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• How to check whether user is login in web application?

  - by Morgan Cheng

  I want to learn the whole details of web application authentication. So, I decided to write a CodeIgniter authentication library from scratch. Now, I have to make design decision about how to determine whether one user is login. Basically, after user input username & password pair. A cookie is set for this session, following navigations in the web application will not require username & password. The server side will check whether the session cookie is valid to determine whether current user is login. The question is: how to determine whether cookie is valid cookie issued from server side? I can image the most simple way is to have the cookie value stored in session status as well. For each HTTP request, compare the value from cookie and the value from server session. (Since CodeIgniter session library store session variables in cookies, it is not applicable without some tweak.) This method requires storage in server side. For huge web application that is deployed in multiple datacenters. It is possible that user input username & password when browsing in one datacenter, while he/she access the web application in another datacenter later. The expected behavior is that user just input username & password once. As a result, all datacenters should be able to access the session status. That is possible not applicable even the session status is stored in external storage such as database. I tried Google. I login Google with Asian proxy which is supposed to direct me to datacenters in Asian. Then I switch to North American proxy which should direct me to datacenters in North America. It recognize my login without asking username and password again. So, is there any way to determine whether user is login without server side session status?

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• c++ exercise question

  - by Djecua

  in need of help on a C++ program that will help a deli owner help her customers.Prompt the user for the number of bagels.if then calculate the customers payment two ways. first it finds the price of the smallest multiple of 13 bagels that is closet to the customer's order. it then calculates the price of the customer's order so that the customer's payment is the smallest amount possible with the customer getting the exact number of bagels ordered. if the first method is the smallest amount the program outputs the number of bagels the customer will receive along with the dollar amount owed and the dollar amount saved. otherwise just print the number of bagels received the dollar amount owed $ 3.80 for a dozen bagels (13 bagels) half a dozen(6 bagels) sing bagel cost $.50 owner of the deli is a honest merchant, customers always get the best price on an order, even if they get more bagels than ordered, for example a customer that orders 10 bagels would pay $ 4.60 (2.60 for a half dozen bagels plus $2.00 for 4 single bagels.merchant gives her customer 13 bagels saving the customer $0.80 If someone can example to me how would i design to give customer extra bagels and how to calculate cost.

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• What is the appropriate HTML 5 element for a hero unit/showcase?

  - by deb

  A lot of marketing and content-heavy sites showcase the page's primary content using large text and/or images, sometimes with a slider, containing a call to action for signing up for a service, or downloading an app, etc.. I'm not sure what this design element is called, I got the term hero unit from twitter bootstrap: http://twitter.github.com/bootstrap/components.html#typography I think most of you know what I'm trying to describe... If it's not clear I can add screenshots or links to this question. I looked at a few different sites, and some put this hero unit inside a ASIDE element, others use SECTION, ARTICLE and even HEADER. Using twitter bootstrap as an example again: <header class="jumbotron masthead"> <div class="inner"> <h1>Bootstrap, from Twitter</h1> <p>Simple and flexible HTML, CSS, and Javascript for popular user interface components and interactions.</p> <p class="download-info"> Is HEADER the most appropriate tag for this type of content? Or should I use ASIDE, ARTICLE or SECTION?

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• Loading Dimension Tables - Methodologies

  - by Nev_Rahd

  Hello, Recently I been working on project, where need to populated Dim Tables from EDW Tables. EDW Tables are of type II which does maintain historical data. When comes to load Dim Table, for which source may be multiple EDW Tables or would be single table with multi level pivoting (on attributes). Mean: There would be 10 records - one for each attribute which need to be pivoted on domain_code to make a single row in Dim. Out of these 10 records there would be some attributes with same domain_code but with different sub_domain_code, which needs further pivoting on subdomain code. Ex: if i got domain code: 01,02, 03 = which are straight pivot on domain code I would also have domain code: 10 with subdomain code / version as 2006,2007,2008,2009 That means I need to split my source table with above attributes into two = one for domain code and other for domain_code + version. so far so good. When it comes to load Dim Table: As per design specs for Dimensions (originally written by third party), what they want is: for every single change in EDW (attribute), it should assemble all the related records (for that NK) mean new one with other attribute values which are current = process them to create a new dim record and insert it. That mean if a single extract contains 100 records updated (one for each NK), it should assemble 100 + (100*9) records to insert / update dim table. How good is this approach. Other way I tried to do is just do a lookup into dim table for that NK get the value's of recent records (attributes which not changed) and insert it and update the current one. What would be the better approach assembling records at source side for one attribute change or looking into dim table's recent record and process it. If this doesn't make sense, would like to elaborate it further. Thanks

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• How can I limit asp.net control actions based on user role?

