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• MYSQL - SELECT ALL FROM TABLE if...

  - by hornetbzz

  Hello I have a (nice) mysql table built like this : Fields Datas id (pk) 1 2 3 4 5 6 master_id 1000 1000 1000 2000 2000 2000 ... master_name home home home shop shop shop ... type_data value common client value common client ... param_a foo_a 1 0 bar_a 0 1 ... param_b foo_b 1 0 bar_b 1 0 ... param_c foo_c 0 1 bar_c 0 1 ... ... ... ... ... ... ... ... ... All these datas are embed in a single table. Each datas are dispatched on 3 "columns" set (1 for the values, 1 for identifying if these are common values and one for identifying client values). It's not the best I got but many other scripts depends on this structure. I'd need sthg like this: SELECT parameters name (eg param_a, param_b..) and their values (eg foo_a, foo_b..) WHEN master_id=? AND type_data=(common or client) (eg for values=1 on the 2nd column) . in order to get the parameters hash like param_a => foo_a param_b => foo_b param_c => foo_c ... I could not succeed in self joining on the same table till now but I guess it should be feasible. (I'd like to avoid to do several queries) Thx in advance

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• Instanced drawing with OpenGL ES 2.0

  - by Mårten Wikström

  In short: Is it possible to use the gl_InstanceID built-in variable in OpenGL ES 2.0? And, if so, how? Some more info: I want to draw multiple instances of an object using glDrawArraysInstanced and gl_InstanceID, and I want my application to run on multiple platforms, including iOS. The specification clearly says that these features require ES 3.0. According to the iOS Device Compatibility Reference ES 3.0 is only available on a few devices (those based on the A7 GPU; so iPhone 5s, but not on iPhone 5 or earlier). So my first assumption was that I needed to avoid using instanced drawing on older iOS devices. However, further down in the compatibility reference document it says that the EXT_draw_instanced extension is supported for all SGX Series 5 processors (that includes iPhone 5 and 4s). This makes me think that I could indeed use instanced drawing on older iOS devices too, by looking up and using the appropriate extension function (EXT or ARB) for glDrawArraysInstanced. I'm currently just running some test code using SDL and GLEW on Windows so I haven't tested anything on iOS yet. However, in my current setup I'm having trouble using the gl_InstanceID built-in variable in a vertex shader. I'm getting the following error message: 'gl_InstanceID' : variable is not available in current GLSL version Enabling the "draw_instanced" extension in GLSL has no effect: #extension GL_ARB_draw_instanced : enable #extension GL_EXT_draw_instanced : enable The error goes away when I specifically declare that I need ES 3.0 (GLSL 300 ES): #version 300 es Although that seem to work fine on my Windows desktop machine in an ES 2.0 context I doubt that this would work on an iPhone 5. So, shall I abandon the idea of being able to use instanced drawing on older iOS devices?

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• Optimize website for touch devices

  - by gregers

  On a touch device like iPhone/iPad/Android it can be difficult to hit a small button with your finger. There is no cross-browser way to detect touch devices with CSS media queries that I know of. So I check if the browser has support for javascript touch events. Until now, other browsers haven't supported them, but the latest Google Chrome on dev channel enabled touch events (even for non touch devices). And I suspect other browser makers will follow, since laptops with touch screens are comming. This is the test I use: function isTouchDevice() { try { document.createEvent("TouchEvent"); return true; } catch (e) { return false; } } The problem is that this only tests if the browser has support for touch events, not the device. Does anyone know of The Correct[tm] way of giving touch devices better user experience? Other than sniffing user agent. Mozilla has a media query for touch devices. But I haven't seen anything like it in any other browser: https://developer.mozilla.org/En/CSS/Media_queries#-moz-touch-enabled Update: I want to avoid using a separate page/site for mobile/touch devices. The solution has to detect touch devices with object detection or similar from JavaScript, or include a custom touch-CSS without user agent sniffing! The main reason I asked, was to make sure it's not possible today, before I contact the css3 working group.

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• What happens when value types are created?

