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• PostGres - run a query in batches?

  - by CaffeineIV

  Is it possible to loop through a query so that if (for example) 500,000 rows are found, it'll return results for the first 10,000 and then rerun the query again? So, what I want to do is run a query and build an array, like this: $result = pg_query("SELECT * FROM myTable"); $i = 0; while($row = pg_fetch_array($result) ) { $myArray[$i]['id'] = $row['id']; $myArray[$i]['name'] = $row['name']; $i++; } But, I know that there will be several hundred thousand rows, so I wanted to do it in batches of like 10,000... 1- 9,999 and then 10,000 - 10,999 etc... The reason why is because I keep getting this error: Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 3 bytes) Which, incidentally, I don't understand how 3 bytes could exhaust 512M... So, if that's something that I can just change, that'd be great, although, still might be better to do this in batches?

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• Django complex queries

  - by Josh K

  I need to craft a filter for an object that checks date ranges. Right now I'm performing a very inefficient loop which checks all the objects. I would like to simplify this to a database call. The logic is you have a start and an end date objects. I need to check if the start OR the end is within the range of an appointment. if (start >= appointment.start && start < appointment.end) || (end > appointment.start && end <= appointment.end) I could do this in SQL, but I'm not as familiar with the Django model structure for more complex queries.

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• Effecient data structure design

  - by Sway

  Hi there, I need to match a series of user inputed words against a large dictionary of words (to ensure the entered value exists). So if the user entered: "orange" it should match an entry "orange' in the dictionary. Now the catch is that the user can also enter a wildcard or series of wildcard characters like say "or__ge" which would also match "orange" The key requirements are: * this should be as fast as possible. * use the smallest amount of memory to achieve it. If the size of the word list was small I could use a string containing all the words and use regular expressions. however given that the word list could contain potentially hundreds of thousands of enteries I'm assuming this wouldn't work. So is some sort of 'tree' be the way to go for this...? Any thoughts or suggestions on this would be totally appreciated! Thanks in advance, Matt

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• Of Models / Entities and N-tier applications

  - by Jonn

  I've only discovered a month ago the folly of directly accessing entities / models from the data access layer of an n-tier app. After reading about ViewModels while studying ASP.NET MVC, I've come to understand that to make a truly extensible application the model that the UI layer interacts with must be different from the one that the Data Access layer has access to. But what about the Business layer? Should I have a different set of models for my business layer as well? For true separation of concern, should I have a specific set of models that are relevant only to my business layer so as not to mess around with any entities (possibly generated by for example, the entity framework, or EJB) in the DAL or would that be overkill?

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• Measuring debug vs release of ASP.NET applications

  - by Alex Angas

  A question at work came up about building ASP.NET applications in release vs debug mode. When researching further (particularly on SO), general advice is that setting <compilation debug="true"> in web.config has a much bigger impact. Has anyone done any testing to get some actual numbers about this? Here's the sort of information I'm looking for (which may give away my experience with testing such things): Execution time | Debug build | Release build -------------------+---------------+--------------- Debug web.config | average 1 | average 2 Retail web.config | average 3 | average 4 Max memory usage | Debug build | Release build -------------------+---------------+--------------- Debug web.config | average 1 | average 2 Retail web.config | average 3 | average 4 Output file size | Debug build | Release build -------------------+---------------+--------------- | size 1 | size 2

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• Why freed struct in C still has data?

  - by kliketa

  When I run this code: #include <stdio.h> typedef struct _Food { char name [128]; } Food; int main (int argc, char **argv) { Food *food; food = (Food*) malloc (sizeof (Food)); snprintf (food->name, 128, "%s", "Corn"); free (food); printf ("%d\n", sizeof *food); printf ("%s\n", food->name); } I still get 128 Corn although I have freed food. Why is this? Is memory really freed?

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• How return a std::string from C's "getcwd" function

  - by rubenvb

  Sorry to keep hammering on this, but I'm trying to learn :). Is this any good? And yes, I care about memory leaks. I can't find a decent way of preallocating the char*, because there simply seems to be no cross-platform way. const string getcwd() { char* a_cwd = getcwd(NULL,0); string s_cwd(a_cwd); free(a_cwd); return s_cwd; } UPDATE2: without Boost or Qt, the most common stuff can get long-winded (see accepted answer)

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• How to properly reserve identity values for usage in a database?

