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Detecting what the target object is when NullReferenceException is thrown.

- by StingyJack

I'm sure we all have received the wonderfully vague "Object reference not set to instance of an Object" exception at some time or another. Identifying the object that is the problem is often a tedious task of setting breakpoints and inspecting all members in each statement. Does anyone have any tricks to easily and efficiently identify the object that causes the exception, either via programmatical means or otherwise? --edit It seems I was vague like the exception =). The point is to _not have to debug the app to find the errant object. The compiler/runtime does know that the object has been allocated, and that the object has not yet been instantiated. Is there a way to extract / identify those details in a caught exception @ W. Craig Trader Your explanation that it is a result of a design problem is probably the best answer I could get. I am fairly compulsive with defensive coding and have managed to get rid of most of these errors after fixing my habits over time. The remaining ones just tweak me to no end, and lead me to posting this question to the community. Thanks for everyone's suggestions.

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The type or namespace name 'WebControls' could not be found (are you missing a using directive or an assembly reference?)

- by user1467175

I encountered this error: The type or namespace name 'WebControls' could not be found (are you missing a using directive or an assembly reference?) Source Error: Line 28: Login Login1 = (WebControls.Login)LoginView1.FindControl("Login1"); // here the error code Line 29: TextBox UserName = (TextBox)Login1.FindControl("UserName"); Line 30: TextBox FailureText = (TextBox)Login1.FindControl("FailureText"); I did some research and the solution was to add this into the source code: System.Web.UI.WebControls.Login but I have no idea where this code can be add into. At first I tried putting it as a namespace, but it was wrong. Anyone can tell me where should I place this code at?? EDIT protected void Login1_LoginError(object sender, System.EventArgs e) { //Login Login1 = (WebControls.Login).LoginView1.FindControl("Login1"); Login Login1 = (System.Web.UI.WebControls.Login)LoginView1.FindControl("Login1"); TextBox UserName = (TextBox)Login1.FindControl("UserName"); TextBox FailureText = (TextBox)Login1.FindControl("FailureText"); //There was a problem logging in the user //See if this user exists in the database MembershipUser userInfo = Membership.GetUser(UserName.Text); if (userInfo == null) { //The user entered an invalid username... FailureText.Text = "There is no user in the database with the username " + UserName.Text; } else { //See if the user is locked out or not approved if (!userInfo.IsApproved) { FailureText.Text = "When you created your account you were sent an email with steps to verify your account. You must follow these steps before you can log into the site."; } else if (userInfo.IsLockedOut) { FailureText.Text = "Your account has been locked out because of a maximum number of incorrect login attempts. You will NOT be able to login until you contact a site administrator and have your account unlocked."; } else { //The password was incorrect (don't show anything, the Login control already describes the problem) FailureText.Text = string.Empty; } } }

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How to know the type of an object in a list?

- by nacho4d

Hi, I want to know the type of object (or type) I have in my list so I wrote this: void **list; //list of references list = new void * [2]; Foo foo = Foo(); const char *not_table [] = {"tf", "ft", 0 }; list[0] = &foo; list[1] = not_table; if (dynamic_cast<LogicProcessor*>(list[0])) { //ERROR here ;( printf("Foo was found\n"); } if (dynamic_cast<char*> (list[0])) { //ERROR here ;( printf("char was found\n"); } but I get : error: cannot dynamic_cast '* list' (of type 'void*') to type 'class Foo*' (source is not a pointer to class) error: cannot dynamic_cast '* list' (of type 'void*') to type 'char*' (target is not pointer or reference to class) Why is this? what I am doing wrong here? Is dynamic_cast what I should use here? Thanks in advance EDIT: I know above code is much like plain C and surely sucks from the C++ point of view but is just I have the following situation and I was trying something before really implementing it: I have two arrays of length n but both arrays will never have an object at the same index. Hence, or I have array1[i]!=NULL or array2[i]!=NULL. This is obviously a waste of memory so I thought everything would be solved if I could have both kind of objects in a single array of length n. I am looking something like Cocoa's (Objective-C) NSArray where you don't care about the type of the object to be put in. Not knowing the type of the object is not a problem since you can use other method to get the class of a certain later. Is there something like it in c++ (preferably not third party C++ libraries) ? Thanks in advance ;)

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Context.Items.Add on an XML return type?