  - by Bob

  I'm developing a game using XNA and C# and was attempting to avoid calling new struct() type code each frame as I thought it would freak the GC out. "But wait," I said to myself, "struct is a value type. The GC shouldn't get called then, right?" Well, that's why I'm asking here. I only have a very vague idea of what happens to value types. If I create a new struct within a function call, is the struct being created on the stack? Will it simply get pushed and popped and performance not take a hit? Further, would there be some memory limit or performance implications if, say, I need to create many instances in a single call? Take, for instance, this code: spriteBatch.Draw(tex, new Rectangle(x, y, width, height), Color.White); Rectangle in this case is a struct. What happens when that new Rectangle is created? What are the implications of having to repeat that line many times (say, thousands of times)? Is this Rectangle created, a copy sent to the Draw method, and then discarded (meaning no memory getting eaten up the more Draw is called in that manner in the same function)? P.S. I know this may be pre-mature optimization, but I'm mostly curious and wish to have a better understanding of what is happening.

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• HTML entity encoding (convert '<' to '&lt;') on iPhone in objective-c

  - by Markus

  I'm developing an application for the iPhone that has inApp-mail sending capabilities. So far so good, but now I want to avoid html-injections as some parts of the mail are user-generated texts. Basically I search for something like this: // inits NSString *sourceString = [NSString stringWithString:@"Hello world! Grüße dich Welt <-- This is in German."]; // ----- THAT'S WHAT I'M LOOKING FOR // pseudo-code | // V NSString *htmlEncodedString = [sourceString htmlEncode]; // log NSLog(@"source string: %@", sourceString); NSLog(@"encoded string: %@", htmlEncodedString); Expected output source string: Hello world! Grüße dich Welt <-- This is in German. encoded string: Hello world! Gr&#252;&#223;e dich Welt &lt;-- This is in German. I already googled and looked through several of SO's questions and answers, but all of them seem to be related to URL-encoding and that's not what I really need (I tried stringByAddingPercentEscapesUsingEncoding with no luck - it creates %C3%BC out of an 'ü' that should be an ü). A code sample would be really great (correcting mine?)... -- Thanks in advance, Markus

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• Problems running XNA game on 64-bit Windows 7

  - by Tesserex

  I'm having problems getting my game engine to run on my brother's machine, which is running 64-bit Windows 7. I'm developing on 32-bit XP SP2. My app uses XNA, FMOD.NET, and another dll I wrote entirely in C#. Everything is targeted to x86, not AnyCPU. I've read that this is required for XNA to work because there is no 64-bit xna framework. I recompiled FMOD.NET as x86 as well and made sure to be using the 32-bit version of the native dll. So I don't see any problems there. However when he tries to run my app, it gives an error which is mysterious, but not unheard of. A FileNotFoundException with an empty file name, and the top of the stack trace is in my main form constructor. The message is The specified module could not be found. (Exception from HRESULT: 0x8007007E) I found some threads online about this error, all with very vague, mixed, and fuzzy responses that don't really help me. Most remind people to target x86. Some say check that they have all the dlls necessary. I gave my brother Microsoft.Xna.Framework.dll, but does he need to install the entire XNA redistributable package? When I take everything I sent him and stick it in a random directory, it still runs fine for me. I developed the game in VS2008, not in game studio, using XNA 3.0 and a Windows Forms control that uses XNA drawing which I found in an msdn tutorial. I would also like to avoid requiring a full installer if possible. Any insight? Please?

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• MySql Query lag time?

  - by Click Upvote

  When there are multiple PHP scripts running in parallel, each making an UPDATE query to the same record in the same table repeatedly, is it possible for there to be a 'lag time' before the table is updated with each query? I have basically 5-6 instances of a PHP script running in parallel, having been launched via cron. Each script gets all the records in the items table, and then loops through them and processes them. However, to avoid processing the same item more than once, I store the id of the last item being processed in a seperate table. So this is how my code works: function getCurrentItem() { $sql = "SELECT currentItemId from settings"; $result = $this->db->query($sql); return $result->get('currentItemId'); } function setCurrentItem($id) { $sql = "UPDATE settings SET currentItemId='$id'"; $this->db->query($sql); } $currentItem = $this->getCurrentItem(); $sql = "SELECT * FROM items WHERE status='pending' AND id > $currentItem'"; $result = $this->db->query($sql); $items = $result->getAll(); foreach ($items as $i) { //Check if $i has been processed by a different instance of the script, and if so, //leave it untouched. if ($this->getCurrentItem() > $i->id) continue; $this->setCurrentItem($i->id); // Process the item here } But despite of all the precautions, most items are being processed more than once. Which makes me think that there is some lag time between the update queries being run by the PHP script, and when the database actually updates the record. Is it true? And if so, what other mechanism should I use to ensure that the PHP scripts always get only the latest currentItemId even when there are multiple scripts running in parrallel? Would using a text file instead of the db help?

