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• Representing xml through a single class

  - by Charles

  I am trying to abstract away the difficulties of configuring an application that we use. This application takes a xml configuration file and it can be a bit bothersome to manually edit this file, especially when we are trying to setup some automatic testing scenarios. I am finding that reading xml is nice, pretty easy, you get a network of element nodes that you can just go through and build your structures quite nicely. However I am slowly finding that the reverse is not quite so nice. I want to be able to build a xml configuration file through a single easy to use interface and because xml is composed of a system of nodes I am having a lot of struggle trying to maintain the 'easy' part. Does anyone know of any examples or samples that easily and intuitively build xml files without declaring a bunch of element type classes and expect the user to build the network themselves? For example if my desired xml output is like so <cook version="1.1"> <recipe name="chocolate chip cookie"> <ingredients> <ingredient name="flour" amount="2" units="cups"/> <ingredient name="eggs" amount="2" units="" /> <ingredient name="cooking chocolate" amount="5" units="cups" /> </ingredients> <directions> <direction name="step 1">Preheat oven</direction> <direction name="step 2">Mix flour, egg, and chocolate</direction> <direction name="step 2">bake</direction> </directions> </recipe> <recipe name="hot dog"> ... How would I go about designing a class to build that network of elements and make one easy to use interface for creating recipes? Right now I have a recipe object, an ingredient object, and a direction object. The user must make each one, set the attributes in the class and attach them to the root object which assembles the xml elements and outputs the formatted xml. Its not very pretty and I just know there has to be a better way. I am using python so bonus points for pythonic solutions

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• devise register confirmation

  - by mattherick

  hello! i have a user and an admin role in my project. i created my authentification with devise, really nice and goot tool for handling the authentification. in my admin role i don´t have any confirmation or something like that. it is really simple and doesn´t make problems. but in my user model i have following things: model: devise :database_authenticatable, :confirmable, :recoverable, :rememberable, :trackable, :validatable, :timeoutable, :registerable # Setup accessible (or protected) attributes for your model attr_accessible :email, :username, :prename, :surname, :phone, :street, :number, :location, :password, :password_confirmation and few validations, but they aren´t relevant this time. my migration looks like following one: class DeviseCreateUsers < ActiveRecord::Migration def self.up create_table(:users) do |t| t.database_authenticatable :null = false t.confirmable t.recoverable t.rememberable t.trackable t.timeoutable t.validateable t.string :username t.string :prename t.string :surname t.string :phone t.string :street t.integer :number t.string :location t.timestamps end add_index :users, :email, :unique => true add_index :users, :confirmation_token, :unique => true add_index :users, :reset_password_token, :unique => true add_index :users, :username, :unique => true add_index :users, :prename, :unique => false add_index :users, :surname, :unique => false add_index :users, :phone, :unique => false add_index :users, :street, :unique => false add_index :users, :number, :unique => false add_index :users, :location, :unique => false end def self.down drop_table :users end end into my route.rb I added following statements: map.devise_for :admins map.devise_for :users, :path_names = { :sign_up = "register", :sign_in = "login" } map.root :controller = "main" and now my problem.. if I register a new user, I fill in all my data in the register form and submit it. After that I get redirected to the controller main with the flash-notice "You have signed up successfully." And I am logged in. But I don´t want to be logged in, because I don´t have confirmed my new user account yet. If I open the console I see the last things in the logs and there I see the confirmation-mail and the text and all stuff, but I am already logged in... I can´t explain why, ... does somebody of you have an idea? If I copy out the confirmation-token from the logs and confirm my account, I can log in, but if I don´t confirm, I also can log in..

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• Unable to get index from jQuery UI slider range

  - by Phil.Wheeler

  I'm having a hell of a time trying to get (what I thought was) a simple index from a collection of multiple sliders. The HTML is as follows: <div id="left-values" class="line"> <span id="l1" style="padding: 0 1.8em;">0</span> <span id="l2" style="padding: 0 1.8em;">0</span> <span id="l3" style="padding: 0 1.8em;">0</span> <span id="l4" style="padding: 0 1.8em;">0</span> <span id="l5" style="padding: 0 1.8em;">0</span> <span id="l6" style="padding: 0 1.8em;">0</span> <span id="l7" style="padding: 0 1.8em;">0</span> <span id="l8" style="padding: 0 1.8em;">0</span> </div> And the jQuery code is: // setup audiometry sliders $("#eq > span").each(function (e) { // read initial values from markup and remove that var value = parseInt($(this).text()); // var index = $(this).index; <- this didn't work. $(this).empty(); $(this).slider({ value: value, slide: function (event, ui) { //console.log($(this).attr('id')); <- neither did this. //console.log(index); $('#left-values span:first').text(ui.value); } }) }); The problem is that jQuery UI - when creating a slider - replaces the existing HTML with its own markup. This includes any ID values and, for whatever reason, I can't get the index for a given slider to surface either. So I'm running out of ideas.