- by Scott Schluer

So I have the following code contained within an HttpModule in an application I've been asked to support: app.Context.Response.ContentType = "text/xml"; app.Context.Items.Add("IpixRoomId", ipixRoomId); app.Context.Items.Add("IpixId", ipixId); app.Context.Response.Cache.SetCacheability(HttpCacheability.NoCache); app.Context.RewritePath(rewriteUrl, true); What's the purpose of adding data to Context.Items when the content type is XML? EDIT: For clarification, I'm calling up this URL: http://website.com/virtualtour/1971/6284/panorama2flash.swf I assume the SWF file (I know very little about Flash) makes another call to http://website.com/virtualtour/config.xml. The code I pasted above only executes on calls to config.xml. So since it's only the SWF file and config.xml being requested from the server, I'm a little confused. Can the .SWF file have access to HttpContext.Current.Items? Other than the HttpModule, there is no .NET involved in the code, it's a straight request to the SWF file which triggers a call to config.xml but it seems that those Context.Items contain the data needed to make the SWF file display the right virtual tour. I'm just missing where that link happens. It can't happen in the XML, so maybe in Flash?

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Convert C# (with typed events) to VB.NET

- by Steven

I have an ASPX page (with VB Codebehind). I would like to extend the GridView class to show the header / footer when no rows are returned. I found a C# example online (link) (source). However, I cannot convert it to VB because it uses typed events (which are not legal in VB). I have tried several free C# to VB.NET converters online, but none have worked. Please convert the example to VB.NET or provide an alternate method of extending the GridView class. Notes / Difficulties: If you get an error with DataView objects, specify the type as System.Data.DataView and the type comparison could be the following: If data.[GetType]() Is GetType(System.Data.DataView) Then Since the event MustAddARow cannot have a type in VB (and RaiseEvent event doesn't have a return value), how can I compare it to Nothing in the function OnMustAddARow()? EDIT: The following is a sample with (hopefully) relevant code to help answer the question. namespace AlwaysShowHeaderFooter { public delegate IEnumerable MustAddARowHandler(IEnumerable data); public class GridViewAlwaysShow : GridView { ////////////////////////////////////// // Various member functions omitted // ////////////////////////////////////// protected IEnumerable OnMustAddARow(IEnumerable data) { if (MustAddARow == null) { throw new NullReferenceException("The datasource has no rows. You must handle the \"MustAddARow\" Event."); } return MustAddARow(data); } public event MustAddARowHandler MustAddARow; } }

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Problem with overlapping divs - CSS

- by Chaitanya

html, body {margin: 0px; padding: 0px;} #pageContainer{ margin: auto; padding: auto;} #contentContainer{ margin:150px; width:1100px; height: 100%; overflow: hidden; } #leftContainer{ width: 80%; min-height: 800px; background: #009900; float:left;} #left1{ margin:80px 0 0 80px; height: 550px; top:0px; z-index:1; background: #000000;} #left2{ margin:80px 0 0 80px; height: 500px; top:110px; z-index:2000; background:#99FFFF; } #rightContainer{ width: 20%; height: inherit; background: black; float:right;} / CSS Document */ I require 2 overlapping divs, which look like the one below. ---------------------- | | | ------------------ | | ' '| | ' '| | ' '| | ' '| | ' '| | ' '| | ' '| ---------------------- ' ' ' ' ------------------- <div id="pageContainer"> <div id="contentContainer"> <div id="leftContainer"> Am the left container <div id="left1"> <div id="left2"> </div> </div> </div> <div id="rightContainer"> </div> </div> </div> The problem is am failing to get the overlap. Where am I going wrong? Edit 1: topx was a typo, corrected it.

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Class lookup structure array in C++

- by wyatt

I'm trying to create a structure array which links input strings to classes as follows: struct {string command; CommandPath cPath;} cPathLookup[] = { {"set an alarm", AlarmCommandPath}, {"send an email", EmailCommandPath}, {"", NULL} }; which will be used as follows: CommandPath *cPath = NULL; string input; getline(cin, input); for(int i = 0; cPathLookup[i] != ""; i++) { if(cPathLookup[i].command == input) cPath = new cPathLookup[i].cPath; } Obviously, this code is meaningless, but I think my intention is apparent - depending on input, I'd like cPath to be initialized as either a new AlarmCommandPath or a new EmailCommandPath. I could handle it with a function returning an instance depending on input, but a whole sequence of ifs just seems inelegant. I should also note that, in case it's not apparent and important, that AlarmCommandPath and EmailCommandPath are derived from CommandPath, and CommandPath is an abstract class. Thanks for any help you can offer. EDIT: I just noticed that, in spite of CommandPath being abstract, I have a declaration: CommandPath *cPath = NULL; in working code. Why does that compile?