  - by esac

  We have some code in which we need to maintain our own identity (PK) column in SQL. We have a table in which we bulk insert data, but we add data to related tables before the bulk insert is done, thus we can not use an IDENTITY column and find out the value up front. The current code is selecting the MAX value of the field and incrementing it by 1. Although there is a highly unlikely chance that two instances of our application will be running at the same time, it is still not thread-safe (not to mention that it goes to the database everytime). I am using the ADO.net entity model. How would I go about 'reserving' a range of id's to use, and when that range runs out, grab a new block to use, and guarantee that the same range will not be used.

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• General approach for posting business logic status messages to UI?

  - by generalt

  Hello all. I have been struggling with this question for awhile now, and I haven't reached a conclusion. I'm not typically a UI programmer, so forgive the noobishness. I'm writing a typical application with a UI layer (WPF) and a business layer. I want to post status messages to the UI from the business layer (perhaps deep within the business layer), but I don't want the business layer to have any knowledge of the UI. Is there a generally accepted pattern for this? I was thinking to have a message queue of some sort to which the business layer posts status messages, and have the view model of the UI subscribe to that queue and intercept messages from the queue and route them to the UI. Is that a good approach? Is there somewhere else I should start? Thank you.

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• If free() knows the length of my array, why can't I ask for it in my own code?

  - by Chris Cooper

  I know that it's a common convention to pass the length of dynamically allocated arrays to functions that manipulate them: void initializeAndFree(int* anArray, int length); int main(){ int arrayLength = 0; scanf("%d", &arrayLength); int* myArray = (int*)malloc(sizeof(int)*arrayLength); initializeAndFree(myArray, arrayLength); } void initializeAndFree(int* anArray, int length){ int i = 0; for (i = 0; i < length; i++) { anArray[i] = 0; } free(anArray); } but if there's no way for me to get the length of the allocated memory from a pointer, how does free() "automagically" know what to deallocate? Why can't I get in on the magic, as a C programmer? Where does free() get its free (har-har) knowledge from?

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• Can I add columns in a QListView in Qt ??

  - by Vic.

  Can I add columns in a QListView object?? here's something I found here: model->setHeaderData( 0, Qt::Horizontal, "numéro" ); model->setHeaderData( 1, Qt::Horizontal, "prénom" ); model->setHeaderData( 2, Qt::Horizontal, "nom" ); //... model->setData( model->index( line, 0 ), contact->num(), Qt::DisplayRole ); model->setData( model->index( line, 1 ), contact->prenom(), Qt::DisplayRole ); model->setData( model->index( line, 2 ), contact->nom(), Qt::DisplayRole ); Since I'm using Qt Creator 2.0.1, I figured my model would be: ui->ObjectName->model() The application builds successfully but I get a: "The program has unexpectedly finished." at runtime. Any Ideas ? Thanks.

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• Adding a Categorylist to all pages with MVC4

  - by Sidriel

  I'm new with MVC4 and just MVC. I have a homecontroller and a categorycontroller. The categorycontroller sends data from the model to the categoryIndex view. That's works fine. But now I want to add the categorylist on all the available controllers. I already fixed this to add in all classes return view(db.categorys.ToList()); and add categoryIndex to the shared folder. In _layout.cshtml I'm adding a@RenderPage("~/")` and this works. But when I have to pass more than only the (db.categorys.ToList()) in the return it goes wrong. How can I fixed this problem. How do I add the categorylist too every controller and page properly?

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• in java, which is better - three arrays of booleans or 1 array of bytes?

