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  • Best Practice With JFrame Constructors?

    - by David Barry
    In both my Java classes, and the books we used in them laying out a GUI with code heavily involved the constructor of the JFrame. The standard technique in the books seems to be to initialize all components and add them to the JFrame in the constructor, and add anonymous event handlers to handle events where needed, and this is what has been advocated in my class. This seems to be pretty easy to understand, and easy to work with when creating a very simple GUI, but seems to quickly get ugly and cumbersome when making anything other than a very simple gui. Here is a small code sample of what I'm describing: public class FooFrame extends JFrame { JLabel inputLabel; JTextField inputField; JButton fooBtn; JPanel fooPanel; public FooFrame() { super("Foo"); fooPanel = new JPanel(); fooPanel.setLayout(new FlowLayout()); inputLabel = new JLabel("Input stuff"); fooPanel.add(inputLabel); inputField = new JTextField(20); fooPanel.add(inputField); fooBtn = new JButton("Do Foo"); fooBtn.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { //handle event } }); fooPanel.add(fooBtn); add(fooPanel, BorderLayout.CENTER); } } Is this type of use of the constructor the best way to code a Swing application in java? If so, what techniques can I use to make sure this type of constructor is organized and maintainable? If not, what is the recommended way to approach putting together a JFrame in java?

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  • Does acts-as-taggable-on work on heroku?

    - by Martin
    Hello, I have a question: does the acts-as-taggable-on gem work on Heroku? I'been trying but it doesn't seem to work. In my development machine works okay. I'm wondering if it's maybe because Heroku uses PostgreSQL and my local env SQLite and for some reason postgresql is not supported by the special tagging "magic"? I couldn't find any related info to this, so I would like to know other experiences with this. I guess is pretty rare since both the gem and heroku are very popular. ActionView::Template::Error (undefined method `interests' for "#<About:0x2b35d6125728>":About): <% unless @user.about.interests.empty? %> <p><strong>interests and passions</strong><br /> <% @user.about.interests.each do |tag| %> <%= tag %>, <% end -%></p><% end %> In my about.rb model i have acts_as_taggable_on :interests, :music, :movies, :books, :tvs Thank you in advance!

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  • SQL Server 2000 intermittent connection exceptions on production server - specific environment probl

    - by StickyMcGinty
    We've been having intermittent problems causing users to be forcibly logged out of out application. Our set-up is ASP.Net/C# web application on Windows Server 2003 Standard Edition with SQL Server 2000 on the back end. We've recently performed a major product upgrade on our client's VMWare server (we have a guest instance dedicated to us) and whereas we had none of these issues with the previous release the added complexity that the new upgrade brings to the product has caused a lot of issues. We are also running SQL Server 2000 (build 8.00.2039, or SP4) and the IIS/ASP.NET (.Net v2.0.50727) application on the same box and connecting to each other via a TCP/IP connection. Primarily, the exceptions being thrown are: System.IndexOutOfRangeException: Cannot find table 0. System.ArgumentException: Column 'password' does not belong to table Table. [This exception occurs in the log in script, even though there is clearly a password column available] System.InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first. [This one is occurring very regularly] System.InvalidOperationException: This SqlTransaction has completed; it is no longer usable. System.ApplicationException: ExecuteReader requires an open and available Connection. The connection's current state is connecting. System.Data.SqlClient.SqlException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. And just today, for the first time: System.Web.UI.ViewStateException: Invalid viewstate. We have load tested the app using the same number of concurrent users as the production server and cannot reproduce these errors. They are very intermittent and occur even when there are only 8/9/10 user connections. My gut is telling me its ASP.NET - SQL Server 2000 connection issues.. We've pretty much ruled out code-level Data Access Layer errors at this stage (we've a development team of 15 experienced developers working on this) so we think its a specific production server environment issue.