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• Remote Postgresql - extremely slow

  - by Muffinbubble

  Hi, I have setup PostgreSQL on a VPS I own - the software that accesses the database is a program called PokerTracker. PokerTracker logs all your hands and statistics whilst playing online poker. I wanted this accessible from several different computers so decided to installed it on my VPS and after a few hiccups I managed to get it connecting without errors. However, the performance is dreadful. I have done tons of research on 'remote postgresql slow' etc and am yet to find an answer so am hoping someone is able to help. Things to note: The query I am trying to execute is very small. Whilst connecting locally on the VPS, the query runs instantly. While running it remotely, it takes about 1 minute and 30 seconds to run the query. The VPS is running 100MBPS and then computer I'm connecting to it from is on an 8MB line. The network communication between the two is almost instant, I am able to remotely connect fine with no lag whatsoever and am hosting several websites running MSSQL and all the queries run instantly, whether connected remotely or locally so it seems specific to PostgreSQL. I'm running their newest version of the software and the newest compatible version of PostgreSQL with their software. The database is a new database, containing hardly any data and I've ran vacuum/analyze etc all to no avail, I see no improvements. I don't understand how MSSQL can query almost instantly yet PostgreSQL struggles so much. I am able to telnet to the post 5432 on the VPS IP with no problems, and as I say the query does execute it just takes an extremely long time. What I do notice is on the router when the query is running that hardly any bandwidth is being used - but then again I wouldn't expect it to for a simple query but am not sure if this is the issue. I've tried connecting remotely on 3 different networks now (including different routers) but the problem remains. Connecting remotely via another machine via the LAN is instant. I have also edited the postgre conf file to allow for more memory/buffers etc but I don't think this is the problem - what I am asking it to do is very simple - it shouldn't be intensive at all. Thanks, Ricky

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• Java loading user-specified classes at runtime

  - by user349043

  I'm working on robot simulation in Java (a Swing application). I have an abstract class "Robot" from which different types of Robots are derived, e.g. public class StupidRobot extends Robot { int m_stupidness; int m_insanityLevel; ... } public class AngryRobot extends Robot { float m_aggression; ... } As you can see, each Robot subclass has a different set of parameters. What I would like to do is control the simulation setup in the initial UI. Choose the number and type of Robots, give it a name, fill in the parameters etc. This is one of those times where being such a dinosaur programmer, and new to Java, I wonder if there is some higher level stuff/thinking that could help me here. So here is what I've got: (1) User Interface Scrolling list of Robot types on the left. "Add " and "<< Remove" buttons in the middle. Default-named scrolling list of Robots on the right. "Set Parameters" button underneath. (So if you wanted an AngryRobot, you'd select AngryRobot on the left list, click "Add" and "AngryRobot1" would show up on the right.) When selecting a Robot on the right, click "Set Parameters..." button which would call yet another model dialog where you'd fill in the parameters. Different dialog called for each Robot type. (2) Data structures an implementation As an end-product I think a HashMap would be most convenient. The keys would be Robot types and the accompanying object would be all of the parameters. The initializer could just retrieve each item one and a time and instantiate. Here's what the data structures would look like: enum ROBOT_TYPE {STUPID, ANGRY, etc} public class RobotInitializer { public ROBOT_TYPE m_type; public string m_name; public int[] m_int_params; public float[] m_float_params; etc. The initializer's constructor would create the appropriate length parameter arrays based on the type: public RobotInitializer(ROBOT_TYPE type, int[] int_array, float[] float_array, etc){ switch (type){ case STUPID: m_int_params = new int[STUPID_INT_PARAM_LENGTH]; System.arraycopy(int_array,0,m_int_params,0,STUPID_INT_PARAM_LENGTH); etc. Once all the RobotInitializers are instantiated, they are added to the HashMap. Iterating through the HashMap, the simulation initializer takes items from the Hashmap and instantiates the appropriate Robots. Is this reasonable? If not, how can it be improved? Thanks

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• Using JSON Data to Populate a Google Map with Database Objects

  - by MikeH

  I'm revising this question after reading the resources mentioned in the original answers and working through implementing it. I'm using the google maps api to integrate a map into my Rails site. I have a markets model with the following columns: ID, name, address, lat, lng. On my markets/index view, I want to populate a map with all the markets in my markets table. I'm trying to output @markets as json data, and that's where I'm running into problems. I have the basic map displaying, but right now it's just a blank map. I'm following the tutorials very closely, but I can't get the markers to generate dynamically from the json. Any help is much appreciated! Here's my setup: Markets Controller: def index @markets = Market.filter_city(params[:filter]) respond_to do |format| format.html # index.html.erb format.json { render :json => @market} format.xml { render :xml => @market } end end Markets/index view: <head> <script type="text/javascript" src="http://www.google.com/jsapi?key=GOOGLE KEY REDACTED, BUT IT'S THERE" > </script> <script type="text/javascript"> var markets = <%= @markets.to_json %>; </script> <script type="text/javascript" charset="utf-8"> google.load("maps", "2.x"); google.load("jquery", "1.3.2"); </script> </head> <body> <div id="map" style="width:400px; height:300px;"></div> </body> Public/javascripts/application.js: function initialize() { if (GBrowserIsCompatible() && typeof markets != 'undefined') { var map = new GMap2(document.getElementById("map")); map.setCenter(new GLatLng(40.7371, -73.9903), 13); map.addControl(new GLargeMapControl()); function createMarker(latlng, market) { var marker = new GMarker(latlng); var html="<strong>"+market.name+"</strong><br />"+market.address; GEvent.addListener(marker,"click", function() { map.openInfoWindowHtml(latlng, html); }); return marker; } var bounds = new GLatLngBounds; for (var i = 0; i < markets.length; i++) { var latlng=new GLatLng(markets[i].lat,markets[i].lng) bounds.extend(latlng); map.addOverlay(createMarker(latlng, markets[i])); } } } window.onload=initialize; window.onunload=GUnload;

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• NHibernate unable to create SessionFactory

