1.5 GB because of "overhead". My understanding was that some/all/none of this virtual memory could be physical memory, and the amount of physical memory used by a process could change over time (memory could be swapped out to disc, etc.)I further thought that, in general, when you allocate memory, the operating system could use physical memory or virtual memory. From this, I conclude that dwAvailVirtual should always be equal to or greater than dwAvailPhys in the call GlobalMemoryStatus. However, I often (always?) see the opposite. What am I missing. I apologize in advance if my question is not well formed. I'm still trying to get my head around the whole memory management system in Windows. Tutorials/Explanations/Book recs are most welcome! Andrew

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• Prefuse: Reloading of XML files

  - by John

  Hello all, I am a new to the prefuse visualization toolkit and have a couple of general questions. For my purpose, I would like to perform an initial visualization using prefuse (graphview / graphml). Once rendered, upon a user click of a node, I would like to completely reload a new xml file for a new visualization. I want to do this in order to allow me to "pre-package" graphs for display. For example. If I search for Ted. I would like to have an xml file relating to Ted load and render a display. Now in the display I see that Ted has nodes associated called Bill and Joe. When I click Joe, I would like to clear the display and load an xml file associated with Joe. And so on. I have looked into loading one very large xml file containing all node and node relationship info and allowing prefuse to handle this using the hops from one level to another. However, eventually I am sure that system performance issues will arise due to the size of data. Thanks in advance for any help, John

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• Python metaclass for enforcing immutability of custom types

  - by Mark Lehmacher

  Having searched for a way to enforce immutability of custom types and not having found a satisfactory answer I came up with my own shot at a solution in form of a metaclass: class ImmutableTypeException( Exception ): pass class Immutable( type ): ''' Enforce some aspects of the immutability contract for new-style classes: - attributes must not be created, modified or deleted after object construction - immutable types must implement __eq__ and __hash__ ''' def __new__( meta, classname, bases, classDict ): instance = type.__new__( meta, classname, bases, classDict ) # Make sure __eq__ and __hash__ have been implemented by the immutable type. # In the case of __hash__ also make sure the object default implementation has been overridden. # TODO: the check for eq and hash functions could probably be done more directly and thus more efficiently # (hasattr does not seem to traverse the type hierarchy) if not '__eq__' in dir( instance ): raise ImmutableTypeException( 'Immutable types must implement __eq__.' ) if not '__hash__' in dir( instance ): raise ImmutableTypeException( 'Immutable types must implement __hash__.' ) if _methodFromObjectType( instance.__hash__ ): raise ImmutableTypeException( 'Immutable types must override object.__hash__.' ) instance.__setattr__ = _setattr instance.__delattr__ = _delattr return instance def __call__( self, *args, **kwargs ): obj = type.__call__( self, *args, **kwargs ) obj.__immutable__ = True return obj def _setattr( self, attr, value ): if '__immutable__' in self.__dict__ and self.__immutable__: raise AttributeError( "'%s' must not be modified because '%s' is immutable" % ( attr, self ) ) object.__setattr__( self, attr, value ) def _delattr( self, attr ): raise AttributeError( "'%s' must not be deleted because '%s' is immutable" % ( attr, self ) ) def _methodFromObjectType( method ): ''' Return True if the given method has been defined by object, False otherwise. ''' try: # TODO: Are we exploiting an implementation detail here? Find better solution! return isinstance( method.__objclass__, object ) except: return False However, while the general approach seems to be working rather well there are still some iffy implementation details (also see TODO comments in code): How do I check if a particular method has been implemented anywhere in the type hierarchy? How do I check which type is the origin of a method declaration (i.e. as part of which type a method has been defined)?

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• django shopping cart as a beginner

  - by Jacques Knie

  Hi, i'm quite new to django and trying to add a shopping cart to a simple webshop. What I need is a simple cart that can be filled and presents its content, which is then sent to the vendor via email. So Satchmo might be too big for this task. Therefore i chose django-cart (http://code.google.com/p/django-cart/) which causes some problems now. 1. Is django-cart the right thing? Or are there any better approaches to this task? 2. As I am a beginner even django-cart makes me struggle. I used the view and the template of the django-cart-website, but writing a form that can be used to add products to the cart took me hours. I probably need help in understanding the general layout of a shopping cart and its integration into a website. 3. Two more specific questions: Is it possible to dynamically populate a formfield in a template (e.g. with {{ object.id }})? Is django-cart able to change (update) the contents of a cart? I hope it's not too many questions at once. Thanks in advance Jacques

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• How to do Basic Authentication using FireWatir on Ubuntu Linux?