  - by joe_shmoe

  I know the question sounds silly, but consider this: I have an array of items and a labelling algorithm. at any point the item is in one of three states. The current version holds these states in a byte array, where 0, 1 and 2 represent the three states. alternatively, I could have three arrays of boolean - one for each state. which is better (consumes less memory) depends on how jvm (sun's version) stores the arrays - is a boolean represented by 1 bit? (p.s. don't start with all that "this is not the way OO/Java works" - I know, but here performance comes in front. plus the algorithm is simple and perfectly readable even in such form). Thanks a lot

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• Freeing a character pointer returns error

  - by Kraffs

  I'm trying to free a character pointer after having used it but it returns a strange error. The error says: "_CrtDbgREport: String too long or IO Error" The debugger itself returns no errors while compiling. The code currently looks like this: void RespondToUser(SOCKET client, SOCKET server) { char buffer[80]; char *temp = malloc(_scprintf("HTTP/1.1 200 OK\r\n%s\r\nServer: %s\r\nConnection: close\r\n\r\nHi!", buffer, SERVER_NAME)); sprintf(temp, "HTTP/1.1 200 OK\r\n%s\r\nServer: %s\r\nConnection: close\r\n\r\nHi!", buffer, SERVER_NAME); send(client, temp, strlen(temp), 0); closesocket(client); free(temp); ListenToUsers(server); } The problem only occurs when I try to free the temp pointer from the memory and not otherwise. What might be causing this?

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• Rails architecture questions

  - by justinbach

  I'm building a Rails site that, among other things, allows users to build their own recipe repository. Recipes are entered either manually or via a link to another site (think epicurious, cooks.com, etc). I'm writing scripts that will scrape a recipe from these sites given a link from a user, and so far (legal issues notwithstanding) that part isn't giving me any trouble. However, I'm not sure where to put the code that I'm writing for these scraper scripts. My first thought was to put it in the recipes model, but it seems a bit too involved to go there; would a library or a helper be more appropriate? Also, as I mentioned, I'm building several different scrapers for different food websites. It seems to me that the elegant way to do this would be to define an interface (or abstract base class) that determines a set of methods for constructing a recipe object given a link, but I'm not sure what the best approach would be here, either. How might I build out these OO relationships, and where should the code go?

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• std::string constructor corrupts pointer

  - by computergeek6

  I have an Entity class, which contains 3 pointers: m_rigidBody, m_entity, and m_parent. Somewhere in Entity::setModel(std::string model), it's crashing. Apparently, this is caused by bad data in m_entity. The weird thing is that I nulled it in the constructor and haven't touched it since then. I debugged it and put a watchpoint on it, and it comes up that the m_entity member is being changed in the constructor for std::string that's being called while converting a const char* into an std::string for the setModel call. I'm running on a Mac, if that helps (I think I remember some problem with std::string on the Mac). Any ideas about what's going on?

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• Rails: How do I run a before_save only if certain conditions are met?

  - by Shpigford

  I have a before_save method that I call that renames an uploaded image. before_save :randomize_file_name def randomize_file_name extension = File.extname(screen_file_name).downcase key = ActiveSupport::SecureRandom.hex(8) self.screen.instance_write(:file_name, "#{key}#{extension}") end That method is part of my Item model. That works great when I create a new item or need to update the image associated with an item...but the problem is that if I need to update an item but NOT the image, the randomize_file_name method still gets run and renames the file in the database (though not the file itself, obviously). So, I'm thinking I need to figure out a way to only run randomize_file_name if a file is included in the form submission...but I'm not sure how to pull that off.

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• Allocated Private Bytes keeps going up in one computer but not the other

  - by Jacob

  OK, this may sound weird, but here goes. There are 2 computers, A (Pentium D) and B (Quad Core) with almost the same amount of RAM. If I run the same code on both computers, the allocated private bytes in A never goes down resulting in a crash later on. In B it looks like the private bytes is constantly deallocated and everything looks fine. In both computers, the working set is deallocated and allocated similarly. Could this be an issue with manifests or DLLs (system)? I'm clueless. Note: I observed the utilized memory with Process Explorer.

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• Garbage Collection in Java

  - by simion

  On the slides I am revising from it says the following: Live objects can be identified either by maintaining a count of the number of references to each object, or by tracing chains of references from the roots. Reference counting is expensive – it needs action every time a reference changes and it doesn’t spot cyclical structures, but it can reclaim space incrementally. Tracing involves identifying live objects only when you need to reclaim space – moving the cost from general access to the time at which the GC runs, typically only when you are out of memory. I understand the principles of why reference counting is expensive but do not understand what "doesn’t spot cyclical structures, but it can reclaim space incrementally." means. Could anyone help me out a little bit please? Thanks

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• Recommendation for tool/framework that follows the naked objects pattern?