  - by Duke

  I have several pages or views in my application which are essentially the same for both authenticated users and anonymous users. I'd like to limit the insert/update/delete actions in formviews and gridviews to authenticated users only, and allow read access for both authed and anon users. I'm using the asp.net configuration system for handling authentication and roles. This system limits access based on path so I've been creating duplicate pages for authed and anon paths. The solution that comes to mind immediately is to check roles in the appropriate event handlers, limiting what possible actions are displayed (insert/update/delete buttons) and also limiting what actions are performed (for users that may know how to perform an action in the absence of a button.) However, this solution doesn't eliminate duplication - I'd be duplicating security code on a series of pages rather than duplicating pages and limiting access based on path; the latter would be significantly less complicated. I could always build some controls that offered role-based configuration, but I don't think I have time for that kind of commitment right now. Is there a relatively easy way to do this (do such controls exist?) or should I just stick to path-based access and duplicate pages? Does it even make sense to use two methods of authorization? There are still some pages which are strictly for either role so I'll be making use of path-based authorization anyway. Finally, would using something other than path-based authorization be contrary to typical asp.net design practices, at least in the context of using the asp.net configuration system?

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• How big can I make an Android application's canvas in terms of pixels?

  - by user279112

  I've determined an estimate of the size of my Android emulator's screen in pixels, although I think its resolution can be changed to other numbers. Quite frankly though that doesn't eliminate the general problem of not knowing how many pixels on each axis I have to work with on my Android applications in general. The main problem I'm trying to solve is this: How do I make sure I don't use a faulty resolution on Android applications if I want to keep things' sizes constant (so that if the application screen shrinks, for instances, objects will still show up just as big - there just won't be as many of them being shown) if I wish to do this with a single universal resolution for each program? Failing that, how do I make sure everything's alright if I try to do everything the same way with maybe a few different pre-set resolutions? Mainly it seems like a relevant question that must be answered before I can come across a complete answer for the general problem is how big can I always make my application in pixels, NOT regarding if and when a user resizes the application's screen to something smaller than the maximum size permitted by the phone and its operating system. I really want to try to keep this simple. If I were doing this for a modern desktop, for instance, I know that if I design the application with a 800x600 canvas, the user can still shrink the application to the point they're not doing themselves any favors, but at least I can basically count on it working right and not being too big for the monitor or something. Is there such a magic resolution for Android, assuming that I'm designing for API levels 3+ (Android 1.5+)? Thanks

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• SIMPLE PHP MVC Framework [closed]

  - by Allen

  I need a simple and basic MVC example to get me started. I don't want to use any of the available packaged frameworks. I am in need of a simple example of a simple PHP MVC framework that would allow, at most, the basic creation of a simple multi-page site. I am asking for a simple example because I learn best from simple real world examples. Big popular frameworks (such as code igniter) are to much for me to even try to understand and any other "simple" example I have found are not well explained or seem a little sketchy in general. I should add that most examples of simple MVC frameworks I see use mod_rewrite (for URL routing) or some other Apache-only method. I run PHP on IIS. I need to be able to understand a basic MVC framework, so that I could develop my own that would allow me to easily extend functionality with classes. I am at the point where I understand basic design patterns and MVC pretty well. I understand them in theory, but when it comes down to actually building a real world, simple, well designed MVC framework in PHP, I'm stuck. Edit: I just want to note that I am looking for a simple example that an experienced programmer could whip up in under an hour. I mean simple as in bare bones simple. I don't want to use any huge frameworks, I am trying to roll my own. I need a decent SIMPLE example to get me going.