  - by Tyler

  I'm having a bit of trouble setting up NHibernate, and I'm not too sure what the problem is exactly. I'm attempting to save a domain object to the database (Oracle 10g XE). However, I'm getting a TypeInitializationException while trying to create the ISessionFactory. Here is what my hibernate.cfg.xml looks like: <?xml version="1.0" encoding="utf-8"?> <hibernate-configuration xmlns="urn:nhibernate-configuration-2.2" > <session-factory name="MyProject.DataAccess"> <property name="connection.driver_class">NHibernate.Driver.OracleClientDriver</property> <property name="connection.connection_string"> User ID=myid;Password=mypassword;Data Source=localhost </property> <property name="show_sql">true</property> <property name="dialect">NHibernate.Dialect.OracleDialect</property> <property name="proxyfactory.factory_class">NHibernate.ByteCode.LinFu.ProxyFactoryFactory, NHibernate.ByteCode.LinFu</property> <mapping resource="MyProject/Domain/User.hbm.xml"/> </session-factory> </hibernate-configuration> I created a DAO which I will use to persist domain objects to the database. The DAO uses a HibernateUtil class that creates the SessionFactory. Both classes are in the DataAccess namespace along with the Hibernate configuration. This is where the exception is occuring. Here's that class: public class HibernateUtil { private static ISessionFactory SessionFactory = BuildSessionFactory(); private static ISessionFactory BuildSessionFactory() { try { // This seems to be where the problem occurs return new Configuration().Configure().BuildSessionFactory(); } catch (TypeInitializationException ex) { Console.WriteLine("Initial SessionFactory creation failed." + ex); throw new Exception("Unable to create SessionFactory."); } } public static ISessionFactory GetSessionFactory() { return SessionFactory; } } The DataAccess namespace references the NHibernate DLLs. This is virtually the same setup I've used with Hibernate in Java, so I'm not entirely sure what I'm doing wrong here. Any ideas? Edit The innermost exception is the following: "Could not find file 'C:\Users\Tyler\Documents\Visual Studio 2010\Projects\MyProject\MyProject\ConsoleApplication\bin\Debug\hibernate.cfg.xml'." ConsoleApplication contains the entry point where I've created a User object and am trying to persist it with my DAO. Why is it looking for the configuration file there? The actual persisting takes place in the DAO, which is in DataAccess. Also, when I add the configuration file to ConsoleApplication, it still does not find it.

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• UIScrollView does not scroll

  - by Preston Cheung

  I got a problem about UIScrollView. I am making a custom view which inherits UIView. The view has a UIScrollView on which there are lots of buttons which should scroll left and right. The UIScrollView and buttons can show normally. But I cannot scroll the buttons. Could someone give me some suggestions? Thanks a lot! MZMPhotoCalenderSwitcher.h #import <UIKit/UIKit.h> @interface MZMPhotoCalenderSwitcher : UIView <UIScrollViewDelegate> @property (strong, nonatomic) UIScrollView *topSwitcher; @end MZMPhotoCalenderSwitcher.m - (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view. self.topSwitcher = [[UIScrollView alloc] initWithFrame:CGRectMake(0, LABEL_HEIGHT + VIEW_Y, self.view.bounds.size.width, TOP_SWITCHER_HEIGHT)]; self.topSwitcher.backgroundColor = [UIColor greenColor]; self.topSwitcher.pagingEnabled = YES; self.topSwitcher.showsHorizontalScrollIndicator = NO; self.topSwitcher.showsVerticalScrollIndicator = NO; [self add:3 ButtonsOnView:self.topSwitcher withButtonWidth:44.8f andHeight:20.0f]; } - (void)add:(int)num ButtonsOnView:(UIScrollView *)view withButtonWidth:(CGFloat)width andHeight:(CGFloat)height { CGFloat totalTopSwitcherWidth = num * width; [view setContentSize:CGSizeMake(totalTopSwitcherWidth, view.bounds.size.height)]; CGFloat xOffset = 0.0f; for (int i=1; i<=num; i++) { UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect]; [button setFrame:CGRectMake(xOffset, 0, width, height)]; xOffset += width; [button setTitle:[NSString stringWithFormat:@"%d", i] forState:UIControlStateNormal]; button.titleLabel.font = [UIFont systemFontOfSize:10]; [button setTitleColor:[UIColor blueColor] forState:UIControlStateNormal]; [button setTitleColor:[UIColor blackColor] forState:UIControlStateSelected]; [button setTag:i]; [button addTarget:self action:@selector(buttonEvent) forControlEvents:UIControlEventTouchUpInside]; if (i % 2 == 0) [button setBackgroundColor:[UIColor yellowColor]]; else [button setBackgroundColor:[UIColor redColor]]; [view addSubview:button]; } }

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• Rails 3: How do I call a javascript function from a js.erb file

  - by user321775

  Now that I've upgraded to Rails 3, I'm trying to figure out the proper way to separate and reuse pieces of javascript. Here's the scenario I'm dealing with: I have a page with two areas: one with elements that should be draggable, the other with droppables. When the page loads I use jQuery to setup the draggables and droppables. Currently I have the script in the head portion of application.html.erb, which I'm sure is not the right solution but at least works. When I press a button on the page, an ajax call is made to my controller that replaces the draggables with a new set of elements that should also be draggable. I have a js.erb file that renders a partial in the correct location. After rendering I need to make the new elements draggable, so I'd like to reuse the code that currently lives in application.html.erb, but I haven't found the right way to do it. I can only make the new elements draggable by pasting the code directly into my js.erb file (yuck). What I'd like to have: - a javascript file that contains the functions prepdraggables() and prepdroppables() - a way to call either function from application.html.erb or from a js.erb file I've tried using :content_for to store and reuse the code, but can't seem to get it working correctly. What I currently have in the head section of application.html.erb <% content_for :drag_drop_prep do %> <script type="text/javascript" charset="utf-8"> $(document).ready(function () { // declare all DOM elements with class draggable to be draggable $( ".draggable" ).draggable( { revert : 'invalid' }); // declare all DOM elements with class legal to be droppable $(".legal").droppable({ hoverClass : 'legal_hover', drop : function(event, ui) { var c = new Object(); c['die'] = ui.draggable.attr("id"); c['cell'] = $(this).attr("id"); c['authenticity_token'] = encodeURIComponent(window._token); $.ajax({ type: "POST", url: "/placeDie", data: c, timeout: 5000 }); }}); }); </script> <% end %> undo.js.erb $("#board").html("<%= escape_javascript(render :partial => 'shared/board', :locals => { :playable => true, :restartable => !session[:challenge]}) %>") // This is where I want to prepare draggables. <%= javascript_include_tag "customdragdrop.js" %> // assuming this file had the draggables code from above in a prepdraggables() function prepdraggables();

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• I need help converting a C# string from one character encoding to another?