  - by lotharsmash

  Hi, I'm trying to use FireWatir (1.6.5) to access a site using Basic Authentication and I've been unable to find a solution that works on Firefox in Linux. Does FireWatir 1.6.5 support Basic Authentication on Linux? I've been searching the web for 2 days and can't get a straight answer anywhere as to how to do this. The only thread I found that seemed helpful was this one ( http://groups.google.com/group/watir-general/browse_thread/thread/d8ab9a177d282ce4/fc1bf2319fb387d8?lnk=gst&q=basic+authentication#fc1bf2319fb387d8). Aedorn Varanis says " Angrez's fork had the solution so I'm using that now. Thanks Angrez, works perfectly!", but he doesn't mention what he did to get things working. Initially I tried to bypass the authentication dialog box by using: browser.goto('http://admin:[email protected]') However, this generates a "Confirm" dialog which says: "You are about to log in to the site "172.20.1.1" with the username "admin"." [Cancel, OK] This dialog blocks, and the goto call won't return until I click "OK". Then I tried adding: browser.startClicker("ok") browser.goto('http://admin:[email protected]') But this ALSO generates the same "Confirm" dialog. I tested out the startClicker functionality using the unit test /var/ lib/gems/1.8/gems/firewatir-1.6.5/unittests/html/JavascriptClick.html and it worked fine, which makes me think that using the startClicker method is NOT the correct way to take care of the Confirm dialog. Anybody else found a way to get Basic Auth to work, or how to click the OK on the confirm dialog? I'm at my wits end...

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• Need help with creating complex UITableView items (like the Settings app).

  - by Craig

  I hoping to create a custom Settings view, similar to the Settings application, but with more control over the UI and access to some settings (i need to lock some of the settings). Obvously, there are a variety of UI elements mixed in each row of application's UITableView. For example, the 'Airplane Mode' setting shows a UISwitch, while the 'Wi-Fi' setting has a text value adjacent to the disclosure symbol (''). Further complicating matters, is the grouping of these settings. I have some general questions about the approach I should take: Seems like i need to save the name of the setting, its current value, its grouping, and the type of UI element(s) needed to modify its value. i would like to make use [NSUserDefault standardUserDefaults], but not have these settings appear in the Settings application. I'm guessing that I will need to create my own settings-persistence class. is it better to build each one of these complex UI-element combinations in code or to create a series of custom views based on the UITableViewCell and load the appropriate one? i'm guessing the latter. some of the setting require that i load another view to select its value. assuming that the application is based on the Utility pattern, should the SettingsView manage the navigation stack, rather than having the app delegate do so? Thanks for your time and comments. Craig Buchanan

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• FluentNHibernate SQLite configuration exception - after switching to .net4

  - by stiank81

  I get an exception thrown when trying to use Fluent to configure my NHibernate connection to SQLite. The code I use to configure is as follows: var cfg = Fluently.Configure(). Database(SQLiteConfiguration.Standard.ShowSql().UsingFile("MyDb.db")). Mappings(m => m.FluentMappings.AddFromAssemblyOf<MappingsPersistenceModel>()); _sessionFactory = cfg.BuildSessionFactory(); A HibernateException is thrown when BuildSessionFactory() is called, saying: Could not create the driver from NHibernate.Driver.SQLite20Driver, NHibernate, Version=2.1.2.4000, Culture=neutral, PublicKeyToken=aa95f207798dfdb4. It has an InnerException: Exception has been thrown by the target of an invocation. Which again has an InnerException: The IDbCommand and IDbConnection implementation in the assembly System.Data.SQLite could not be found. Ensure that the assembly System.Data.SQLite is located in the application directory or in the Global Assembly Cache. If the assembly is in the GAC, use element in the application configuration file to specify the full name of the assembly. Now - to me it sounds like it doesn't find System.Data.SQLite.dll, but I can't understand this. Everywhere this is referenced I have "Copy Local", and I have verified that it is in every build folder for projects using SQLite. I have also copied it manually to every Debug folder of the solution - without luck. What can be causing this? Any ideas? My suspicion is that it is related to .Net4 somehow. The reason is that it worked just fine when I used .Net3.5, and then I changed to .Net4, and the problem started. You can also check out this other question for a more general approach towards Fluent-.Net4 compatibility.