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• JasperReports reports access image via authenticated URL

  - by user363115

  Hi all, I am hoping that this is a simple issue with a simple solution and that I have missed something obvious. Let me explain the problem; We have an application that generates PDF reports (using Jasper). These reports contain data from our database, as well as imagery (photographs). These photographs are stored in S3. We use signed URLs to access these photographs. We link these photographs into our Jasper reports using these S3 URLs. Because the S3 URLs are signed and time-limited (by design), the process is as follows; User requests a report to be generated, Report is filled, and goes to our database (at which time UUIDs to any required images are retrieved), For each UUID an S3 signed URL must be generated, To do this the URL behind each report image is a call to an authenticated URL in our app (/get_img?uuid=foo), The controller behind this URL generates a signed S3 URL and returns it, Reports loads the image. The problem is with step (4) - the call to the authenticated URL fails because Jasper does not pass any authentication information with the request. Is there a solution here? Thanks all for your time. Ben

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• Assembly-wide / root-level styles in WPF class library

  - by WarpedBoard

  I have a C# (2008/.NET 3.5) class library assembly that supports WPF (based on http://dotupdate.wordpress.com/2007/12/05/how-to-add-a-wpf-control-library-template-to-visual-c-express-2008/). I've created several windows, and am now attempting to create a common style set for them. However, as it's a class library (instead of a WPF app), I don't have an app.xaml (and its contained Application & corresponding Application.Resources) in which to store these styles for global access. So: How can I create a top-level set of style definitions that'll be seen by all xaml files in the assembly, given that I do not have app.xaml (see above)? And/or is it possible to add a working app.xaml to a class library? FYI, I did try creating a ResourceDictionary in a ResourceDictionary.xaml file, and include it in each window within a "Window.Resources" block. That turned out to solve the styling of Buttons, etc... but not for the enclosing Window. I can put 'Style="{StaticResource MyWindowStyle}"' in the Window's opening block, and it compiles and shows up in the VS Design window fine, but during actual runtime I get a parse exception (MyWindowStyle could not be found; I'm guessing Visual Studio sees the dictionary included after the line in question, but the CRL does things more sequentially and therefore hasn't loaded the ResourceDictionary yet).

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• Changing <object> height and width works in Chrome but not Firefox or IE. Why?

  - by Michael Hopkins

  I am making a site with two Youtube videos. These videos use the raw embed code from Youtube. The site's design doesn't work with any of the default Youtube sizes, so I am writing code to automatically resize the video. Here is my code. There will never be more than these two tags on the page, otherwise I'd do a better job selecting the videos. <script language='JavaScript' type='text/javascript'> var x=document.getElementsByTagName('object'); x.[0].width='350'; x.[0].height='350'; x.[1].width='350'; x.[1].height='350'; </script> For reference, here's a sample default Youtube embed that the code might alter: <object width="480" height="385"> <param name="movie" value="http://www.youtube-nocookie.com/v/zSgiXGELjbc&hl=en_US&fs=1&rel=0"></param> <param name="allowFullScreen" value="true"></param> <param name="allowscriptaccess" value="always"></param> <embed src="http://www.youtube-nocookie.com/v/zSgiXGELjbc&hl=en_US&fs=1&rel=0" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed> </object> In Chrome, the video players sit perfectly in a 350x350 box. In IE and FF (latest versions), the videos are the unchanged, normal size. I cannot find anything in Google that explans why this won't work. I have tried using setattribute, for loops, adjusting both and , single-quotes and double-quotes, etc. Any ideas what is going wrong?

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• Error handling in C++, constructors vs. regular methods

  - by Dennis Ritchie

  I have a cheesesales.txt CSV file with all of my recent cheese sales. I want to create a class CheeseSales that can do things like these: CheeseSales sales("cheesesales.txt"); //has no default constructor cout << sales.totalSales() << endl; sales.outputPieChart("piechart.pdf"); The above code assumes that no failures will happen. In reality, failures will take place. In this case, two kinds of failures could occur: Failure in the constructor: The file may not exist, may not have read-permissions, contain invalid/unparsable data, etc. Failure in the regular method: The file may already exist, there may not be write access, too little sales data available to create a pie chart, etc. My question is simply: How would you design this code to handle failures? One idea: Return a bool from the regular method indicating failure. Not sure how to deal with the constructor. How would seasoned C++ coders do these kinds of things?