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• Transitioning from desktop app written in C++ to a web-based app

  - by Karim

  We have a mature Windows desktop application written in C++. The application's GUI sits on top of a windows DLL that does most of the work for the GUI (it's kind of the engine). It, too, is written in C++. We are considering transitioning the Windows app to be a web-based app for various reasons. What I would like to avoid is having to writing the CGI for this web-based app in C++. That is, I would rather have the power of a 4G language like Python or a .NET language for creating the web-based version of this app. So, the question is: given that I need to use a C++ DLL on the backend to do the work of the app what technology stack would you recommend for sitting between the user's browser and are C++ dll? We can assume that the web server will be Windows. Some options: Write a COM layer on top of the windows DLL which can then be access via .NET and use ASP.NET for the UI Access the export DLL interface directly from .NET and use ASP.NET for the UI. Write a custom Python library that wraps the windows DLL so that the rest of the code can be written. Write the CGI using C++ and a C++-based MVC framework like Wt Concerns: I would rather not use C++ for the web framework if it can be avoided - I think languages like Python and C# are simply more powerful and efficient in terms of development time. I'm concerned that my mixing managed and unmanaged code with one of the .NET solutions I'm asking for lots of little problems that are hard to debug (purely anecdotal evidence for that) Same is true for using a Python layer. Anything that's slightly off the beaten path like that worries me in that I don't have much evidence one way or the other if this is a viable long term solution.

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• can javascript process binary data?

  - by Johnny

  admit me describe my questions in situation-oriented way: assume IE is still the dominate web browser(the firefox have document for binary processing): the XMLHttpRequest.responseText or XMLHttpRequest.responseXML in ie desire txt or xml/xhtml/html,but what about the server response the xmlHttprequest whith MIME TYPE application/octet ? would the response string all little than 256 ?(every char of that string < 256), thanks very much for a straight answer, i have no webserver env,so i don't know how to test it out. because use txt or xml have a issue of character set encode, and i don't know how to process #[[[CDDATA node of one encoded xml(ex : utf-8,ascii,gb18030) with javascript, when i getNodeText, does the docObj return me byte or decoded char ? if it was decoded char which according to the header indicated charSet in the httpresponse , it would be all wrong. to avoid mess up with charSet ,i would like the server to response octet data and force strings data to be encoded as utf-8 but another charSet in the binary format. if the response is octal, so i guess the browser would not try to decode the response"txt" does this weird? or miss understanding the fundamental things? EDIT: I believe the question is asking this: Can Javascript safely process strings that aren't encoded in Unicode? What are the problems with trying to do so? EDIT: no no no , i means if http-header: content-type is "application/octet" , would the ie try to decoded it as (16bits Unicode | ie local setting charset ) when i get XMLHttpRequestobj.responseText use javascript ? or it(ie) just wrap every single byte of the response body as a javascript string, then every char in that string little than or equal 256 (char<=256), am i talking Mars language? sadly, if i were Marsizen,i would come as tourist without fuzzy questions. however i am in a country which share at least one property with Mars : RED

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• Ordering by formula fields in NHibernate

  - by Darin Dimitrov

  Suppose that I have the following mapping with a formula property: <class name="Planet" table="planets"> <id name="Id" column="id"> <generator class="native" /> </id> <!-- somefunc() is a native SQL function --> <property name="Distance" formula="somefunc()" /> </class> I would like to get all planets and order them by the Distance calculated property: var planets = session .CreateCriteria<Planet>() .AddOrder(Order.Asc("Distance")) .List<Planet>(); This is translated to the following query: SELECT Id as id0, somefunc() as formula0 FROM planets ORDER BY somefunc() Desired query: SELECT Id as id0, somefunc() as formula0 FROM planets ORDER BY formula0 If I set a projection with an alias it works fine: var planets = session .CreateCriteria<Planet>() .SetProjection(Projections.Alias(Projections.Property("Distance"), "dist")) .AddOrder(Order.Asc("dist")) .List<Planet>(); SQL: SELECT somefunc() as formula0 FROM planets ORDER BY formula0 but it populates only the Distance property in the result and I really like to avoid projecting manually over all the other properties of my object (there could be many other properties). Is this achievable with NHibernate? As a bonus I would like to pass parameters to the native somefunc() SQL function. Anything producing the desired SQL is acceptable (replacing the formula field with subselects, etc...), the important thing is to have the calculated Distance property in the resulting object and order by this distance inside SQL.