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  • vb.net documentation and exception question

    - by dcp
    Let's say I have this sub in vb.net: ''' <summary> ''' Validates that <paramref name="value"/> is not <c>null</c>. ''' </summary> ''' ''' <param name="value">The object to validate.</param> ''' ''' <param name="name">The variable name of the object.</param> ''' ''' <exception cref="ArgumentNullException">If <paramref name="value"/> is <c>null</c>.</exception> Sub ValidateNotNull(ByVal value As Object, ByVal name As String) If value Is Nothing Then Throw New ArgumentNullException(name, String.Format("{0} cannot be null.", name)) End If End Sub My question is, is it proper to call this ValidateNotNull (which is what I would call it in C#) or should I stick with VB terminology and call it ValidateNotNothing instead? Also, in my exception, is it proper to say "cannot be null", or would it be better to say "cannot be Nothing"? I sort of like the way I have it, but since this is VB, maybe I should use Nothing. But since the exception itself is called ArgumentNullException, it feels weird to make the message say "cannot be Nothing". Anyway, I guess it's pretty nitkpicky, just wondered what you folks thought.

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  • Password Cracking in 2010 and Beyond

    - by mttr
    I have looked a bit into cryptography and related matters during the last couple of days and am pretty confused by now. I have a question about password strength and am hoping that someone can clear up my confusion by sharing how they think through the following questions. I am becoming obsessed about these things, but need to spend my time otherwise :-) Let's assume we have an eight-digit password that consists of upper and lower-case alphabetic characters, numbers and common symbols. This means we have 8^96 ~= 7.2 quadrillion different possible passwords. As I understand there are at least two approaches to breaking this password. One is to try a brute-force attack where we try to guess each possible combination of characters. How many passwords can modern processors (in 2010, Core i7 Extreme for eg) guess per second (how many instructions does a single password guess take and why)? My guess would be that it takes a modern processor in the order of years to break such a password. Another approach would consist of obtaining a hash of my password as stored by operating systems and then search for collisions. Depending on the type of hash used, we might get the password a lot quicker than by the bruteforce attack. A number of questions about this: Is the assertion in the above sentence correct? How do I think about the time it takes to find collisions for MD4, MD5, etc. hashes? Where does my Snow Leopard store my password hash and what hashing algorithm does it use? And finally, regardless of the strength of file encryption using AES-128/256, the weak link is still my en/decryption password used. Even if breaking the ciphered text would take longer than the lifetime of the universe, a brute-force attack on my de/encryption password (guess password, then try to decrypt file, try next password...), might succeed a lot earlier than the end of the universe. Is that correct? I would be very grateful, if people could have mercy on me and help me think through these probably simple questions, so that I can get back to work.

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  • In facebook connect, how can I check if a user is a fan of my facebook page? Is it possible to track

    - by Tony
    I am trying to determine if a user is a facebook fan. I load the facebook JS library and initialize: <script type="text/javascript" src="http://static.ak.connect.facebook.com/js/api_lib/v0.4/FeatureLoader.js.php/en_US"></script> FB_RequireFeatures(["XFBML","Connect","Api"], function() { FB.init("my_api_key","xd_receiver.htm") }); FB.ensureInit(function () { FB.ApiClient.pages_isFan(my_profile_id,"some_UID",callback); }); However when I call the API client with FB.ApiClient.pages_isFan, I get a JS error - FB.ApiClient is undefined I am also using the FBML fan tag to display the "become a fan" button: <fb:fan profile_id="my_profile_id" stream="0" connections="10" logobar="1" width="300"></fb:fan> And would like to be notified when either the "become a fan" button is clicked or a user has successfully become a fan. The business logic is pretty simple - If they become a fan, track it in my database. Then if they try to become a fan again, check with the library if they are a fan and say "You are already a fan" if they are a fan, show the widget if not.