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Rails Nested Attributes Doesn't Insert ID Correctly

- by MunkiPhD

I'm attempting to edit a model's nested attributes, much as outline here, replicated here: <%= form_for @person do |person_form| %> <%= person_form.text_field :name %> <% for address in @person.addresses %> <%= person_form.fields_for address, :index => address do |address_form|%> <%= address_form.text_field :city %> <% end %> <% end %> <% end %> In my code, I have the following: <%= form_for(@meal) do |f| %>  <% for subitem in @meal.meal_line_items %> <%= f.fields_for subitem, :index => subitem do |line_item_form| %> <%= line_item_form.label :servings %><br/> <%= line_item_form.text_field :servings %><br/> <%= line_item_form.label :food_id %><br/> <%= line_item_form.text_field :food_id %><br/> <% end %> <% end %> <%= f.submit %> <% end %> This works great, except, when I look at the HTML, it's creating the inputs that look like the following, failing to input the correct id and instead placing the memory representation(?) of the model: <input type="text" value="2" size="30" name="meal[meal_line_item][#<MealLineItem:0x00000005c5d618>][servings]" id="meal_meal_line_item_#<MealLineItem:0x00000005c5d618>_servings">

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Finding and displaying image src attributes

- by MIF

Hi I have a problem and an try to find in one week. I need help :( I have a script work with : <script type="text/javascript"> var list_images = 'image1.jpg|image2.jpg|image3.jpg|image4.jpg|image5.jpg|image6.jpg|image7.jpg'; var images = list_images.split('|'); </script> but know in blogger always give more tab in my script <script type="text/javascript"> var list_images = ' <div></div> <a href="anything"><img src="image1.jpg"><a/> <a href="anything"><img src="image2.jpg"><a/> <a href="anything"><img src="image3.jpg"><a/> <a href="anything"><img src="image4.jpg"><a/> <a href="anything"><img src="image5.jpg"><a/> <li><img src="image6.jpg"></li> <li><img src="image7.jpg"></li> <br/> '; var images = list_images.split('|'); </script> I want remove all tabs and convert them like var list_images = 'image1.jpg|image2.jpg|image3.jpg|image4.jpg|image5.jpg|image6.jpg|image7.jpg'; Finding and displaying image src attributes - Old question http://manhualayout.blogspot.com/2011/03/httpcomic.html - Work good http://manhualayout.blogspot.com/2011/02/blog-post.html - My problem make my crazy in one week :( I Can't just edit the variable of the block HTML ...I want find a way to search and replace to remove all and got the link with the | Please help me . Very very thanks .. Me so stupid :(

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Sorting a very large text file in Java

- by Alice

Hi, I have a large text file I need to sort in Java. The format is: word [tab] frequency [new line] The algorithm for sorting is: Read some of the file, filtering for purlely alphabetic words. Once you have X number of alphabetic words, call Collections.sort and write the result to a file. Repeat until you have finished reading the file. Start reading two sorted files, comparing line by line for the word with higher frequency, and writing at the same time to a new file as to not load much into your memory Repeat until all files are merged into one large file Right now I've divided the large file into smaller ones (sorted by descending frequency) with 10,000 lines each. I know I need to somehow merge these files back together, but I'm not sure how to go about this. I've created a LinkedList to keep track of all the files created. The algorithm says to compare each line in the two files, except I've tried a case where , say file1 = 8,6,5,3,1 and file2 = 9,8,8,8,8. Then if I compare them line by line I would get file3 = 9,8,8,6,8,5,8,3,8,1 which is incorrectly sorted (they should be in decreasing order). I think I'm misunderstanding some part of the algorithm. If someone could point out what I should do instead, I'd greatly appreciate it. Thanks. edit: Yes this is an assignment. We aren't allowed to increase memory unfortunately :(

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NullPointerException on Activity Testint Tutorial