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• Retrieve 2 last posts for each category.

  - by Savageman

  Hello, Lets say I have 2 tables: blog_posts and categories. Each blog post belongs to only ONE category, so there is basically a foreign key between the 2 tables here. I would like to retrieve the 2 lasts posts from each category, is it possible to achieve this in a single request? GROUP BY would group everything and leave me with only one row in each category. But I want 2 of them. It would be easy to perform 1 + N query (N = number of category). First retrieve the categories. And then retrieve 2 posts from each category. I believe it would also be quite easy to perform M queries (M = number of posts I want from each category). First query selects the first post for each category (with a group by). Second query retrieves the second post for each category. etc. I'm just wondering if someone has a better solution for this. I don't really mind doing 1+N queries for that, but for curiosity and general SQL knowledge, it would be appreciated! Thanks in advance to whom can help me with this.

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• iPhone offline reading

  - by Andy

  Hi, first of all - I am quite new to iPhone App development (3 months). I am working for a software company that offers a content management system. Our customers are for the main part publishing houses for magazines. They use our software to write articles to their homepages. Now we want to offer iPhone Applications to go with our cms. What I have accomplished so far is an RSS reader that shows newly published articles in a list view. The user selects one article and is redirected to a specially formatted detail view of this article. The next step is to add offline reading capabilities. I have searched the internet up and down but couldn't find anything like a best practice for that. I get it that there are two possibilities in general: Store the contents of the uiwebview locally on the iPhone/iPad (including css, images, js and so on). There would be the need to rework the basic html to use the downloaded css, images and js. Also I would have to somehow edit hyperlinks to following pages in multipage articles - Sounds like a lot of work ;) Create a PDF on the server side and download that to the mobile device. Rework the RSS Source to point to the locally saved pdf instead of the website on the server. My question is - what is the better way to go? Are there any downsides for either of the possibilities? Are there other (simple ;)) ways to implement offline reading features? Are there possibly any howto's that I could've missed? Thanks y'all!

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• Database warehoue design: fact tables and dimension tables

  - by morpheous

  I am building a poor man's data warehouse using a RDBMS. I have identified the key 'attributes' to be recorded as: sex (true/false) demographic classification (A, B, C etc) place of birth date of birth weight (recorded daily): The fact that is being recorded My requirements are to be able to run 'OLAP' queries that allow me to: 'slice and dice' 'drill up/down' the data and generally, be able to view the data from different perspectives After reading up on this topic area, the general consensus seems to be that this is best implemented using dimension tables rather than normalized tables. Assuming that this assertion is true (i.e. the solution is best implemented using fact and dimension tables), I would like to see some help in the design of these tables. 'Natural' (or obvious) dimensions are: Date dimension Geographical location Which have hierarchical attributes. However, I am struggling with how to model the following fields: sex (true/false) demographic classification (A, B, C etc) The reason I am struggling with these fields is that: They have no obvious hierarchical attributes which will aid aggregation (AFAIA) - which suggest they should be in a fact table They are mostly static or very rarely change - which suggests they should be in a dimension table. Maybe the heuristic I am using above is too crude? I will give some examples on the type of analysis I would like to carryout on the data warehouse - hopefully that will clarify things further. I would like to aggregate and analyze the data by sex and demographic classification - e.g. answer questions like: How does male and female weights compare across different demographic classifications? Which demographic classification (male AND female), show the most increase in weight this quarter. etc. Can anyone clarify whether sex and demographic classification are part of the fact table, or whether they are (as I suspect) dimension tables.? Also assuming they are dimension tables, could someone elaborate on the table structures (i.e. the fields)? The 'obvious' schema: CREATE TABLE sex_type (is_male int); CREATE TABLE demographic_category (id int, name varchar(4)); may not be the correct one.

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• .NET WF4: Should it be in the middle of everything?