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  • Rails - handling global site settings

    - by egarcia
    I'm developing a new rails application which is supposed to be installed several times in order to implement several sites. There are some things, like the "Site Title" or the "Default Number of Items per Page" that clearly belong to a "global settings" table / config file. I've made a list of the things I think I'll need: ActiveRecord model that is capable of: Storing different kinds of data. I suppose this would be accomplished encoding the values on a string on the db, probably with a "type" field. Indexing settings by name Validations based on a "type" attribute (i.e. don't accept invalid dates on "date" settings) Validations based on a allows_nil property. A controller that allows me to change settings via views. I'm pretty sure I could implement this myself, but I'm not willing to reinvent the wheel. I've done some searching, but I could only find rails-settings, which doesn't really serve me: I need a proper model & controller so I can use declarative-authorization, and it does not provide any controller or view facilities. Is there a gem or plugin out there that implements what I want, or any library I should look at? Thanks a lot.

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  • How Does MVC Handle Missing Data Requirements

    - by Don Bakke
    I'm teaching myself MVC concepts in hopes of applying them to a non-OO/procedural development environment. I am pretty sure I understand simple View - Request - Controller - Request - Model - Response - Controller - Response - View flow. What I am struggling with is understanding more complex scenarios. For instance, let's say I have a shopping cart form with a button for 'Calculate Shipping'. Normally a click on this button will follow the above flow. But what if there is missing data, like the zip code? Should the View verify this first and alert the user before making a 'Calculate Shipping' request? Or should the request be made and the Model returns a notification that critical data is missing? If the latter, does the Controller instruct the View to alert the user? What if I wanted to prompt the user for the missing zip code (perhaps in a popup input display) and then automatically request the 'Calculate Shipping' method again? I suppose this gets into the question of how smart a View ought to be. It seems that MVC has evolved due to richer UI and automation (such as with data-binding) and this muddies the water from a purist MVC perspective. Any thoughts are greatly appreciated.

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  • Drawing only part of a

    - by Ben Reeves
    ..Continued on from my previous question I have a 320*480 RGB565 framebuffer which I wish to draw using OpenGL ES 1.0 on the iPhone. - (void)setupView { glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_CROP_RECT_OES, (int[4]){0, 0, 480, 320}); glEnable(GL_TEXTURE_2D); } // Updates the OpenGL view when the timer fires - (void)drawView { // Make sure that you are drawing to the current context [EAGLContext setCurrentContext:context]; //Get the 320*480 buffer const int8_t * frameBuf = [source getNextBuffer]; //Create enough storage for a 512x512 power of 2 texture int8_t lBuf[2*512*512]; memcpy (lBuf, frameBuf, 320*480*2); //Upload the texture glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 512, 512, 0, GL_RGB, GL_UNSIGNED_SHORT_5_6_5, lBuf); //Draw it glDrawTexiOES(0, 0, 1, 480, 320); [context presentRenderbuffer:GL_RENDERBUFFER_OES]; } If I produce the original texture in 512*512 the output is cropped incorrectly but other than that looks fine. However using the require output size of 320*480 everything is distorted and messed up. I'm pretty sure it's the way I'm copying the framebuffer into the new 512*512 buffer. I have tried this routine int8_t lBuf[512][512][2]; const char * frameDataP = frameData; for (int ii = 0; ii < 480; ++ii) { memcpy(lBuf[ii], frameDataP, 320); frameDataP += 320; } Which is better, but the width appears to be stretched and the height is messed up. Any help appreciated.

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  • Improving field get and set performance with ASM or Javassist

    - by ng
    I would like to avoid reflection in an open source project I am developing. Here I have classes like the following. public class PurchaseOrder { @Property private Customer customer; @Property private String name; } I scan for the @Property annotation to determine what I can set and get from the PurchaseOrder reflectively. There are many such classes all using java.lang.reflect.Field.get() and java.lang.reflect.Field.set(). Ideally I would like to generate for each property an invoker like the following. public interface PropertyAccessor<S, V> { public void set(S source, V value); public V get(S source); } Now when I scan the class I can create a static inner class of PurchaseOrder like so. static class customer_Field implements PropertyAccessor<PurchaseOrder, Customer> { public void set(PurchaseOrder order, Customer customer) { order.customer = customer; } public Customer get(PurchaseOrder order) { return order.customer; } } With these I totally avoid the cost of reflection. I can now set and get from my instances with native performance. Can anyone tell me how I would do this. A code example would be great. I have searched the net for a good example but can find nothing like this. The ASM and Javasist examples are pretty poor also. The key here is that I have an interface that I can pass around. So I can have various implementations, perhaps one with Java Reflection as a default, one with ASM, and one with Javassist? Any help would be greatly appreciated.