- by Bendik

Hello, I am currently trying the activity testing tutorial (Found here), and have a problem. It seems that whenever I try to call something inside the UIThread, I get a java.lang.NullPointerException. public void testSpinnerUI() { mActivity.runOnUiThread( new Runnable() { public void run() { mSpinner.requestFocus(); } }); } This gives me: Incomplete: java.lang.NullPointerException and nothing else. I have tried this on two different samples now, with the same result. I tried with a try/catch clause around the mSpinner.requestFocus() call, and it seems that mSpinner is null inside the thread. I have set it properly up with the setUp() function found in the same sample, and a quick assertNotNull( mSpinner ) shows me that mSpinner is in fact not null after the setUp() function. What can be the cause of this? EDIT; ok, some more testing has been done. It seems that the application that is being tested resets between each test. This essentially makes me have to reinstantiate all variables between each test. Is this normal?

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ListView and wrap_content

- by gaiapac

I want to create an activity with a ListView on the left and a TextView on the right, side by side. I write the following xml, but the ListView occupies the entire page and it don't worry about the wrap_content. Why? How can I resolve it? <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="horizontal" > <ListView android:id="@+id/lv" android:layout_width="wrap_content" android:layout_height="match_parent" /> <TextView android:id="@+id/tv" android:layout_width="wrap_content" android:layout_height="match_parent" /> </LinearLayout> EDIT: my onCreate @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); ListView lv = (ListView) findViewById(R.id.lv); String[] values = new String[] { "Test1", "Test2", "Test3", "Test4" }; ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,R.layout.left, R.id.tv1, values); lv.setAdapter(adapter); }

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(Python/Pyramid) Better ways to have standard list/form editors?

- by badcat

I'm working on a number of Pyramid (former Pylons) projects, and often I have the need to display a list of some content (let's say user accounts, log entries or simply some other data). A user should be able to paginate through the list, click on a row and get a form where he/she can edit the contents of that row. Right now I'm always re-inventing the wheel by having Mako templates which use webhelpers for the pagination, Jquery UI for providing a dialog and I craft the editor form and AJAX requests on the client and server side by hand. As you may know, this eats up painfully much time. So what I'm wondering is: Is there a better way of providing lists, editor dialog and server/client communication about this, without having to re-invent the wheel every time? I heard Django takes off a big load of that by providing user accounts and other stuff out of the box; but in my case it's not just about user accounts, it can be any kind of data that is stored on the server-side in a SQL database, which should be able to be edited by a user. Thanks in advance!

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Using NHibernate's HQL to make a query with multiple inner joins

- by Abu Dhabi

The problem here consists of translating a statement written in LINQ to SQL syntax into the equivalent for NHibernate. The LINQ to SQL code looks like so: var whatevervar = from threads in context.THREADs join threadposts in context.THREADPOSTs on threads.thread_id equals threadposts.thread_id join posts1 in context.POSTs on threadposts.post_id equals posts1.post_id join users in context.USERs on posts1.user_id equals users.user_id orderby posts1.post_time where threads.thread_id == int.Parse(id) select new { threads.thread_topic, posts1.post_time, users.user_display_name, users.user_signature, users.user_avatar, posts1.post_body, posts1.post_topic }; It's essentially trying to grab a list of posts within a given forum thread. The best I've been able to come up with (with the help of the helpful users of this site) for NHibernate is: var whatevervar = session.CreateQuery("select t.Thread_topic, p.Post_time, " + "u.User_display_name, u.User_signature, " + "u.User_avatar, p.Post_body, p.Post_topic " + "from THREADPOST tp " + "inner join tp.Thread_ as t " + "inner join tp.Post_ as p " + "inner join p.User_ as u " + "where tp.Thread_ = :what") .SetParameter<THREAD>("what", threadid) .SetResultTransformer(Transformers.AliasToBean(typeof(MyDTO))) .List<MyDTO>(); But that doesn't parse well, complaining that the aliases for the joined tables are null references. MyDTO is a custom type for the output: public class MyDTO { public string thread_topic { get; set; } public DateTime post_time { get; set; } public string user_display_name { get; set; } public string user_signature { get; set; } public string user_avatar { get; set; } public string post_topic { get; set; } public string post_body { get; set; } } I'm out of ideas, and while doing this by direct SQL query is possible, I'd like to do it properly, without defeating the purpose of using an ORM. Thanks in advance! EDIT: The database looks like this: http://i41.tinypic.com/5agciu.jpg (Can't post images yet.)