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• Recommended/Standard image thumbnails sizes

  - by Rakan

  Hello, Based on my experience in web development, HTML page designs for every website may have different < img tags showing the same image in different sizes. i.e: the website's main page might show the image in 500*500, the listing page might show it in 200*150 and other sections of the website will use different sizes. As you can see the ratios of the images to be displayed to the user are different and therefore will cause a problem in having the image stretched, pixelated and/or other quality problems. How i currently handle this issue is by using a jQuery cropping tool to make the user select the area to crop the thumbnail from. the cropped image is then resized using imagick to fit every position in the page design. However, although i can force the user to crop an image based on a pre-defined ratio, the HTML will still contain image placeholders which require a different ratio for its thumbnail. So for every image, i will have to force the user to crop the uploaded image based on every possible ratio my site requires. I don't think this is a friendly solution, how would you do it? is there a standard/recommended ratio for the web? Thanks

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• Javascript and jQuery (Fancybox) question

  - by songdogtech

  Javascript and jQuery (Fancybox) question I'm using the Javascript function below for Twitter sharing (as well as other services; the function code is simplified to just Twitter for this question) that grabs the to-be-shared page URL and title and it is invoked in the link with onclick. That results in the Twitter share page loading in a pop up browser window, i.e.<img src="/images/twitter_16.png" onclick="share.tw()" /> In order to be consistent with other design aspects of the site, what I'd like to be able to do is have the Twitter share page open not in a standard browser window but in a Fancybox (jQuery) window. Fancybox can load an external page in an iFrame when the img or href link contains a class (in this case class="iframe" ) in the link and in the document ready function in the header. Right now, of course, when I give the iframe class to the link that also has the onclick share.tw(), I get two popups: one browser window popup with the correct Twitter share page loaded, and a Fancybox jQuery popup that shows a site 404. How can I change the function to use Fancybox to present the Twitter share page? Is that a correct way to approach it? Or is there a better way, such as implementing the share function in jQuery, too? Thanks... Javascript share function: var share = { tw:function(title,url) { this.share('http://twitter.com/home?status=##URL##+##TITLE##',title,url); }, share:function(tpl,title,url) { if(!url) url = encodeURIComponent(window.location); if(!title) title = encodeURIComponent(document.title); tpl = tpl.replace("##URL##",url); tpl = tpl.replace("##TITLE##",title); window.open(tpl,"sharewindow"+tpl.substr(6,15),"width=640,height=480"); } }; It is invoked, i.e.: <img src="/images/twitter_16.png" onclick="share.tw()" /> Fancybox function, invoked by adding class="iframe" in the img or href link $(".iframe").fancybox({ 'width' : '100%', 'height' : '100%', 'autoScale' : false, 'transitionIn' : 'none', 'transitionOut' : 'none', 'type' : 'iframe' });

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• Modelling in Agile Development

  - by bertzzie

  I'm writing a bachelor dissertation report where I'm developing a system with Agile methodology. Given that the development is an one man show, of course the "Agile" I did was not really agile at all (from my understanding at least). So I want some perspective from SO crowds, who is of course a professional, real world, developer with tons of experience. I think real world experience is better than the theory and experiments that I did. My question is: Do we model during development time when using Agile? UML? DFD? Or a Functional Specification is enough1? If modelling is not really necessary, what do we use to communicate to the user, as the user almost always won't understand UML or DFD? For my system, I use UI & UX Design with heavy prototyping, but then I don't have time to draw UML any more. Which one is better? 1 http://www.joelonsoftware.com/articles/fog0000000036.html I hope the question's not "subjective and argumentative" as I know this question exist because of my lack of understanding in the agile development. If it is, could someone just give me a pointer or reference about that? Possible duplicate: Do you use UML in Agile development practices?

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• How should I return different types in a method based on the value of a string in Java?