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  • Is F# a good language for card game AI?

    - by Anthony Brien
    I'm writing a Mahjong Game in C# (the Chinese traditional game, not the solitaire kind). While writing the code for the bot player's AI, I'm wondering if a functional language like F# would be a more suitable language than what I currently use which is C# with a lot of Linq. I don't know much about F# which is why I ask here. To illustrate what I try to solve, here's a quick summary of Mahjong: Mahjong plays a bit like Gin Rummy. You have 13 tiles in your hand, and each turn, you draw a tile and discard another one, trying to improve your hand towards a winning Mahjong hand, which consists or 4 sets and a pair. Sets can be a 3 of a kind (pungs), 4 of a kind (kongs) or a sequence of 3 consecutive tiles (chows). You can also steal another player's discard, if it can complete one of your sets. The code I had to write to detect if the bot can declare 3 consecutive tiles set (chow) is pretty tedious. I have to find all the unique tiles in the hand, and then start checking if there's a sequence of 3 tiles that contain that one in the hand. Detecting if the bot can go Mahjong is even more complicated since it's a combination of detecting if there's 4 sets and a pair in his hand. And that's just a standard Mahjong hand. There's also numerous "special" hands that break those rules but are still a Mahjong hand. For example, "13 unique wonders" consists of 13 specific tiles, "Jade Empire" consists of only tiles colored green, etc. In a perfect world, I'd love to be able to just state the 'rules' of Mahjong, and have the language be able to match a set of 13 tiles against those rules to retrieve which rules it fulfills, for example, checking if it's a Mahjong hand or if it includes a 4 of a kind. Is this something F#'s pattern matching feature can help solve?

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  • AJAX vs ActiveX/Flash for browser-based game

    - by iconiK
    I have been following the usage of JavaScript for the past few years, and with the release of extremely fast scripting engines (V8, SquirrelFish Extrene, TraceMonkey, etc.) the possibilities of JavaScript have increased dramatically. However, the usage share of Internet Explorer coupled with it's total lack of support for recent standards makes me want to drop a bomb on Microsoft's HQ, as it creates a huge amount of problems for any website. The game will need to be pretty dynamic client-side, with animations and other eye-candy things, but not a full-blown game like those that run directly in the OS using DirectX or OpenGL. However, this might be a little stretch for JavaScript and will certainly feel extremely slow in Internet Explorer (given that the current IE engine can be hundreds of times slower than SFX; gotta see what IE9 will bring), would it be better to just do the whole thing in Flash? I know this means requiring the plug-in AND I have no experience whatsoever with Flash (other than browsing YouTube :P). It also means I can't just output directly from PHP, I would have to use XML or some other format to pass data to it (JSON is directly integrated in JS and PHP can deal with it easily). Another idea would be to provide an alternative interface just for IE, though I don't know how (ActiveX maybe? or with Flash, then why not just provide it to all browsers) or totally not supporting it and requiring the use of other browsers, although this is plain stupid from a business perspective. So here am I, wondering what approach to take and thus asking for your advice. How should I build the client-side? AJAX in all browsers, Flash in all browsers or a mix (AJAX for "modern" browsers and something else for the "grandpa": IE).