  - by Handleman

  According to Spolsky I can't call myself a developer, so there is a lot of shame behind this question... Scenario: From a C# application, I would like to take a string value from a SQL db and use it as the name of a directory. I have a secure (SSL) FTP server on which I want to set the current directory using the string value from the DB. Problem: Everything is working fine until I hit a string value with a "special" character - I seem unable to encode the directory name correctly to satisfy the FTP server. The code example below uses "special" character é as an example uses WinSCP as an external application for the ftps comms does not show all the code required to setup the Process "_winscp". sends commands to the WinSCP exe by writing to the process standardinput for simplicity, does not get the info from the DB, but instead simply declares a string (but I did do a .Equals to confirm that the value from the DB is the same as the declared string) makes three attempts to set the current directory on the FTP server using different string encodings - all of which fail makes an attempt to set the directory using a string that was created from a hand-crafted byte array - which works Process _winscp = new Process(); byte[] buffer; string nameFromString = "Sinéad O'Connor"; _winscp.StandardInput.WriteLine("cd \"" + nameFromString + "\""); buffer = Encoding.UTF8.GetBytes(nameFromString); _winscp.StandardInput.WriteLine("cd \"" + Encoding.UTF8.GetString(buffer) + "\""); buffer = Encoding.ASCII.GetBytes(nameFromString); _winscp.StandardInput.WriteLine("cd \"" + Encoding.ASCII.GetString(buffer) + "\""); byte[] nameFromBytes = new byte[] { 83, 105, 110, 130, 97, 100, 32, 79, 39, 67, 111, 110, 110, 111, 114 }; _winscp.StandardInput.WriteLine("cd \"" + Encoding.Default.GetString(nameFromBytes) + "\""); The UTF8 encoding changes é to 101 (decimal) but the FTP server doesn't like it. The ASCII encoding changes é to 63 (decimal) but the FTP server doesn't like it. When I represent é as value 130 (decimal) the FTP server is happy, except I can't find a method that will do this for me (I had to manually contruct the string from explicit bytes). Anyone know what I should do to my string to encode the é as 130 and make the FTP server happy and finally elevate me to level 1 developer by explaining the only single thing a developer should understand?

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• Load/Store Objects in file in Java

  - by brain_damage

  I want to store an object from my class in file, and after that to be able to load the object from this file. But somewhere I am making a mistake(s) and cannot figure out where. May I receive some help? public class GameManagerSystem implements GameManager, Serializable { private static final long serialVersionUID = -5966618586666474164L; HashMap<Game, GameStatus> games; HashMap<Ticket, ArrayList<Object>> baggage; HashSet<Ticket> bookedTickets; Place place; public GameManagerSystem(Place place) { super(); this.games = new HashMap<Game, GameStatus>(); this.baggage = new HashMap<Ticket, ArrayList<Object>>(); this.bookedTickets = new HashSet<Ticket>(); this.place = place; } public static GameManager createManagerSystem(Game at) { return new GameManagerSystem(at); } public boolean store(File f) { try { FileOutputStream fos = new FileOutputStream(f); ObjectOutputStream oos = new ObjectOutputStream(fos); oos.writeObject(games); oos.writeObject(bookedTickets); oos.writeObject(baggage); oos.close(); fos.close(); } catch (IOException ex) { return false; } return true; } public boolean load(File f) { try { FileInputStream fis = new FileInputStream(f); ObjectInputStream ois = new ObjectInputStream(fis); this.games = (HashMap<Game,GameStatus>)ois.readObject(); this.bookedTickets = (HashSet<Ticket>)ois.readObject(); this.baggage = (HashMap<Ticket,ArrayList<Object>>)ois.readObject(); ois.close(); fis.close(); } catch (IOException e) { return false; } catch (ClassNotFoundException e) { return false; } return true; } . . . } public class JUnitDemo { GameManager manager; @Before public void setUp() { manager = GameManagerSystem.createManagerSystem(Place.ENG); } @Test public void testStore() { Game g = new Game(new Date(), Teams.LIONS, Teams.SHARKS); manager.registerGame(g); File file = new File("file.ser"); assertTrue(airport.store(file)); } }

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• HTTP Handlers in Win2008/IIS7

  - by Keith Barrows

  We are migrating our web sites from Win2003/IIS6 to Win2008/IIS7. Our .NET code is in a WAP form with compiled binaries. I do dev work on a Win7/IIS7 box so had to learn early how to set up HTTP Handlers in this newer environment. What I have that has worked fine on my box is: <system.webServer> <handlers> <remove name="WebServiceHandlerFactory-Integrated" /> <remove name="ScriptHandlerFactory" /> <remove name="ScriptHandlerFactoryAppServices" /> <remove name="ScriptResource" /> <add name="RivWorks" path="*.riv" verb="*" modules="IsapiModule" scriptProcessor="C:\Windows\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" preCondition="classicMode,runtimeVersionv2.0,bitness32" /> <add name="RivWorks2" path="*.riv2" verb="*" modules="IsapiModule" scriptProcessor="C:\Windows\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" preCondition="classicMode,runtimeVersionv2.0,bitness32" /> <add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" /> <add name="ScriptHandlerFactoryAppServices" verb="*" path="*_AppService.axd" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" /> <add name="ScriptResource" verb="GET,HEAD" path="ScriptResource.axd" preCondition="integratedMode" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" /> </handlers> All I am getting on the new web site when I try to call into the *.riv handler is: 404 - File or directory not found. The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable. OK. You see interesting things when writing out these questions. Our server is setup in integrated mode and runs on a x64 system. So, I changed the precondition clause to: preCondition="integratedMode,runtimeVersionv2.0,bitness64" Now I get this instead: 500 - Internal server error. There is a problem with the resource you are looking for, and it cannot be displayed. Any ideas of what I should be doing, where I should be looking? TIA