  - by Siracuse

  I'm new to Java and I have come to having the following problem: I have created several classes which all implement the interface "Parser". I have a JavaParser, PythonParser, CParser and finally a TextParser. I'm trying to write a method so it will take either a File or a String (representing a filename) and return the appropriate parser given the extension of the file. Here is some psuedo-code of what I'm basically attempting to do: public Parser getParser(String filename) { String extension = filename.substring(filename.lastIndexOf(".")); switch(extension) { case "py": return new PythonParser(); case "java": return new JavaParser(); case "c": return new CParser(); default: return new TextParser(); } } In general, is this the right way to handle this situation? Also, how should I handle the fact that Java doesn't allow switching on strings? Should I use the .hashcode() value of the strings? I feel like there is some design pattern or something for handling this but it eludes me. Is this how you would do it?

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• Need help tuning a SQL statement

  - by jeffself

  I've got a table that has two fields (custno and custno2) that need to be searched from a query. I didn't design this table, so don't scream at me. :-) I need to find all records where either the custno or custno2 matches the value returned from a query on the same table based on a titleno. In other words, the user types in 1234 for the titleno. My query searches the table to find the custno associated with the titleno. It also looks for the custno2 for that titleno. Then it needs to do a search on the same table for all other records that have either the custno or custno2 returned in the previous search in the custno or custno2 fields for those other records. Here is what I've come up with: SELECT BILLYR, BILLNO, TITLENO, VINID, TAXPAID, DUEDATE, DATEPIF, PROPDESC FROM TRCDBA.BILLSPAID WHERE CUSTNO IN (select custno from trcdba.billspaid where titleno = '1234' union select custno2 from trcdba.billspaid where titleno = '1234' and custno2 != '') OR CUSTNO2 IN (select custno from trcdba.billspaid where titleno = '1234' union select custno2 from trcdba.billspaid where titleno = '1234' and custno2 != '') The query takes about 5-10 seconds to return data. Can it be rewritten to work faster?

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• Using a CMS with an external database

  - by George Reith

  I am looking at building an external site with a CMS, probably Drupal or ExpressionEngine. The problem is that our company already has a membership database that is designed to work with our existing enterprise software. Migrating data from the database manually is not an option as modifications and new data must be accessible in real-time. Because the design of the external database will differ from the CMS's own I have decided the best way forward is to use two databases and force the CMS to use the external to read user information (cannot write to) and a local for everything else the CMS needs to do (read + write). Is this feasible with these Drupal or ExpressionEngine? Ideally I need to be able to use hooks as I do not wan't to modify core CMS files. Sifting through the docs I am not able to find what I would hook into for ether CMS. (Note: I know it is possible, but I want to know if it's feasible). Finally if there is a better way of handling this situation please also chime in. Perhaps there is something at the database level to reference a field or table in an external database? I'm clutching at straws someone can point me in the right direction I'm sure.

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• QT NOOB: Add action handler for multiple objects of same type.

  - by what

  I have a simple QT application with 10 radio buttons with names radio_1 through radio_10. It is a ui called Selector, and is part of a class called TimeSelector In my header file for this design, i have this: //! [1] class TimeSelector : public QWidget { Q_OBJECT public: TimeSelector(QWidget *parent = 0); private slots: //void on_inputSpinBox1_valueChanged(int value); //void on_inputSpinBox2_valueChanged(int value); private: Ui::Selector ui; }; //! [1] the commented out void_on_inputSpinBox1_valueChanged(int value) is from the tutorial for the simple calculator. I know i can do: void on_radio_1_valueChanged(int value); but I would need 10 functions. I want to be able to make one function that works for everything, and lets me pass in maybe a name of the radio button that called it, or a reference to the radio button so i can work with it and determine who it was. I am very new to QT but this seems like it should be very basic and doable, thanks.

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• Help with mysql sum and group query and managing results for jquery graph.