  - by stimpy77

  I am aware that WF4 (Windows Workflow 4.0, part of .NET 4.0) is a significant rework and redesign of WF3, where much of what made WF3 a poor technology choice has been cleaned up in WF4. For example, as far as I can tell, WF4 (Windows Workflow 4.0) activities are more or less testable with [TestMethod] and mocking. This among other things, like improved performance, has grabbed my attention about the technology again, whereas I had previously pooh-poohed WF3. I'm working on a new architecture for essentially an n-tier collaborative application (not enterprise-class, just a smallish project with potential to grow significantly) where I'm already trying to discipline myself to use IoC and, to some extent, TDD, and I'm wondering, in general terms, whether it is wiser to just hand-code workflow logic or if I should delve into learning and integrating WF4 so that WF becomes literally the controller of the entire application, i.e. the practical C in "MVC" (not ASP.NET MVC but rather the pattern). So should workflow activities in WF4 be the primary controller for a highly expandable/growable web-based collaborative application? Or am I asking entirely the wrong question? This is a vague question, I'm sure, so abstract answers are as welcome as specific ones.

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• Reset.css and then a Set.css

  - by Sixfoot Studio

  I have, for a while now been using a reset.css file to reset everything before I start laying out my html designs. The reset is great in that it allows one to better control attributes such as margins, padding, line-height etc for all browsers. In essence the flatliner of css files. Now to get the heart beating again, I need a "set.css" file. So what I have done is created an Html file with all the possible elements on the page to then go and set the padding, margins etc of the h1, h2, p, td etc. I need some help with this as I am not sure what the defaults normally are. I had a look at the Firefox default css file that's used to generate all these attributes on a raw html file but it doesn't cover all the scenarios I could come up with when developing a site. Here's an example of the set.html file (a work in progress) which can be used as a lorem ipsum filler to add to your first page in a cms and then to style with a "set.css" file http://www.sixfoot.co.za/labs/Html-Css/set.html I'd appreciate it if someone knows if something like a set.css file exists or if someone could tell me what the general padding and margins are in cases like this when you have reset the css. Cheers, James

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• Django Models / SQLAlchemy are bloated! Any truly Pythonic DB models out there?

  - by Luke Stanley

  "Make things as simple as possible, but no simpler." Can we find the solution/s that fix the Python database world? from someAmazingDB import * class Task (model): title = '' isDone = False db.taskList = [] #or db.taskList = expandableTypeCollection(Task) #not sure what this syntax would be db['taskList'].append(Task(title='Beat old sql interfaces',done=False)) db.taskList.append(Task('Illustrate different syntax modes',True)) #at this point it should autosave #we should be able to reload the console and access like: >> from someAmazingDB import * >> print 'Done tasks:' >> for task in db.taskList: >> if task.done: >> print task 'Illustrate different syntax modes' I'm a fan of Python, webPy and Cherry Py, and KISS in general. We're talking automatic Python to SQL type translation or NoSQL. We don't have to totally be SQL compatible! Just a scalable subset or ignore it! Re:model changes, it's ok to ask the developer when they try to change it or have a set of sensible defaults. Here is the challenge: The above code should work with very little modification or thinking required. Why must we put up with compromise when we know better? It's 2010, we should be able to code scalable, simple databases in our sleep. If you think this is important, please upvote!

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• Optimal Serialization of Primitive Types

  - by Greg Dean

  We are beginning to roll out more and more WAN deployments of our product (.Net fat client w/ IIS hosted Remoting backend). Because of this we are trying to reduce the size of the data on the wire. We have overridden the default serialization by implementing ISerializable (similar to this), we are seeing anywhere from 12% to 50% gains. Most of our efforts focus on optimizing arrays of primitive types. I would like to know if anyone knows of any fancy way of serializing primitive types, beyond the obvious? For example today we serialize an array of ints as follows: [4-bytes (array length)][4-bytes][4-bytes] Can anyone do significantly better? The most obvious example of a significant improvement, for boolean arrays, is putting 8 bools in each byte, which we already do. Note: Saving 7 bits per bool may seem like a waste of time, but when you are dealing with large magnitudes of data (which we are), it adds up very fast. Note: We want to avoid general compression algorithms because of the latency associated with it. Remoting only supports buffered requests/responses(no chunked encoding). I realize there is a fine line between compression and optimal serialization, but our tests indicate we can afford very specific serialization optimizations at very little cost in latency. Whereas reprocessing the entire buffered response into new compressed buffer is too expensive.