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• General workflow to allow multiple OpenIDs to be associated with one app account

  - by BobTodd

  I have a (typical?) scenario: that my app's users can use multiple openids mapped to one app account (like stackoverflow). For me the unique thing on the account is the email address, so this binds openids to the profile. Question is, how to allow a user to start using a second openid once one is setup. I am asking as I have read that it is a security hole to allow automatic account openid syncing simply based on the provider-supplied email address as someone could easily spoof someone's email address to create a spoof openid and falsely access the account (how I am not sure) - although this seems to be exactly how stack operates. See options a. and b. below. Problem for me with a. is what happens if the original openid no longer works for whatever reason - how would you set-up a new openid? Would b. be more acceptable if we used email verification? Does anyone have an article detailing a "standard" way (set of user stories) for this - it seems to be an increasingly popular way to authenticate. I have tried to detail this in a rough decision tree... 1. My Site > authentication landing page - user chooses an openid (facebook, google, myopenid etc), redirection > 2. Provider site returns with token (includes user registering a new openid, logging in or is already logged in to Provider site) 3. My Site > use token id to lookup user 3.1 Profile exists? Yes > authenticate. ends. No > 3.1.1 was email address supplied by provider? Yes > lookup user by email address 3.1.1.1 Profile exists? Yes > a. error message - please login with existing openid and associate this openid (from special page) Yes > b. or associate this openid with existing profile automatically. authenticate. ends. No > Register profile. With registration email address follow 3.1.1, except this time where email is unique, we will associate openid. ends

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• Adding user role constraint redirects Browser to jsf.js script?

  - by simgineer

  My JSF form login was working with Constraint 1 however when I added Constraint 2 to my web.xml doing a submit on the form now takes me to a jsf javascript page. Can someone tell me what I am doing wrong? I would like only administrators to be able to access the /admin/* pages and only registered users to access the entire site included admin files. BTW after I see the java script page I can still navigate to the intended page in the browser, I just don't want the user to see the intermediate js page or need to know the target page URL. Constraint 1 <security-constraint> <display-name>Admin</display-name> <web-resource-collection> <url-pattern>/admin/*</url-pattern> </web-resource-collection> <auth-constraint> <role-name>ADMIN</role-name> </auth-constraint> </security-constraint> Constraint 2 <security-constraint> <display-name>Users</display-name> <web-resource-collection> <url-pattern>/*</url-pattern> </web-resource-collection> <auth-constraint> <role-name>USER</role-name> </auth-constraint> </security-constraint> Here is the undesired url I am being redirected to: javax.faces.resource/jsf.js.xhtml?ln=javax.faces&stage=Development Here is the start of the jsf.js.xhtml... /** @project JSF JavaScript Library @version 2.0 @description This is the standard implementation of the JSF JavaScript Library. */ /** * Register with OpenAjax */ if (typeof OpenAjax !== "undefined" && typeof OpenAjax.hub.registerLibrary !== "undefined") { OpenAjax.hub.registerLibrary("jsf", "www.sun.com", "2.0", null); } // Detect if this is already loaded, and if loaded, if it's a higher version if (!((jsf && jsf.specversion && jsf.specversion >= 20000 ) && (jsf.implversion && jsf.implversion >= 3))) { ... Notes I'm using Firefox 10.0.4, Glassfish 3.1 w JSF2.0 lib, j_security_check, and my login realm setup is similar to this

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• JPanels, JFrames, and Windows, Oh my!

  - by Jonathan

  Simply stated, I am trying to make a game I am working on full-screen. I have the following code I am trying to use: GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment(); GraphicsDevice gs = ge.getDefaultScreenDevice(); if(!gs.isFullScreenSupported()) { System.out.println("full-screen not supported"); } Frame frame = new Frame(gs.getDefaultConfiguration()); Window win = new Window(frame); try { // Enter full-screen mode gs.setFullScreenWindow(win); win.validate(); } Problem with this is that I am working within a class that extends JPanel, and while I have a variable of type Frame, I have none of type Window within the class. My understanding of JPanel is that it is a Window of sorts, but I cannot pass 'this' into gs.setFullScreenWindow(Window win)... How should I go about doing this? Is there any easy way of calling that, or a similar method, using a JPanel? Is there a way I can get something of type Window from my JPanel? - EDIT: The following method changes the state of JFrame and is called every 10ms: public void paintScreen() { Graphics g; try{ g = this.getGraphics(); //get Panel's graphic context if(g == null) { frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setExtendedState(frame.getExtendedState()|JFrame.MAXIMIZED_BOTH); frame.add(this); frame.pack(); frame.setResizable(false); frame.setTitle("Game Window"); frame.setVisible(true); } if((g != null) && (dbImage != null)) { g.drawImage(dbImage, 0, 0, null); } Toolkit.getDefaultToolkit().sync(); //sync the display on some systems g.dispose(); } catch (Exception e) { if(blockError) { blockError = false; } else { System.out.println("Graphics context error: " + e); } } } I anticipate that there may be a few redundancies or unnecessary calls after the if(g==null) statement (all the frame.somethingOrOther()s), any cleanup advice would be appreciated... Also, the block error is what it seems. I am ignoring an error. The error only occurs once, and this works fine when setup to ignore the first instance of the error... For anyone interested I can post additional info there if anyone wants to see if that block can be removed, but i'm not concerned... I might look into it later.