  - by Scarface

  I have a system I am trying to design that will retrieve information from a database, so that it can be plotted in a jquery graph. I need to retrieve the information and somehow put it in the necessary coordinates format (for example two coordinates var d = [[1269417600000, 10],[1269504000000, 15]];). My table that I am selecting from is a table that stores user votes with fields: points_id (1=vote up, 2=vote down), user_id, timestamp, topic_id What I need to do is select all the votes and somehow group them into respective days and then sum the difference between 1 votes and 2 votes for each day. I then need to somehow display the data in the appropriate plotting format shown earlier. For example April 1, 4 votes. The data needs to be separated by commas, except the last plot entry, so I am not sure how to approach that. I showed an example below of the kind of thing I need but it is not correct, echo "var d=["; $query=mysql_query( "SELECT *, SUM(IF(points_id = \"1\", 1,0))-SUM(IF([points_id = \"2\", 1,0)) AS 'total' FROM points LEFT JOIN topic ON topic.topic_id=points.topic_id WHERE topic.creator='$user' GROUP by timestamp HAVING certain time interval" ); while ($row=mysql_fetch_assoc($query)){ $timestamp=$row['timestamp']; $votes=$row['total']; echo "[$timestamp,$vote],"; } echo "];";

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• Custom Android control with children

  - by Gromix

  Hi, I'm trying to create a custom Android control that contains a LinearLayout. You can think of it as an extended LinearLayout with fancy borders, a background, an image on the left... I could do it all in XML (works great) but since I have dozens of occurences in my app it's getting hard to maintain. I thought it would be nicer to have something like this: /* Main.xml */ <MyFancyLayout> <TextView /> /* what goes inside my control's linear layout */ </MyfancyLayout> How would you approach this? I'd like to avoid re-writing the whole linear layout onMeasure / onLayout methods. This is what I have for the moment: /* MyFancyLayout.xml */ <TableLayout> <ImageView /> <LinearLayout id="container" /> /* where I want the real content to go */ </TableLayout> and /* MyFancyLayout.java */ public class MyFancyLayout extends LinearLayout { public MyFancyLayout(Context context) { super(context); View.inflate(context, R.layout.my_fancy_layout, this); } } How would you go about inserting the user-specified content (the TextView in main.xml) in the right place (id=container)? Cheers! Romain ----- edit ------- Still no luck on this, so I changed my design to use a simpler layout and decided to live with a bit of repeated XML. Still very interested in anyone knows how to do this though!

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• DotNetOpeId - Using OpenIdButton for Google Apps

  - by JediPotPie

  I am an ASP.NET newbie and I am trying to design an OpenID/SSO system for an internal web application. The web application is pretty simple and the authentication is currently being managed by a database with usernames and passwords. I want to replace the existing accounts stored in the database with Google Apps accounts. I have downloaded the latest DotNetOpenAuth-3.4.3.10103 package and got the OpenIdRelyingPartyWebForms sample up and running on IIS. I have built my own login page using just a OpenIdButton object that points to a development Google domain. The button seems to work fine in FireFox, at least it is forwarding me to the Google Apps login, but nothing happens when I load the same page in IE. When I click the Google button, nothing happens, zip. The same is true for the Yahoo button in the login.aspx page given in the sample. Here is the .aspx code I am using... <rp:OpenIdButton runat="server" ImageUrl="http://www.google.com/accounts/google_transparent.gif" Text="Login with Google!" ID="googleLoginButton" Identifier="https://www.google.com/accounts/o8/site-xrds?hd=dev.connexcloud.com" />

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• PHP Combo Box AJAX Refresh

  - by bhs

  I have a PHP page that currently has 4 years of team positions in columns on the page. The client wants to select the players in positions and have first, second and thrid choices. Currently the page shows 4 columns with sets of combos for each position. Each combo has a full set of players in it and the user chooses the player he wants from the combos. On submit the player positions are stored in the database. However, the client now wants to change the page so that when he selects a player in a year and position then the player is removed from the combo and can no longer be selected for that year. I've used a bit of AJAX but was wondering if anyone had any thoughts/suggestions. The page is currently quite slow so they want it speeded up as well. The page layout is currently like this POISTION YEAR1 YEAR2 YEAR3 YEAR4 1 COMBOC1 COMBOC1 COMBOC1 COMBOC1 COMBOC2 COMBOC2 COMBOC2 COMBOC2 COMBOC3 COMBOC3 COMBOC3 COMBOC3 2 same as above COMBOC1, 2 and 3 all currently have the same players - when a player is selected it needs to be removed for all combos below it. I was thinking of starting by changing the page design and having text boxes for the players and a single player select under each year like this: POISTION YEAR1 YEAR2 YEAR3 YEAR4 1 <PLAYER><POSITION><CHOICE> ... [TEXT BOX CHOICE1] [TEXT BOX CHOICE2] [TEXT BOX CHOICE3] 2 ... Then I only have 1 combo box for each year to worry about - I do however have the same problem of refreshing the combo box and removing the player that has been selected, and I'd prefer to do it withough a page submit. Sorry for the long posting - cheers