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• How do I do MongoDB console-style queries in PHP?

  - by Zoe Boles

  I'm trying to get a MongoDB query from the javascript console into my PHP app. What I'm trying to avoid is having to translate the query into the PHP "native driver"'s format... I don't want to hand build arrays and hand-chain functions any more than I want to manually build an array of MySQL's internal query structure just to get data. I already have a string producing the exact content I want in the Mongo console: db.intake.find({"processed": {"$exists": "false"}}).sort({"insert_date": "1"}).limit(10); The question is, is there a way for me to hand this string, as is, to MongoDB and have it return a cursor with the dataset I request? Right now I'm at the "write your own parser because it's not valid json to kinda turn a subset of valid Mongo queries into the format the PHP native driver wants" state, which isn't very fun. I don't want an ORM or a massive wrapper library; I just want to give a function my query string as it exists in the console and get an Iterator back that I can work with. I know there are a couple of PHP-based Mongo manager applications that apparently take console-style queries and handle them, but initial browsing through their code, I'm not sure how they handle the translation. I absolutely love working with mongo in the console, but I'm rapidly starting to loathe the thought of converting every query into the format the native writer wants...

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• Why does this test fail?

  - by Tomas Lycken

  I'm trying to test/spec the following action method public virtual ActionResult ChangePassword(ChangePasswordModel model) { if (ModelState.IsValid) { if (MembershipService.ChangePassword(User.Identity.Name, model.OldPassword, model.NewPassword)) { return RedirectToAction(MVC.Account.Actions.ChangePasswordSuccess); } else { ModelState.AddModelError("", "The current password is incorrect or the new password is invalid."); } } // If we got this far, something failed, redisplay form return RedirectToAction(MVC.Account.Actions.ChangePassword); } with the following MSpec specification: public class When_a_change_password_request_is_successful : with_a_change_password_input_model { Establish context = () => { membershipService.Setup(s => s.ChangePassword(Param.IsAny<string>(), Param.IsAny<string>(), Param.IsAny<string>())).Returns(true); controller.SetFakeControllerContext("POST"); }; Because of = () => controller.ChangePassword(inputModel); ThenIt should_be_a_redirect_result = () => result.ShouldBeARedirectToRoute(); ThenIt should_redirect_to_success_page = () => result.ShouldBeARedirectToRoute().And().ShouldRedirectToAction<AccountController>(c => c.ChangePasswordSuccess()); } where with_a_change_password_input_model is a base class that instantiates the input model, sets up a mock for the IMembershipService etc. The test fails on the first ThenIt (which is just an alias I'm using to avoid conflict with Moq...) with the following error description: Machine.Specifications.SpecificationException: Should be of type System.RuntimeType but is [null] But I am returning something - in fact, a RedirectToRouteResult - in each way the method can terminate! Why does MSpec believe the result to be null?

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• When downloading a file using FileStream, why does page error message refers to aspx page name, not

  - by StuperUser

  After building a filepath (path, below) in a string (I am aware of Path in System.IO, but am using someone else's code and do not have the opportunity to refactor it to use Path). I am using a FileStream to deliver the file to the user (see below): FileStream myStream = new FileStream(path, FileMode.Open, FileAccess.Read); long fileSize = myStream.Length; byte[] Buffer = new byte[(int)fileSize + 1]; myStream.Read(Buffer, 0, (int)myStream.Length); myStream.Close(); Response.ContentType = "application/csv"; Response.AddHeader("content-disposition", "attachment; filename=" + filename); Response.BinaryWrite(Buffer); Response.Flush(); Response.End(); I have seen from: http://stackoverflow.com/questions/736301/asp-net-how-to-stream-file-to-user reasons to avoid use of Response.End() and Response.Close(). I have also seen several articles about different ways to transmit files and have diagnosed and found a solution to the problem (https and http headers) with a colleague. However, the error message that was being displayed was not about access to the file at path, but the aspx file. Edit: Error message is: Internet Explorer cannot download MyPage.aspx from server.domain.tld Internet Explorer was not able to open this Internet site. The requested site is either unavailable or cannot be found. Please try again later. (page name and address anonymised) Why is this? Is it due to the contents of the file coming from the HTTP response .Flush() method rather than a file being accessed at its address?