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  • Google Maps Terms of Service - saving some data to a database

    - by R.M.
    I've read the terms of service, and, from what I understand, I'm not allowed to store any information I retrieve from the Google Maps API. Are there any exceptions to this? More to the point, I'm planning on building an application that shows the user several points of interest (like restaurants, libraries etc) at a certain distance around a location he chooses (it can be in one city or more, depending on the distance he chooses). There are two problems: The first problem is that (at least for my country) the geocoder doesn't locate exact addresses, at best it only locates street names (but completely ignores street numbers) in larger cities. It is even worse for smaller rural areas. So the only way to accurately show the places on the map is by storing their coordinates in the database. Another problem seems to be with calculating distances. To show the points located below a certain distance from the user, it would mean I would have to use GDirections to get all distances between the user's location and the other points, to see which ones to show. That would be really slow for the user (since I also have to set a small delay between requests), and it would also send a pretty large amount of requests to google. Would I be allowed to store those distances in a database? The users would not be able to access a list of all the stored information, they would only see the names of the places, and a map with some markers on it. Thank you.

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  • Java SSH2 libraries in depth: Trilead/Ganymed/Orion [/other?]

    - by Bernd Haug
    I have been searching for a pure Java SSH library to use for a project. The single most important needed feature is that it has to be able to work with command-line git, but remote-controlling command-line tools is also important. A pretty common choice, e.g. used in the IntelliJ IDEA git integration (which works very well), seems to be Trilead SSH2. Looking at their website, it's not being maintained any more. Trilead seems to have been a fork of Ganymed SSH2, which was a ETH Zurich project that didn't see releases for a while, but had a recent release by its new owner, Christian Plattner. There is another actively maintained fork from that code base, Orion SSH, that saw an even more recent release, but which seems to get mentioned online much less than the other 2 forks. Has anybody here worked with any of (or, if possible, both) of Ganymed and Orion and could kindly describe the development experience with either/both? Accuracy of documentation [existence of documentation?], stability, buggyness... - all of these would be highly interesting to me. Performance is not so important for my current project. If there is another pure-Java SSH implementation that should be used instead, please feel free to mention it, but please don't just mention a name...describe your judgment from actual experience. Sorry if this question may seem a bit "do my homework"-y, but I've really searched for reviews. Everything out there seems to be either a listing of implementations or short "use this! it's great!" snippets.

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  • jQuery slideDown: Parent element doesn't expand?

    - by bobsoap
    Hi, This issue occurs in our beloved Internet Explorer 7: I have a list of items, each with a hidden child div. When user clicks the "expand" button in any list item, the hidden div will expand downwards and push all content beneith it lower. This works just as it should in FF, Chrome, IE8 - but IE7 will not expand the parent element along with its children. This is noticeable because the top-most parent container has an absolutely positioned image at the very bottom (yea... rounded corners) - that doesn't get pushed down when the content expands. I'm guessing that's because of the absolute positioning... Just wondering whether I should attempt to code up some huge workaround in jQuery just for that (assuming I'm able to), or if this is a known issue of some sort. My HTML: <div id="container"> <ul> <li>Click here to expand <div class="hide"></div> </li> <li>Click here to expand <div class="hide"></div> </li> <li>Click here to expand <div class="hide"></div> </li> </ul> <div id="containerbottom"></div> </div> The CSS: #container { position:relative; } #container #containerbottom { position:absolute; bottom:0px; left:0px; } The jQuery is pretty much your everyday slide function: $('ul li').click(function() { $(this).children('.hide').slideDown(200); }); Any ideas?

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  • How does jquery display an image received from an ajax request?

    - by Gnee
    I have this working great, but I'd like a deeper understanding of what is actually going on behind the scenes. I am using Jquery's Ajax method to pull 5 blog posts (returning only the title and first photo). A PHP script grabs the blog posts' title and first photo and sticks it in an array and sends it back to my browser as JSON. Upon receiving the JSON object, Jquery grabs the first member of the JSON object and displays it's title and photo. In a gallery I made, using buttons – the user can iterate the 1-5 posts. So the actual AJAX call happens right away, and only once. I am basically using this kind of setup: $('my_div').html(json_obj[i]) and each click does a i++. So jquery is plucking these blog posts from my computers memory, my web browsers cache, or some kind of cache in the Javascript engine? One of the things it's returning is a pretty gnarly animated gif. I just wonder if it constantly running in the background (but not visible), stealing processing cycles...etc. Or Javascript just inserting (say a flash movie) into the DOM, but before hand does nothing but take up a little memory (no processing). Anyway, I'm just curious. If someone is a guru on this, I'd love to hear your take. Thanks!!