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• Appropriate uses of Monad `fail` vs. MonadPlus `mzero`

  - by jberryman

  This is a question that has come up several times for me in the design code, especially libraries. There seems to be some interest in it so I thought it might make a good community wiki. The fail method in Monad is considered by some to be a wart; a somewhat arbitrary addition to the class that does not come from the original category theory. But of course in the current state of things, many Monad types have logical and useful fail instances. The MonadPlus class is a sub-class of Monad that provides an mzero method which logically encapsulates the idea of failure in a monad. So a library designer who wants to write some monadic code that does some sort of failure handling can choose to make his code use the fail method in Monad or restrict his code to the MonadPlus class, just so that he can feel good about using mzero, even though he doesn't care about the monoidal combining mplus operation at all. Some discussions on this subject are in this wiki page about proposals to reform the MonadPlus class. So I guess I have one specific question: What monad instances, if any, have a natural fail method, but cannot be instances of MonadPlus because they have no logical implementation for mplus? But I'm mostly interested in a discussion about this subject. Thanks! EDIT: One final thought occured to me. I recently learned (even though it's right there in the docs for fail) that monadic "do" notation is desugared in such a way that pattern match failures, as in (x:xs) <- return [] call the monad's fail. It seems like the language designers must have been strongly influenced by the prospect of some automatic failure handling built in to haskell's syntax in their inclusion of fail in Monad.

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• Code Contracts: Do we have to specify Contract.Requires(...) statements redundantly in delegating me

  - by herzmeister der welten

  I'm intending to use the new .NET 4 Code Contracts feature for future development. This made me wonder if we have to specify equivalent Contract.Requires(...) statements redundantly in a chain of methods. I think a code example is worth a thousand words: public bool CrushGodzilla(string weapon, int velocity) { Contract.Requires(weapon != null); // long code return false; } public bool CrushGodzilla(string weapon) { Contract.Requires(weapon != null); // specify contract requirement here // as well??? return this.CrushGodzilla(weapon, int.MaxValue); } For runtime checking it doesn't matter much, as we will eventually always hit the requirement check, and we will get an error if it fails. However, is it considered bad practice when we don't specify the contract requirement here in the second overload again? Also, there will be the feature of compile time checking, and possibly also design time checking of code contracts. It seems it's not yet available for C# in Visual Studio 2010, but I think there are some languages like Spec# that already do. These engines will probably give us hints when we write code to call such a method and our argument currently can or will be null. So I wonder if these engines will always analyze a call stack until they find a method with a contract that is currently not satisfied? Furthermore, here I learned about the difference between Contract.Requires(...) and Contract.Assume(...). I suppose that difference is also to consider in the context of this question then?

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• How do I implement a collection in Scala 2.8?

  - by Simon Reinhardt

  In trying to write an API I'm struggling with Scala's collections in 2.8(.0-beta1). Basically what I need is to write something that: adds functionality to immutable sets of a certain type where all methods like filter and map return a collection of the same type without having to override everything (which is why I went for 2.8 in the first place) where all collections you gain through those methods are constructed with the same parameters the original collection had (similar to how SortedSet hands through an ordering via implicits) which is still a trait in itself, independent of any set implementations. Additionally I want to define a default implementation, for example based on a HashSet. The companion object of the trait might use this default implementation. I'm not sure yet if I need the full power of builder factories to map my collection type to other collection types. I read the paper on the redesign of the collections API but it seems like things have changed a bit since then and I'm missing some details in there. I've also digged through the collections source code but I'm not sure it's very consistent yet. Ideally what I'd like to see is either a hands-on tutorial that tells me step-by-step just the bits that I need or an extensive description of all the details so I can judge myself which bits I need. I liked the chapter on object equality in "Programming in Scala". :-) But I appreciate any pointers to documentation or examples that help me understand the new collections design better.

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