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• wxGraphicsContext dreadfully slow on Windows

  - by Jonatan

  I've implemented a plotter using wxGraphicsContext. The development was done using wxGTK, and the graphics was very fast. Then I switched to Windows (XP) using wxWidgets 2.9.0. And the same code is extremely slow. It takes about 350 ms to render a frame. Since the user is able to drag the plotter with the mouse to navigate it feels very sluggish with such a slow update rate. I've tried to implement some parts using wxDC and benchmarked the difference. With wxDC the code runs just about 100 times faster. As far as I know both Cairo and GDI+ are implemented in software at this point, so there's no real reason Cairo should be so much faster than GDI+. Am I doing something wrong? Or is the GDI+ implementation just not up on par with Cairo? One small note: I'm rendering to a wxBitmap now, with the wxGraphicsContext created from a wxMemoryDC. This is to avoid flicker on XP, since double buffering doesn't work there.

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• What's a way for a client to automatically resolve the ip address of a server?

  - by zooropa

  The project I am working on is a client/server architecture. In a LAN environment, I want the client's to be able to automatically determine the server's address. I want to avoid having to manually configure each client with the ip address of the server. What is the best way to do this? Some alternatives I have thought about doing are: The server could listen for broadcast packets from the clients. The message from the client would be a request for the IP address of the server. The server would respond with its address. The machine running my project's server could also have a bind server running. The LAN's router could be configured to use it as one of its DNS servers. I think I saw that there is a bind library. Does that mean I can build the bind service into my server so that bind doesn't have to be installed on the server? Any other ideas? What have you done in the past? What are the pros/cons of these approaches and others that might be suggested? Thanks for your help!

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• Strange layout behaviour

  - by andrii

  I am a little bit confused. Here is an small web page. There are two main div-s: top and mainBlock. First contain image. The problem is that firebug shows that div#top's height is equal to 0 and because of that the next div mainBlock moves up. If I would delete this peace of code: div#logo{ float: left; } everything will start working fine and div#mainBlock will be below the div#top. Could you, please, explain me how it works and how to avoid this in proper way? Here is my code: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/> <title>Paralab - Website &amp; interface design, web apps development, usability</title> <style text="text/css"> html, body{ } div#logo{ float: left; } #top{ width: 100%; } #mainBlock{ width:100%; } </style> </head> <body> <div id="top"> <div id="logo"> <img alt="logo" src="img/logo.png" /> </div> </div> <div id="mainBlock"> Contact Us </div> </body> </html>

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• what does this attempted trojan horse code do?

  - by bstullkid

  It looks like this just sends a ping, but whats the point of that when you can just use ping? /* WARNING: this is someone's attempt at writing a malware trojan. Do not compile and *definitely* don't install. I added an exit as the first line to avoid mishaps - msw */ int main (int argc, char *argv[]) { exit(1); unsigned int pid = 0; char buffer[2]; char *args[] = { "/bin/ping", "-c", "5", NULL, NULL }; if (argc != 2) return 0; args[3] = strdup(argv[1]); for (;;) { gets(buffer); /* FTW */ if (buffer[0] == 0x6e) break; switch (pid = fork()) { case -1: printf("Error Forking\n"); exit(255); case 0: execvp(args[0], args); exit(1); default: break; } } return 255; }

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• Threshold of blurry image - part 2

  - by 1''

  How can I threshold this blurry image to make the digits as clear as possible? In a previous post, I tried adaptively thresholding a blurry image (left), which resulted in distorted and disconnected digits (right): Since then, I've tried using a morphological closing operation as described in this post to make the brightness of the image uniform: If I adaptively threshold this image, I don't get significantly better results. However, because the brightness is approximately uniform, I can now use an ordinary threshold: This is a lot better than before, but I have two problems: I had to manually choose the threshold value. Although the closing operation results in uniform brightness, the level of brightness might be different for other images. Different parts of the image would do better with slight variations in the threshold level. For instance, the 9 and 7 in the top left come out partially faded and should have a lower threshold, while some of the 6s have fused into 8s and should have a higher threshold. I thought that going back to an adaptive threshold, but with a very large block size (1/9th of the image) would solve both problems. Instead, I end up with a weird "halo effect" where the centre of the image is a lot brighter, but the edges are about the same as the normally-thresholded image: Edit: remi suggested morphologically opening the thresholded image at the top right of this post. This doesn't work too well. Using elliptical kernels, only a 3x3 is small enough to avoid obliterating the image entirely, and even then there are significant breakages in the digits:

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• Tree data in MySql database table

  - by Robert Koritnik

  I have a table that uses Adjacency list model for hierarchy storage. My most relevant columns in this table are therefore: ItemId // is auto_increment ParentId Level ParentTrail // in the form of "parentId/../parentId/itemId" then I created a before insert tigger, that populates columns Level and ParentTrail. Since the last column also includes current item's ID I had to use a trick in my trigger because auto_increment columns are not available in the before insert trigger. So I get that value from the information_schema.tables table. All works fine, until I try to write an update trigger, that would update my item and its descendants when the item changes its parent (ParentId has changed). But I can't make an update on my table inside the update trigger. All I can do is to change current record's values but not other's. I could use a separate table for hierarchy data, but that would mean that I would also have to create a view that would combine these two tables (1:1 relation) and I would like to avoid this is at all possible. Is there a way to have all these in the same table so that these fields (Level and ParetTrail) set/update themselves automagically using triggers?

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• Is there a standard pattern for scanning a job table executing some actions?

  - by Howiecamp

  (I realize that my title is poor. If after reading the question you have an improvement in mind, please either edit it or tell me and I'll change it.) I have the relatively common scenario of a job table which has 1 row for some thing that needs to be done. For example, it could be a list of emails to be sent. The table looks something like this: ID Completed TimeCompleted anything else... ---- --------- ------------- ---------------- 1 No blabla 2 No blabla 3 Yes 01:04:22 ... I'm looking either for a standard practice/pattern (or code - C#/SQL Server preferred) for periodically "scanning" (I use the term "scanning" very loosely) this table, finding the not-completed items, doing the action and then marking them completed once done successfully. In addition to the basic process for accomplishing the above, I'm considering the following requirements: I'd like some means of "scaling linearly", e.g. running multiple "worker processes" simultaneously or threading or whatever. (Just a specific technical thought - I'm assuming that as a result of this requirement, I need some method of marking an item as "in progress" to avoid attempting the action multiple times.) Each item in the table should only be executed once. Some other thoughts: I'm not particularly concerned with the implementation being done in the database (e.g. in T-SQL or PL/SQL code) vs. some external program code (e.g. a standalone executable or some action triggered by a web page) which is executed against the database Whether the "doing the action" part is done synchronously or asynchronously is not something I'm considering as part of this question.

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• Trouble creating a SQL query

  - by JoBu1324

  I've been thinking about how to compose this SQL query for a while now, but after thinking about it for a few hours I thought I'd ask the SO community to see if they have any ideas. Here is a mock up of the relevant portion of the tables: contracts id date ar (yes/no) term payments contract_id payment_date The object of the query is to determine, per month, how many payments we expect, vs how many payments we received. conditions for expecting a payment Expected payments begin on contracts.term months after contracts.date, if contracts.ar is "yes". Payments continue to be expected until the month after the first missed payment. There is one other complication to this: payments might be late, but they need to show up as if they were paid on the date expected. The data is all there, but I've been having trouble wrapping my head around the SQL query. I am not an SQL guru - I merely have a decent amount of experience handling simpler queries. I'd like to avoid filtering the results in code, if possible - but without your help that may be what I have to do. Expected Output Month Expected Payments Received Payments January 500 450 February 498 478 March 234 211 April 987 789 ... SQL Fiddle I've created an SQL Fiddle: http://sqlfiddle.com/#!2/a2c3f/2

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• floating point equality in Python and in general