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• What are the steps to convert this function to a model/controller in Zend Framework?

  - by Joel

  Hi guys, I'm learning Zend Framework MVC, and I have a website that is mainly static php pages. However one of the pages is using functions, etc, and I'm trying to figure out what the process is for converting this to an OOP setup. Within the <body> I have this function (and more, but this is the first function): function filterEventDetails($contentText) { $data = array(); foreach($contentText as $row) { if(strstr($row, 'When: ')) { ##cleaning "when" string to get date in the format "May 28, 2009"## $data['duration'] = str_replace('When: ','',$row); list($when, ) = explode(' to ',$data['duration']); $data['when'] = substr($when,4); if(strlen($data['when'])>13) $data['when'] = trim(str_replace(strrchr($data['when'], ' '),'',$data['when'])); $data['duration'] = substr($data['duration'], 0, strlen($data['duration'])-4); //trimming time zone identifier (UTC etc.) } if(strstr($row, 'Where: ')) { $data['where'] = str_replace('Where: ','',$row); //pr($row); //$where = strstr($row, 'Where: '); //pr($where); } if(strstr($row, 'Event Description: ')) { $event_desc = str_replace('Event Description: ','',$row); //$event_desc = strstr($row, 'Event Description: '); ## Filtering event description and extracting venue, ticket urls etc from it. //$event_desc = str_replace('Event Description: ','',$contentText[3]); $event_desc_array = explode('|',$event_desc); array_walk($event_desc_array,'get_desc_second_part'); //pr($event_desc_array); $data['venue_url'] = $event_desc_array[0]; $data['details'] = $event_desc_array[1]; $data['tickets_url'] = $event_desc_array[2]; $data['tickets_button'] = $event_desc_array[3]; $data['facebook_url'] = $event_desc_array[4]; $data['facebook_icon'] = $event_desc_array[5]; } } return $data; } ?> So right now I have this in my example.phtml view page. I understand this needs to be a model and acted on by the controller, but I'm really not sure where to start with this conversion? This is a function tht is taking info from a Google calendar and parsing it for the view. Thanks for any help!

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• Pure sine wave inverter

  - by Nick

  Not exactly programming (sorry) but I think it's pretty close and can be of interest to other programmers. I'm trying to setup a battery power station so that I can work from anywhere. I go surfing a lot and my idea is to be able to work from wherever I can park my car (given there's coverage). So, I'm getting a deep cycle battery, a 240V charger (I'm in Australia), and an inverter. At the back of my laptop it says 19V and 4.62A. From the people I've spoken to that means it consumes about 90W at most. So my inverter needs to be able to output about 100W. Most of them seem to be 200W and up so this shouldn't be a problem. I want to be able run my laptop for 10 hours (plus the 2 hours I get from the laptop battery) straight. According to the people I've spoken to and from what I gather online I need a battery that has the amp hours for my "amp draw". I have no idea how to calculate this but I've been guesstimating. Most deep cycle batteries seem to be classified using amp hours (Ah)... 35Ah, 50Ah, 75Ah, 100Ah, and so on. However the amp hours on those batteries is for a 240V and I seem to be using 19V. According to an expert I spoke to you'd need a 100Ah battery to power a 5A appliance at 240V for 10 hours (you only get about 50% useful power). That to me is 5A * 240V = 100Ah battery. So, naive as I might be I take 240V and divide that by my 19V and reach the conclusion that I can get away with a battery that's about 12 times smaller than that 100Ah. The expert told me I needed a 50Ah battery so that's probably what I'll be getting, but it would be interesting to know what I theoretically would need to power my laptop for 10 hours. As for charging the battery the expert I spoke to said I needed a 3-5A charger to be able to charge that 50Ah battery from flat to full in about 10 hours (I will just leave it plugged in over night). Now to my question. The expert said it's not a matter of "if" more like a guaranteed "when" my computer will stuff up if I don't use a "pure sine wave inverter". From what I gather the power that comes out of that battery is not as clean as the power we get in the socket at home. Apparently it's "square" and the one in the socket is nice and smooth. I've already got an inverter, but it's not "pure". Do I really need to buy the $200-300 pure sine wave inverter or can I get away with something else? Perhaps the laptop adapter that sits in the middle of my laptop power cable already fixes that wave to be nice and smooth? Thanks!

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• TestNG - Factories and Dataproviders

  - by Tim K

  Background Story I'm working at a software firm developing a test automation framework to replace our old spaghetti tangled system. Since our system requires a login for almost everything we do, I decided it would be best to use @BeforeMethod, @DataProvider, and @Factory to setup my tests. However, I've run into some issues. Sample Test Case Lets say the software system is a baseball team roster. We want to test to make sure a user can search for a team member by name. (Note: I'm aware that BeforeMethods don't run in any given order -- assume that's been taken care of for now.) @BeforeMethod public void setupSelenium() { // login with username & password // acknowledge announcements // navigate to search page } @Test(dataProvider="players") public void testSearch(String playerName, String searchTerm) { // search for "searchTerm" // browse through results // pass if we find playerName // fail (Didn't find the player) } This test case assumes the following: The user has already logged on (in a BeforeMethod, most likely) The user has already navigated to the search page (trivial, before method) The parameters to the test are associated with the aforementioned login The Problems So lets try and figure out how to handle the parameters for the test case. Idea #1 This method allows us to associate dataproviders with usernames, and lets us use multiple users for any specific test case! @Test(dataProvider="players") public void testSearch(String user, String pass, String name, String search) { // login with user/pass // acknowledge announcements // navigate to search page // ... } ...but there's lots of repetition, as we have to make EVERY function accept two extra parameters. Not to mention, we're also testing the acknowledge announcements feature, which we don't actually want to test. Idea #2 So lets use the factory to initialize things properly! class BaseTestCase { public BaseTestCase(String user, String password, Object[][] data); } class SomeTest { @Factory public void ... } With this, we end up having to write one factory per test case... Although, it does let us have multiple users per test-case. Conclusion I'm about fresh out of ideas. There was another idea I had where I was loading data from an XML file, and then calling the methods from a program... but its getting silly. Any ideas?