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Jquery Selecting Multiple Classes, Loading External Files

- by WillingLearner

I have 2 links, with the class dynamicLoad. <ul class="navbar"> <li><a href="Page3-News.html" class="dynamicLoad news">NEWS</a></li> <li><a href="Page2-Events.html" class="dynamicLoad">EVENTS</a></li> </ul> and then I have this already working code, which loads external pages into a div named #MainWrapper: <script type="text/javascript"> $( document ).ready( function() { $( 'a.dynamicLoad' ).click( function( e ) { e.preventDefault(); // prevent the browser from following the link e.stopPropagation(); // prevent the browser from following the link $( '#MainWrapper' ).load( $( this ).attr( 'href' ) ); }); }); </script> How do I edit this code and my links, so that i can target the 1st link with the classes of both dynamicLoad and news, and then, load another script and/or pages into the main wrapper, without breaking its already working functionality?

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Is a web-server (e.g servlets) a good solution for an IM server?

- by John

I'm looking at a new app, broadly speaking an IM application with a strong client-server model - all communications go through a server so they can be logged centrally. The server will be Java in some form, clients could at this point be anything from a .NET Desktop app to Flex/Silverlight, to a simple web-interface using JS/AJAX. I had anticipated doing the server using standard J2EE so I get a thread-safe, multi-user server for 'free'... to make things simple let's say using Servlets (but in practice SpringMVC would be likely). This all seemed very neat but I'm concerned if the stateless nature of Servlets is the best approach. If my memory of servlets (been a year or two) is right, each time a client sent a HTTP request, typically a new message entered by the user, the servlet could not assume it had the user/chat in memory and might have to get it from the DB... regardless it has to look it up. Then it either has to use some PUSH system to inform other members of the chat, or cache that there are new messages, for other clients who poll the server using AJAX or similar - and when they poll it again has to lookup the chat, including new messages, and send the new data. I'm wondering if a better system would be the server is running core Java, and implements a socket-based communication with clients. This allows much more immediate data transfer and is more flexible if say the IM client included some game you could play. But then you're writing a custom server and sockets don't sound very friendly to a browser-based client on current browsers. Am I missing some big piece of the puzzle here, it kind of feels like I am? Perhaps a better way to ask the question would simply be "if the client was browser-based using HTML/JS and had to run on IE7+,FF2+ (i.e no HTML5), how would you implement the server?" edit: if you are going to suggest using XMPP, I have been trying to get my head around this in another question, so please consider if that's a more appropriate place to discuss this specifically.

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Turning logical expression around

- by BluePrint

I have the following code: bool s = true; for (...; ...; ...) { // code that defines A, B, C, D // and w, x, y, z if (!(A < w) && s == true) { s = false; } if (!(B < x) && s == true) { s = false; } if (!(C < y) && s == true) { s = false; } if (!(D < z) && s == true) { s = false; } } This code is working well. However, I want to, for several (unimportant) reasons, change the code so that I can initiate s = false; and set it to true inside the if-statement. It tried the following: bool s = false; for (...; ...; ...) { // code that defines A, B, C, D // and w, x, y, z if (A >= w && s == false) { s = true; } if (B >= x && s == false) { s = true; } if (C >= y && s == false) { s = true; } if (D >= z && s == false) { s = true; } } However, this is not working properly as the code above is working. I know thought wrong somewhere in the logic, but I can't figure out where. Does anbyone see my probably obvious error? EDIT: Added three more if-statemets. Missed them since they were commented away.

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What is the point of having a key_t if what will be the key to access shared memory is the return value of shmget()?