  - by Marcus Munzert

  I am searching for a tool/framework that follows the naked objects pattern and is written in Java. I know about tools like for instance JMatter, Naked Objects and Domain Object Explorer. That's not exactly what I am searching for, though. Open source would be great, but doesn't need to be. My intention is to use that tool/framework for the purpose of model-driven software development to do the modeling part. Ideally, such a tool/framework would provide the option to use JPA to store/load objects.

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• release viewcontroller after presenting modally

  - by Jonathan

  I was watching CS193P Stanford course on Itunes, and in one of the lectures a demo was given and There it was said you could present the viewcontroller modally and then release it. Roughly like this (I know this isn't perfect but I'm on my PC atm) [self.view presentcontentmodally:myVC] [myVC release]; However this seems to produce problems. If I put a NSLog(@"%d", [myVC retainCount]) between those two lines then it returns 2 implying it is ok to release. However when I dismiss the myVC the app crashes. Nothing in the NSlog and the debugger won't show where it stopped. But I used malloc-history or something that some blog said would help. And found that it was the myVC. So should I be releasing myVC? (also when the modalVC has been dissmissed should the app's memory usuage go back to before the modalVC was presented?)

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• Conditional value for ActiveRecord create method only

  - by Steve Wright

  I have a form where I have an administrator creating new users. The form uses the User model I created (login, password, first_name, etc...). For the last field on the form, I want to have a checkbox that doesn't need to be stored as part of the User record, but it is needed for the controller. This will control whether or not the newly created user will receive a welcome email or not. def create @user = User.new(params[:user]) if @user.save if @user.send_welcome_email UserMailer.welcome_email(@user).deliver end redirect_to(admin_users_url, :notice => "User #{@user.name} was successfully created.") else render :action => "new" end end In my view (haml) I am trying to access it like this: %p Send Welcome Email? = f.check_box :send_welcome_email I tried to make this an attr_accessible: :send_welcome_email but the controller does not recognize it. I get an undefined method 'send_welcome_email' for #&lt;User:0x00000100d080a8&gt; I would like it to look like this: What is the best way to get this working:

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• What is the fastest way to write hundreds of files to disk using C#?

  - by Ehsan

  My program should write hundreds of files to disk, received by external resources (network) each file is a simple document that I'm currently store it with the name of GUID in a specific folder but creating hundred files, writing, closing is a lengthy process. Is there any better way to store these amount of files to disk? I've come to a solution, but I don't know if it is the best. First, I create 2 files, one of them is like allocation table and the second one is a huge file storing all the content of my documents. But reading from this file would be a nightmare; maybe a memory-mapped file technique could help. Could working with 30GB or more create a problem?

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• Force an object to be allocated on the heap

  - by Warren Seine

  A C++ class I'm writing uses shared_from_this() to return a valid boost::shared_ptr<>. Besides, I don't want to manage memory for this kind of object. At the moment, I'm not restricting the way the user allocates the object, which causes an error if shared_from_this() is called on a stack-allocated object. I'd like to force the object to be allocated with new and managed by a smart pointer, no matter how the user declares it. I thought it could be done through a proxy or an overloaded new operator, but I can't find a proper way of doing that. Is there a common design pattern for such usage? If it's not possible, how can I test it at compile time?

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• In what circumstances can large pages produce a speedup ?

  - by timday

  Modern x86 CPUs have the ability to support larger page sizes than the legacy 4K (ie 2MB or 4MB), and there are OS facilities (Linux, Windows) to access this functionality. The Microsoft link above states large pages "increase the efficiency of the translation buffer, which can increase performance for frequently accessed memory". Which isn't very helpful in predicting whether large pages will improve any given situation. I'm interested in concrete, preferably quantified, examples of where moving some program logic (or a whole application) to use huge pages has resulted in some performance improvement. Anyone got any success stories ? There's one particular case I know of myself: using huge pages can dramatically reduce the time needed to fork a large process (presumably as the number of TLB records needing copying is reduced by a factor on the order of 1000). I'm interested in whether huge pages can also benefit more mundane applications though.

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