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• How should I ethically approach user password storage for later plaintext retrieval?

  - by Shane

  As I continue to build more and more websites and web applications I am often asked to store user's passwords in a way that they can be retrieved if/when the user has an issue (either to email a forgotten password link, walk them through over the phone, etc.) When I can I fight bitterly against this practice and I do a lot of ‘extra’ programming to make password resets and administrative assistance possible without storing their actual password. When I can’t fight it (or can’t win) then I always encode the password in some way so that it at least isn’t stored as plaintext in the database—though I am aware that if my DB gets hacked that it won’t take much for the culprit to crack the passwords as well—so that makes me uncomfortable. In a perfect world folks would update passwords frequently and not duplicate them across many different sites—unfortunately I know MANY people that have the same work/home/email/bank password, and have even freely given it to me when they need assistance. I don’t want to be the one responsible for their financial demise if my DB security procedures fail for some reason. Morally and ethically I feel responsible for protecting what can be, for some users, their livelihood even if they are treating it with much less respect. I am certain that there are many avenues to approach and arguments to be made for salting hashes and different encoding options, but is there a single ‘best practice’ when you have to store them? In almost all cases I am using PHP and MySQL if that makes any difference in the way I should handle the specifics. Additional Information for Bounty I want to clarify that I know this is not something you want to have to do and that in most cases refusal to do so is best. I am, however, not looking for a lecture on the merits of taking this approach I am looking for the best steps to take if you do take this approach. In a note below I made the point that websites geared largely toward the elderly, mentally challenged, or very young can become confusing for people when they are asked to perform a secure password recovery routine. Though we may find it simple and mundane in those cases some users need the extra assistance of either having a service tech help them into the system or having it emailed/displayed directly to them. In such systems the attrition rate from these demographics could hobble the application if users were not given this level of access assistance, so please answer with such a setup in mind. Thanks to Everyone This has been a fun questions with lots of debate and I have enjoyed it. In the end I selected an answer that both retains password security (I will not have to keep plain text or recoverable passwords), but also makes it possible for the user base I specified to log into a system without the major drawbacks I have found from normal password recovery. As always there were about 5 answers that I would like to have marked correct for different reasons, but I had to choose the best one--all the rest got a +1. Thanks everyone!

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• Problem with character encoding on email sent via PHP?

  - by cosgrove

  Hi everybody, Having some trouble sending properly formatted HTML e-mail from a PHP script. I am running PHP 5.3.0 and Apache 2.2.11 on Windows XP Professional. The output looks like this: Agent Summary for Support on Tuesday April 20 2010=20 Ext. Name Time Volume 137 Agent Name 01:27:25 1 138 =09 00:00:00 0 139 =09 00:00:00 0 You see the =20 and =09 in there? If you look at the HTML you also see = signs being turned into =3D. I figure this is a character encoding issue as I read the following at Wikipedia: ISO-8859-1 and Windows-1252 confusion It is very common to mislabel text data with the charset label ISO-8859-1, even though the data is really Windows-1252 encoded. In Windows-1252, codes between 0x80 and 0x9F are used for letters and punctuation, whereas they are control codes in ISO-8859-1. Many web browsers and e-mail clients will interpret ISO-8859-1 control codes as Windows-1252 characters in order to accommodate such mislabeling but it is not standard behaviour and care should be taken to avoid generating these characters in ISO-8859-1 labeled content. This looks like the problem but I don't know how to fix. My code looks like this: ob_start(); report_queue_summary($yesterday,$yesterday,$first_extension,$last_extension,$queue); $body_report = ob_get_contents(); ob_end_clean(); $body_footer = "This is an automatically generated e-mail."; $message = new Mail_mime(); $html = $body_header.$body_report.$body_footer; $message->setHTMLBody($html); $body = $message->get(); $extraheaders = array("From"=>"***redacted***","To"=>$recipient, "Subject"=>"Agent Summary for $yesterday [$queue]", "Content-type"=>"text/html; charset=iso-8859-1"); $headers = $message->headers($extraheaders); # setup e-mail; $host = "*********"; $port = "26"; $username = "*****"; $password = "*****"; # Send e-mail $smtp = Mail::factory('smtp', array ('host' => $host, 'port' => $port, 'auth' => true, 'username' => $username, 'password' => $password)); $mail = $smtp->send($recipient, $extraheaders, $body); if (PEAR::isError($mail)) { echo("" . $mail->getMessage() . ""); } else { echo("Message successfully sent!"); } Is the problem that I'm using output buffering?