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  • PHP - Best practice to retain form values across postback

    - by Adam
    Hello, Complete PHP novice here, almost all my previous work was in ASP.NET. I am now working on a PHP project, and the first rock I have stumbled upon is retaining values across postback. For the most simple yet still realistic example, i have 10 dropdowns. They are not even databound yet, as that is my next step. They are simple dropdowns. I have my entire page inclosed in a tag. the onclick() event for each dropdown, calls a javascript function that will populate the corrosponding dropdowns hidden element, with the dropdowns selected value. Then, upon page reload, if that hidden value is not empty, i set the selected option = that of my hidden. This works great for a single postback. However, when another dropdown is changed, the original 1'st dropdown loses its value, due to its corrosponding hidden value losing its value as well! This draws me to look into using querystring, or sessions, or... some other idea. Could someone point me in the right direction, as to which option is the best in my situation? I am a PHP novice, however I am being required to do some pretty intense stuff for my skill level, so I need something flexable and preferribly somewhat easy to use. Thanks! -----edit----- A little more clarification on my question :) When i say 'PostBack' I am referring to the page/form being submitted. The control is passed back to the server, and the HTML/PHP code is executed again. As for the dropdowns & hiddens, the reason I used hidden variables to retain the "selected value" or "selected index", is so that when the page is submitted, I am able to redraw the dropdown with the previous selection, instead of defaulting back to the first index. When I use the $_POST[] command, I am unable to retrieve the dropdown by name, but I am able to retrieve the hidden value by name. This is why upon dropdown-changed event, I call javascript which sets the selected value from the dropdown into its corrosponding hidden.

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  • 2 different routes on one page?

    - by Dejan.S
    Hi I'm pretty new with MVC2 or MVC in general. If it's one thing I get caught up with it's routes. Like now I got this scenario. Im going from the regular site to Admin. My navigation is the same partialview on both I just do a check which data to render something like this. <% if (!Request.RawUrl.Contains("Admin")){%> <% foreach (var site in Model) { %> <%= Html.MenuItem(site.BelongSite, "Sida", "Site", site.BelongSite) %> | <%} %> <%} else {%> <%= Html.ActionLink("Konfig", "Konfigurera", "Admin") %> <% } %> My route looks like this routes.MapRoute( "Admin", // Route name "Admin/{action}/{name}", // URL with parameters new { controller = "Admin", action = "konfigurera", name = UrlParameter.Optional } // Parameter defaults ); On my View called Konfigurera I got Edit sites and they use the route above and it works great. The navigation tho dont get no action assigned to it. It's just <a href='Admin/'> The navigation is in the shared folder, and it is a strongly typed. Any Ideas? I been struggling with this for about a hour now Thanks for any input

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  • Foreach loop returning null values in PHP?

    - by Jascha
    Hello, I have a pretty simple problem. Basically I have an array called $list that is a list of titles. If I do a print_r($list) I get these results: Array ( [0] => Another New Title [1] => Awesome Movies and stuff [2] => Jascha's Title ) Now, I'm running a foreach loop to retrieve their values and format them in an <ul> like so... function get_film_list(){ global $categories; $list = $categories->get_film_list(); if(count($list)==0){ echo 'No films are in this category'; }else{ echo '<ul>'; foreach($list as $title){ echo '<li>' . $title . '<li>'; } echo '</ul>'; } } The problem I'm having is my loop is returning two values per value (is it the key value?) The result of the preceding function looks like this: Another New Title   Awesome Movies and stuff   Jascha's Title   I even tried: foreach($list as $key => $title){ echo '<li>' . $title . '<li>'; } With the same results: Another New Title   Awesome Movies and stuff   Jascha's Title   What am I missing here? Thanks in advance.