- by devoured elysium

When using shared memory, why should we care about creating a key key_t ftok(const char *path, int id); in the following bit of code? key_t key; int shmid; key = ftok("/home/beej/somefile3", 'R'); shmid = shmget(key, 1024, 0644 | IPC_CREAT); From what I've come to understand, what is needed to access a given shared memory is the shmid, not the key. Or am I wrong? If what we need is the shmid, what is the point in not just creating a random key every time? Edit @link text one can read: What about this key nonsense? How do we create one? Well, since the type key_t is actually just a long, you can use any number you want. But what if you hard-code the number and some other unrelated program hardcodes the same number but wants another queue? The solution is to use the ftok() function which generates a key from two arguments. Reading this, it gives me the impression that what one needs to attach to a shared-memory block is the key. But this isn't true, is it? Thanks

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Generate Spring bean definition from a Java object

- by joeslice

Let's suggest that I have a bean defined in Spring: <bean id="neatBean" class="com..." abstract="true">...</bean> Then we have many clients, each of which have slightly different configuration for their 'neatBean'. The old way we would do it was to have a new file for each client (e.g., clientX_NeatFeature.xml) that contained a bunch of beans for this client (these are hand-edited and part of the code base): <bean id="clientXNeatBean" parent="neatBean"> <property id="whatever" value="something"/> </bean> Now, I want to have a UI where we can edit and redefine a client's neatBean on the fly. My question is: given a neatBean, and a UI that can 'override' properties of this bean, what would be a straightforward way to serialize this to an XML file as we do [manually] today? For example, if the user set property whatever to be "17" for client Y, I'd want to generate: <bean id="clientYNeatBean" parent="neatBean"> <property id="whatever" value="17"/> </bean> Note that moving this configuration to a different format (e.g., database, other-schema'd-xml) is an option, but not really an answer to the question at hand.

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Need help optimizing this Django aggregate query

- by Chris Lawlor

I have the following model class Plugin(models.Model): name = models.CharField(max_length=50) # more fields which represents a plugin that can be downloaded from my site. To track downloads, I have class Download(models.Model): plugin = models.ForiegnKey(Plugin) timestamp = models.DateTimeField(auto_now=True) So to build a view showing plugins sorted by downloads, I have the following query: # pbd is plugins by download - commented here to prevent scrolling pbd = Plugin.objects.annotate(dl_total=Count('download')).order_by('-dl_total') Which works, but is very slow. With only 1,000 plugins, the avg. response is 3.6 - 3.9 seconds (devserver with local PostgreSQL db), where a similar view with a much simpler query (sorting by plugin release date) takes 160 ms or so. I'm looking for suggestions on how to optimize this query. I'd really prefer that the query return Plugin objects (as opposed to using values) since I'm sharing the same template for the other views (Plugins by rating, Plugins by release date, etc.), so the template is expecting Plugin objects - plus I'm not sure how I would get things like the absolute_url without a reference to the plugin object. Or, is my whole approach doomed to failure? Is there a better way to track downloads? I ultimately want to provide users some nice download statistics for the plugins they've uploaded - like downloads per day/week/month. Will I have to calculate and cache Downloads at some point? EDIT: In my test dataset, there are somewhere between 10-20 Download instances per Plugin - in production I expect this number would be much higher for many of the plugins.

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Heap Algorithmic Issue

- by OberynMarDELL

I am having this algorithmic problem that I want to discuss about. Its not about find a solution but about optimization in terms of runtime. So here it is: Suppose we have a race court of Length L and a total of N cars that participate on the race. The race rules are simple. Once a car overtakes an other car the second car is eliminated from the race. The race ends when no more overtakes are possible to happen. The tricky part is that the k'th car has a starting point x[k] and a velocity v[k]. The points are given in an ascending order, but the velocities may differ. What I've done so far: Given that a car can get overtaken only by its previous, I calculated the time that it takes for each car to reach its next one t = (x[i] - x[i+1])/(v[i] - v[i+1]) and I insert these times onto a min heap in O(n log n). So in theory I have to pop the first element in O(logn), find its previous, pop it as well , update its time and insert it in the heap once more, much like a priority queue. My main problem is how I can access specific points of a heap in O(log n) or faster in order to keep the complexity in O(n log n) levels. This program should be written on Haskell so I would like to keep things simple as far as possible EDIT: I Forgot to write the actual point of the race. The goal is to find the order in which cars exit the game

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Controlling race condition at startup.