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• Determing size of visible area of a ScrollingGraphicalViewer in Eclipse GEF

  - by Simon

  I am attempting to create a simple application using Eclipse GEF that displays a diagram inside a ScrollingGraphicalViewer. On start-up I want the diagram to be centered inside the viewer (imagine a star layout where the center of the star is in the center of the view). Here are what I think are the relevant code sections: My view: public class View extends ViewPart { public static final String ID = "GEFProofOfConcept.view"; ... public void createPartControl(Composite parent) { viewer.createControl(parent); viewer.setRootEditPart(rootEditPart); viewer.setEditPartFactory(editPartFactory); viewer.setContents(DummyModelCreator.generateModel()); } The edit part code: @Override protected void refreshVisuals() { Project project = (Project) getModel(); // This is where the actual drawing is done, // Simply a rectangle with text Rectangle bounds = new Rectangle(50, 50, 75, 50); getFigure().setBounds(bounds); Label label = new Label(project.getName()); projectFont = new Font(null, "Arial", 6, SWT.NORMAL); label.setFont(projectFont); label.setTextAlignment(PositionConstants.CENTER); label.setBounds(bounds.crop(IFigure.NO_INSETS)); getFigure().add(label); setLocation(); } private void setLocation() { Project project = (Project) getModel(); if (project.isRoot()) { // Place in centre of the layout Point centrePoint = new Point(0, 0); // This is where I need the center of the view getFigure().setLocation(centrePoint); } else { ... } } And the parent of the above edit part: public class ProjectDependencyModelEditPart extends AbstractGraphicalEditPart { @Override protected IFigure createFigure() { Figure f = new FreeformLayer(); f.setLayoutManager(new XYLayout()); return f; } ... Alternative solutions to the problem also welcome, I am most certainly a GEF (and Eclipse in general) newbie.

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• How do I detect if there is already a similar document stored in Lucene index.

  - by Jenea

  Hi. I need to exclude duplicates in my database. The problem is that duplicates are not considered exact match but rather similar documents. For this purpose I decided to use FuzzyQuery like follows: var fuzzyQuery = new global::Lucene.Net.Search.FuzzyQuery( new Term("text", queryText), 0.8f, 0); hits = _searcher.Search(query); The idea was to set the minimal similarity to 0.8 (that I think is high enough) so only similar documents will be found excluding those that are not sufficiently similar. To test this code I decided to see if it finds already existing document. To the variable queryText was assigned a value that is stored in the index. The code from above found nothing, in other words it doesn't detect even exact match. Index was build by this code: doc.Add(new global::Lucene.Net.Documents.Field( "text", text, global::Lucene.Net.Documents.Field.Store.YES, global::Lucene.Net.Documents.Field.Index.TOKENIZED, global::Lucene.Net.Documents.Field.TermVector.WITH_POSITIONS_OFFSETS)); I followed recomendations from bellow and the results are: TermQuery doesn't return any result. Query contructed with var _analyzer = new RussianAnalyzer(); var parser = new global::Lucene.Net.QueryParsers .QueryParser("text", _analyzer); var query = parser.Parse(queryText); var _searcher = new IndexSearcher (Settings.General.Default.LuceneIndexDirectoryPath); var hits = _searcher.Search(query); Returns several results with the maximum score the document that has exact match and other several documents that have similar content.

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• What is the relationship between Turing Machine & Modern Computer ? [closed]

  - by smwikipedia

  I heard a lot that modern computers are based on Turing machine. I just cannot build a bridge between a conceptual Turing Machine and a modern computer. Could someone help me build this bridge? Below is my current understanding. I think the computer is a big general-purpose Turing machine. Each program we write is a small specific-purpose Turing machine. The classical Turing machine do its job based on the input and its current state inside and so do our programs. Let's take a running program (a process) as an example. We know that in the process's address space, there's areas for stack, heap, and code. A classical Turing machine doesn't have the ability to remember many things, so we borrow the concept of stack from the push-down automaton. The heap and stack areas contains the state of our specific-purpose Turing machine (our program). The code area represents the logic of this small Turing machine. And various I/O devices supply input to this Turing machine.