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  • Counting computers for each lab

    - by Irvin
    Alright I have a problem with having to count PCs, and Macs from different labs. In each lab I need to display how many PC and Macs there is available. The data is coming from a SQL server, right am trying sub queries and the use of union, this the closest I can get to what I need. The query below shows me the number of PCs, and Macs in two different columns, but of course, the PCs will be in one row and the Macs on another right below it. Having the lab come up twice. EX: LabName -- PC / MAC Lab1 -- 5 / 0 Lab1 -- 0 / 2 Query SELECT Labs.LabName, COUNT(*),0 AS Mac FROM HardWare INNER JOIN Labs ON HardWare.LabID = Labs.LabID WHERE ComputerStatus = 'AVAILABLE' GROUP BY Labs.LabName UNION SELECT Labs.LabName, COUNT(*), (SELECT COUNT(Manufacturer)) AS Mac FROM HardWare INNER JOIN Labs ON HardWare.LabID = Labs.LabID WHERE ComputerStatus = 'AVAILABLE' AND Manufacturer = 'Apple' GROUP BY Labs.LabName ORDER BY Labs.LabName So is there any way to get them together in one row as in Lab1 -- 5 / 2 or is there a different way to write the query? anything will be a big help, am pretty much stuck here. Cheers

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  • proper use of volatile keyword

    - by luke
    I think i have a pretty good idea about the volatile keyword in java, but i'm thinking about re-factoring some code and i thought it would be a good idea to use it. i have a class that is basically working as a DB Cache. it holds a bunch of objects that it has read from a database, serves requests for those objects, and then occasionally refreshes the database (based on a timeout). Heres the skeleton public class Cache { private HashMap mappings =....; private long last_update_time; private void loadMappingsFromDB() { //.... } private void checkLoad() { if(System.currentTimeMillis() - last_update_time > TIMEOUT) loadMappingsFromDB(); } public Data get(ID id) { checkLoad(); //.. look it up } } So the concern is that loadMappingsFromDB could be a high latency operation and thats not acceptable, So initially i thought that i could spin up a thread on cache startup and then just have it sleep and then update the cache in the background. But then i would need to synchronize my class (or the map). and then i would just be trading an occasional big pause for making every cache access slower. Then i thought why not use volatile i could define the map reference as volatile private volatile HashMap mappings =....; and then in get (or anywhere else that uses the mappings variable) i would just make a local copy of the reference: public Data get(ID id) { HashMap local = mappings; //.. look it up using local } and then the background thread would just load into a temp table and then swap the references in the class HashMap tmp; //load tmp from DB mappings = tmp;//swap variables forcing write barrier Does this approach make sense? and is it actually thread-safe?

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  • Merging sql queries to get different results by date

    - by pedalpete
    I am trying to build a 'recent events' feed and can't seem to get either my query correct, or figure out how to possible merge the results from two queries to sort them by date. One table holds games/, and another table holds the actions of these games/. I am trying to get the recent events to show users 1) the actions taken on games that are publicly visible (published) 2) when a new game is created and published. So, my actions table has actionId, gameid, userid, actiontype, lastupdate My games table has gameid, startDate, createdby, published, lastupdate I currently have a query like this (simplified for easy understanding I hope). SELECT actionId, actions.gameid, userid, actiontype, actions.lastupdate FROM actions JOIN ( SELECT games.gameid, startDate, createdby, published, games.lastupdate FROM games WHERE published=1 AND lastupdate>today-2 ) publishedGames on actions.gameid=games.gameid WHERE actions.type IN (0,4,5,6,7) AND actions.lastupdate>games.lastupdate and published=1 OR games.lastupdate>today-2 AND published=1 This query is looking for actions from published games where the action took place after the game was published. That pretty much takes care of the first thing that needs to be shown. However, I also need to get the results of the SELECT games.gameid, startDate, createdby, published, games.lastupdate FROM games WHERE published=1 AND startDate>today-2 so I can include in the actions list, when a new game has been published. When I run the query as I've got it written, I get all the actionids, and their gameids, but I don't get a row which shows the gameid when it was published. I understand that it may be possible that I need to run two seperate queries, and then somehow merge the results afterword with php, but I'm completely lost on where to start with that as well.