  - by eric.frederich

  I have a piece of code that behaves differently depending on whether I go through a dictionary to get conversion factors or whether I use them directly. The following piece of code will print 1.0 == 1.0 -> False But if you replace factors[units_from] with 10.0 and factors[units_to ] with 1.0 / 2.54 it will print 1.0 == 1.0 -> True #!/usr/bin/env python base = 'cm' factors = { 'cm' : 1.0, 'mm' : 10.0, 'm' : 0.01, 'km' : 1.0e-5, 'in' : 1.0 / 2.54, 'ft' : 1.0 / 2.54 / 12.0, 'yd' : 1.0 / 2.54 / 12.0 / 3.0, 'mile' : 1.0 / 2.54 / 12.0 / 5280, 'lightyear' : 1.0 / 2.54 / 12.0 / 5280 / 5.87849981e12, } # convert 25.4 mm to inches val = 25.4 units_from = 'mm' units_to = 'in' base_value = val / factors[units_from] ret = base_value * factors[units_to ] print ret, '==', 1.0, '->', ret == 1.0 Let me first say that I am pretty sure what is going on here. I have seen it before in C, just never in Python but since Python in implemented in C we're seeing it. I know that floating point numbers will change values going from a CPU register to cache and back. I know that comparing what should be two equal variables will return false if one of them was paged out while the other stayed resident in a register. Questions What is the best way to avoid problems like this?... In Python or in general. Am I doing something completely wrong? Side Note This is obviously part of a stripped down example but what I'm trying to do is come with with classes of length, volume, etc that can compare against other objects of the same class but with different units. Rhetorical Questions If this is a potentially dangerous problem since it makes programs behave in an undetermanistic matter, should compilers warn or error when they detect that you're checking equality of floats Should compilers support an option to replace all float equality checks with a 'close enough' function? Do compilers already do this and I just can't find the information.

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• How can I sign a Windows Mobile application for internal use?

  - by AR

  I'm developing a Windows Mobile application for internal company use, using the Windows Mobile 6 Professional SDK. Same old story: I've developed and tested on the emulator and all is well, but as soon as I deploy to advice I get an UnauthorizedAccessException when writing files or creating directories. I'm aware that an application installed to a device needs to be signed but I'm running into roadblocks at every turn: Using the project properties 'Devices' window I select 'Sign the project output with this certificate, and choose one of the sample certificates from the SDK. This results in a build error: "The signer's certificate is not valid for signing" when running SignTool. If I try to run SignTool.exe from the commandline, I get an error telling me to run SignTool.exe from a location in the system's PATH. I can't use the 'Signing' tab in the Project Properties to create a test certificate - this is greyed out (presumably for WinMobile projects?). If at all possible, I would like to avoid having to go through Versign or the like to get a Mobile2Market certificate. If I have to go this route for a final version that's fine, but I need to at least be able to test the app on real devices. Any advice would be most welcome!

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• How do i programmatically access the face cache in Windows Live Photo Gallery?

  - by acorderob

  I'm not talking about the "people tags" embeded in the XMP packets of JPEGs. I'm talking about the face database used to recognize new faces. I want to add to my program the option to recognize faces using the already trained database of WLPG. I managed to use the API (a type library dll) to detect faces, but to recognize them it needs an Exemplar Cache object that is not available in the same API. I could create my own object, but i want to use the already existing one to avoid duplicate training for the user. I know the database is in C:\Users\\AppData\Local\Microsoft\Windows Live Photo Gallery and that it is in an SQL Server Compact format. I tried to open the database with Visual Studio 2010, but it says that it is in an older version (pre-3.5) and needs to be upgraded. I don't want to change the database, just read it. I don't know how the WPLG reads it since apparently i don't have the correct OLEDB provider version. I would also prefer to read it without accesing the database directly but i don't see any DLL that exports that functionality. BTW, i'm using Delphi 2010. Any ideas?

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• C++: is it safe to work with std::vectors as if they were arrays?

  - by peoro

  I need to have a fixed-size array of elements and to call on them functions that require to know about how they're placed in memory, in particular: functions like glVertexPointer, that needs to know where the vertices are, how distant they are one from the other and so on. In my case vertices would be members of the elements to store. to get the index of an element within this array, I'd prefer to avoid having an index field within my elements, but would rather play with pointers arithmetic (ie: index of Element *x will be x - & array[0]) -- btw, this sounds dirty to me: is it good practice or should I do something else? Is it safe to use std::vector for this? Something makes me think that an std::array would be more appropriate but: Constructor and destructor for my structure will be rarely called: I don't mind about such overhead. I'm going to set the std::vector capacity to size I need (the size that would use for an std::array, thus won't take any overhead due to sporadic reallocation. I don't mind a little space overhead for std::vector's internal structure. I could use the ability to resize the vector (or better: to have a size chosen during setup), and I think there's no way to do this with std::array, since its size is a template parameter (that's too bad: I could do that even with an old C-like array, just dynamically allocating it on the heap). If std::vector is fine for my purpose I'd like to know into details if it will have some runtime overhead with respect to std::array (or to a plain C array): I know that it'll call the default constructor for any element once I increase its size (but I guess this won't cost anything if my data has got an empty default constructor?), same for destructor. Anything else?

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