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• When optimizing database queries, what exactly is the relationship between number of queries and siz

  - by williamjones

  To optimize application speed, everyone always advises to minimize the number of queries an application makes to the database, consolidating them into fewer queries that retrieve more wherever possible. However, this also always comes with the caution that data transferred is still data transferred, and just because you are making fewer queries doesn't make the data transferred free. I'm in a situation where I can over-include on the query in order to cut down the number of queries, and simply remove the unwanted data in the application code. Is there any type of a rule of thumb on how much of a cost there is to each query, to know when to optimize number of queries versus size of queries? I've tried to Google for objective performance analysis data, but surprisingly haven't been able to find anything like that. Clearly this relationship will change for factors such as when the database grows in size, making this somewhat individualized, but surely this is not so individualized that a broad sense of the landscape can't be drawn out? I'm looking for general answers, but for what it's worth, I'm running an application on Heroku.com, which means Ruby on Rails with a Postgres database.

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• Implementing crossover in genetic programming

  - by Name

  Hi, I'm writing a genetic programming (GP) system (in C but that's a minor detail). I've read a lot of the literature (Koza, Poli, Langdon, Banzhaf, Brameier, et al) but there are some implementation details I've never seen explained. For example: I'm using a steady state population rather than a generational approach, primarily to use all of the computer's memory rather than reserve half for the interim population. Q1. In GP, as opposed to GA, when you perform crossover you select two parents but do you create one child or two, or is that a free choice you have? Q2. In steady state GP, as opposed to a generational system, what members of the population do the children created by crossover replace? This is what I haven't seen discussed. Is it the two parents, or is it two other, randomly-selected members? I can understand if it's the latter, and that you might use negative tournament selection to choose members to replace, but would that not create premature convergence? (After a crossover event the population contains the two original parents plus two children of those parents, and two other random members get removed. Elitism is inherent.) Q3. Is there a Web forum or mailing list focused on GP? Oddly I haven't found one. Yahoo's GP group is used almost exclusively for announcements, the Poli/Langdon Field Guide forum is almost silent, and GP discussions on general/game programming sites like gamedev.net are very basic. Thanks for any help you can provide!

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• What is the correct date format for a Date column in YUI DataTable ?

  - by giulio

  I have produced a data table. All the columns are sortable. It has a date in one column which I formatted dd/mm/yyyy hh:mm:ss . This is different from the default format as defined in the doco, but I should be able to define my own format for non-american formats. (See below) The DataTable class provides a set of built-in static functions to format certain well-known types of data. In your Column definition, if you set a Column's formatter to YAHOO.widget.DataTable.formatDate, that function will render data of type Date with the default syntax of "MM/DD/YYYY". If you would like to bypass a built-in formatter in favor of your own, you can point a Column's formatter to a custom function that you define. The table is generated from HTML Markup, so the data is held within "" tags. This gives me some more clues about compatible string dates for javascript: In general, the RecordSet expects to hold data in native JavaScript types. For instance, a date is expected to be a JavaScript Date instance, not a string like "4/26/2005" in order to sort properly. Converting data types as data comes into your RecordSet is enabled through the parser property in the fields array of your DataSource's responseSchema I suspect that the I'm missing something in the date format. So what is an acceptable string date for javascript, that Yui dataTable will recognise, given that I want format it as "dd/mm/yyyy hh:mm:ss" ?

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• Quadratic Bezier Curve: Calculate Tangent

  - by stefan.at.wpf

  I have a quadratic bezier curve and I want to calculate the slope of the tangent in a given point. For example, let it be the middlepoint of the quadratic bezier curve, therefore t=0.5 (please see the link below for a picture of this). I've calculated the first derivative of the formula for the quadratic bezier curve; however I get 400 as value for the slope, though it should be 0. Maybe I'm using the first derivative in a wrong way? I know I could also calculate the tangents using trigonometric functions; however I'd like to do it using the first derivative, shouldn't this be possible? Thanks for any hint! For clarification / please note: I'm interested in a general way to get the slope in a arbitrary given point on a quadratic bezier curve, not only to get the tangent in the start- and end point. A picture of my problem including the text above: http://cid-0432ee4cfe9c26a0.skydrive.live.com/self.aspx/%c3%96ffentlich/Quadratic%20Bezier%20Curve.pdf Thank you very much for any hint!

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