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  • CSS Design question, I've got myself completely turned around.

    - by Matt Dawdy
    Okay, I have a couple of other questions out there, but I think I'd better just ask from the beginning how you CSS experts would do this. Client's page is split into 2 rows -- header has some info, some aligned to left of page, some to right, some in the middle. This is currently done using a table. I'm fine with leaving this alone or changing it. My real question is that I need a page layout to handle the following: 2 columns - column on left is 200px, but can be "close" down to to 10px (not a slider, it's either 200 or 10 px). The column on the right needs to be as big as it needs to be -- which might be larger than the width of the page. When left column is "closed" then the right column slides over of course. Again, this right column might be 300px or it might be 4000 pixels (it's a reporting interface). Now, to add another wrinkle, SOME pages have 3 columns. The first 2 columns are each exactly 200px, and both can be "closed" down to 10 px each. But, the user may not close both columns, maybe just 1. Or none. Or both. The third column needs to act just like I described above, being able to be larger than the page width, and sliding over to take advantage of any of the "closed" left columns. Whew! I'm pretty confused as to how to go about this, as either I get it right but I can't scroll over to the right at all (overflow: hidden) and information is lost, or the right column jumps down below the left 2 columns and just looks plain stupid. My minimum browser requirements are IE8, FF3.5, Chrome and Safari (latest versions of all). Any and all pointers are gladly accepted.

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  • Help understanding .NET delegates, events, and eventhandlers

    - by Seth Spearman
    Hello, In the last couple of days I asked a couple of questions about delegates HERE and HERE. I confess...I don't really understand delegates. And I REALLY REALLY REALLY want to understand and master them. (I can define them--type safe function pointers--but since I have little experience with C type languages it is not really helpful.) Can anyone recommend some online resource(s) that will explain delegates in a way that presumes nothing? This is one of those moments where I suspect that VB actually handicaps me because it does some wiring for me behind the scenes. The ideal resource would just explain what delegates are, without reference to anything else like (events and eventhandlers), would show me how all everything is wired up, explain (as I just learned) that delegates are types and what makes them unique as a type (perhaps using a little ildasm magic)). That foundation would then expand to explain how delegates are related to events and eventhandlers which would need a pretty good explanation in there own right. Finally this resource could tie it all together using real examples and explain what wiring DOES happen automatically by the compiler, how to use them, etc. And, oh yeah, when you should and should not use delegates, in other words, downsides and alternatives to using delegates. What say ye? Can any of you point me to resource(s) that can help me begin my journey to mastery? EDIT One last thing. The ideal resource will explain how you can and cannot use delegates in an interface declaration. That is something that really tripped me up. Thanks for your help. Seth

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  • Sql query - selecting top 5 rows and further selecting rows only if User is present

    - by Gublooo
    Hello, I kind of stuck on how to implement this query - this is pretty similar to the query I posted earlier but I'm not able to crack it. I have a shopping table where everytime a user buys anything, a record is inserted. Some of the fields are * shopping_id (primary key) * store_id * user_id Now what I need is to pull only the list of those stores where he's among the top 5 visitors: When I break it down - this is what I want to accomplish: * Find all stores where this UserA has visited * For each of these stores - see who the top 5 visitors are. * Select the store only if UserA is among the top 5 visitors. The corresponding queries would be: select store_id from shopping where user_id = xxx select user_id,count(*) as 'visits' from shopping where store_id in (select store_id from shopping where user_id = xxx) group by user_id order by visits desc limit 5 Now I need to check in this resultset if UserA is present and select that store only if he's present. For example if he has visited a store 5 times - but if there are 5 or more people who have visited that store more than 5 times - then that store should not be selected. So I'm kind of lost here. Thanks for your help

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