- by Will Hartung

I have some code that I want to have some one time initialisation performed. But this code doesn't have a definite lifecycle, so my logic can be potentially invoked by multiple threads before my initialisation is done. So, I want to basically ensure that my logic code "waits" until initialisation is done. This is my first cut. public class MyClass { private static final AtomicBoolean initialised = new AtomicBoolean(false); public void initialise() { synchronized(initialised) { initStuff(); initialised.getAndSet(true); initialised.notifyAll(); } } public void doStuff() { synchronized(initialised) { if (!initialised.get()) { try { initialised.wait(); } catch (InterruptedException ex) { throw new RuntimeException("Uh oh!", ex); } } } doOtherStuff(); } } I basically want to make sure this is going to do what I think it's going to do -- block doStuff until the initialised is true, and that I'm not missing a race condition where doStuff might get stuck on a Object.wait() that will never arrive. Edit: I have no control over the threads. And I want to be able to control when all of the initialisation is done, which is why doStuff() can't call initialise(). I used an AtomicBoolean as it was a combination of a value holder, and an object I could synchronize. I could have also simply had a "public static final Object lock = new Object();" and a simple boolean flag. AtomicBoolean conveniently gave me both. A Boolean can not be modified. The CountDownLatch is exactly what I was looking for. I also considered using a Sempahore with 0 permits. But the CountDownLatch is perfect for just this task.

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How to iteratively generate k elements subsets from a set of size n in java?

- by Bea Metitiri

Hi, I'm working on a puzzle that involves analyzing all size k subsets and figuring out which one is optimal. I wrote a solution that works when the number of subsets is small, but it runs out of memory for larger problems. Now I'm trying to translate an iterative function written in python to java so that I can analyze each subset as it's created and get only the value that represents how optimized it is and not the entire set so that I won't run out of memory. Here is what I have so far and it doesn't seem to finish even for very small problems: public static LinkedList<LinkedList<Integer>> getSets(int k, LinkedList<Integer> set) { int N = set.size(); int maxsets = nCr(N, k); LinkedList<LinkedList<Integer>> toRet = new LinkedList<LinkedList<Integer>>(); int remains, thresh; LinkedList<Integer> newset; for (int i=0; i<maxsets; i++) { remains = k; newset = new LinkedList<Integer>(); for (int val=1; val<=N; val++) { if (remains==0) break; thresh = nCr(N-val, remains-1); if (i < thresh) { newset.add(set.get(val-1)); remains --; } else { i -= thresh; } } toRet.add(newset); } return toRet; } Can anybody help me debug this function or suggest another algorithm for iteratively generating size k subsets? EDIT: I finally got this function working, I had to create a new variable that was the same as i to do the i and thresh comparison because python handles for loop indexes differently.

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Call from a singleton class to a function which in turn calls that class's method

- by dare2be

Hello, I am still looking for a way to phrase it properly (I'm not a native speaker...). So I have this class SQL which implements the singleton pattern (for obvious reasons) and I also have this function, checkUsr(), which queries the database using one of SQL's methods. Everything works fine as long as I don't call checkUsr() from within the SQL class. When I do so, the scripts just exits and a blank page is displayed - no errors are returned, no exception is thrown... What's happening? And how do I work around this problem? EDIT: class SQL { public static function singleton() { static $instance; if(!isset($instance)) $instance = new SQL; return $instance; } public function tryLoginAuthor( $login, $sha1 ) { (...) } } function checkUsr() { if (!isset($_SESSION['login']) || !isset($_SESSION['sha1'])) throw new Exception('Not logged in', 1); $SQL = SQL::singleton(); $res = $SQL->tryLoginAuthor($_SESSION['login'], $_SESSION['sha1']); if (!isset($res[0])) throw new Exception('Not logged in', 1); }

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jQuery trigger custom event synchronously?

- by Miguel Angelo

I am using the jQuery trigger method to call an event... but it behaves inconsistently. Sometimes it call the event, sometimes it does not. <a href="#" onclick=" $(this).trigger('custom-event'); window.location.href = 'url'; return false; ">text</a> The custom-event has lots of listeners added to it. It is as if the trigger method is not synchronous, allowing the window.location.href be changed before executing the events. And when window.location.href is changed a navigation occurs, interrupting everything. How can I trigger events synchronously? Using jQuery 1.8.1. Thanks! EDIT I have found that the event, when called has a stack trace like this: jQuery.fx.tick (jquery-1.8.1.js:9021) tick (jquery-1.8.1.js:8499) jQuery.Callbacks.self.fireWith (jquery-1.8.1.js:1082) jQuery.Callbacks.fire (jquery-1.8.1.js:974) jQuery.speed.opt.complete (jquery-1.8.1.js:8991) $.customEvent (myfile.js:28) This proves that jQuery trigger method is asynchronous. Oohhh my... =\

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