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• PHP, MySQL - My own version of SALT (I call salty) - Login Issue

  - by Fabio Anselmo

  Ok I wrote my own version of SALT I call it salty lol don't make fun of me.. Anyway the registration part of my script as follows is working 100% correctly. //generate SALTY my own version of SALT and I likes me salt.. lol function rand_string( $length ) { $chars = "ABCDEFGHIJKLMNOPQRSTUWXYZabcdefghijklmnopqrstuwxyz1234567890"; $size = strlen( $chars ); for( $i = 0; $i < $length; $i++ ) { $str .= $chars[ rand( 0, $size - 1 ) ]; } return $str; } $salty = rand_string( 256 ); //generate my extra salty pw $password = crypt('password'); $hash = $password . $salty; $newpass = $hash; //insert the data in the database include ('../../scripts/dbconnect.php'); //Update db record with my salty pw ;) // TESTED WITH AND WITHOUT SALTY //HENCE $password and $newpass mysql_query("UPDATE `Register` SET `Password` = '$password' WHERE `emailinput` = '$email'"); mysql_close($connect); However my LOGIN script is failing. I have it setup to TEST and echo if its login or not. It always returns FAILED. I entered the DB and changed the crypted salty pw to "TEST" and I got a SUCCESS. So my problem is somewhere in this LOGIN script I assume. Now I am not sure how to implement my $Salty in this. But also be advised that even without SALTY (just using crypt to store my pass) - I was still unable to perform a login successfully. And if you're gonna suggest i use blowfish - note that my webhost doesn't have it supported and i don't know how to install it. here's my login script: if (isset($_POST['formsubmitted'])) { include ('../../scripts/dbconnect.php'); $username = mysql_real_escape_string($_POST['username']); $password = crypt(mysql_real_escape_string($_POST['password'])); $qry = "SELECT ID FROM Register WHERE emailinput='$username' AND Password='$password'"; $result = mysql_query($qry); if(mysql_num_rows($result) > 0) { echo 'SUCCESS'; //START SESSION } else { echo 'FAILED'; //YOU ARE NOT LOGGED IN } } So what's wrong with this login? Why isn't it working just using the crypt/storing only crypt? How can i make it work storing both the crypt and randomly generated SALTY :) ? Ty advance

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• Action Cache for root URL not working

  - by askegg

  Here's the setup. I have web site which is essentially a simple CMS. Here is the routes file: map.connect ':url', :controller => :pages, :action => :show map.root :controller => :pages, :action => :show, :url => "/" The page controller is thus: class PagesController < ApplicationController before_filter :verify_access, :except => [:show] # Cache show action if we are not logged in. caches_action :show, :layout => false, :unless => Proc.new { |controller| controller.logged_in? } def update @page = Page.find(params[:id]) respond_to do |format| expire_action :action => :show, :url => @page.url So when a visitor hits "/" it maps to :controller = "pages, :action = "show, :url = "/". This generates a cached version on first try, then returns the appropriate result there after. The log files show: Processing PagesController#show (for 127.0.0.1 at 2009-08-02 14:15:01) [GET] Parameters: {"action"=>"show", "url"=>"/", "controller"=>"pages"} Cached fragment hit: views/out.local// (0.1ms) Rendering template within layouts/application Filter chain halted as [#<ActionController::Filters::AroundFilter:0x23eb03c @identifier=nil, @method=#<Proc:0x01904858@/Library/Ruby/Gems/1.8/gems/actionpack-2.3.3/lib/action_controller/caching/actions.rb:64>, @kind=:filter, @options={:only=>#<Set: {"show"}>, :if=>nil, :unless=>#<Proc:0x025137ac@/Users/askegg/Sites/out/app/controllers/pages_controller.rb:6>}>] did_not_yield. Completed in 2ms (View: 1, DB: 0) | 200 OK [http://out.local/] OK - all good so far. When I update the page, it should expire the cache (see above). The logs show: Page Load (0.2ms) SELECT * FROM "pages" WHERE ("pages"."id" = 3) Page Load (0.1ms) SELECT "pages".id FROM "pages" WHERE ("pages"."url" = '/' AND "pages".domain_id = 1 AND "pages".id <> 3) LIMIT 1 Expired fragment: views/out.local/index (0.1ms) Redirected to http://out.local/pages/3 Completed in 9ms (DB: 0) | 302 Found [http://out.local/pages/3] See the problem? Rails is clearing the cache named "index", but it sets it as "/". Naturally this results in the cache NOT being cleared, so visitors are now seeing the old version.

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• C++ linked list based tree structure. Sanely move nodes between lists.

  - by krunk

  The requirements: Each Node in the list must contain a reference to its previous sibling Each Node in the list must contain a reference to its next sibling Each Node may have a list of child nodes Each child Node must have a reference to its parent node Basically what we have is a tree structure of arbitrary depth and length. Something like: -root(NULL) --Node1 ----ChildNode1 ------ChildOfChild --------AnotherChild ----ChildNode2 --Node2 ----ChildNode1 ------ChildOfChild ----ChildNode2 ------ChildOfChild --Node3 ----ChildNode1 ----ChildNode2 Given any individual node, you need to be able to either traverse its siblings. the children, or up the tree to the root node. A Node ends up looking something like this: class Node { Node* previoius; Node* next; Node* child; Node* parent; } I have a container class that stores these and provides STL iterators. It performs your typical linked list accessors. So insertAfter looks like: void insertAfter(Node* after, Node* newNode) { Node* next = after->next; after->next = newNode; newNode->previous = after; next->previous = newNode; newNode->next = next; newNode->parent = after->parent; } That's the setup, now for the question. How would one move a node (and its children etc) to another list without leaving the previous list dangling? For example, if Node* myNode exists in ListOne and I want to append it to listTwo. Using pointers, listOne is left with a hole in its list since the next and previous pointers are changed. One solution is pass by value of the appended Node. So our insertAfter method would become: void insertAfter(Node* after, Node newNode); This seems like an awkward syntax. Another option is doing the copying internally, so you'd have: void insertAfter(Node* after, const Node* newNode) { Node *new_node = new Node(*newNode); Node* next = after->next; after->next = new_node; new_node->previous = after; next->previous = new_node; new_node->next = next; new_node->parent = after->parent; } Finally, you might create a moveNode method for moving and prevent raw insertion or appending of a node that already has been assigned siblings and parents. // default pointer value is 0 in constructor and a operator bool(..) // is defined for the Node bool isInList(const Node* node) const { return (node->previous || node->next || node->parent); } // then in insertAfter and friends if(isInList(newNode) // throw some error and bail I thought I'd toss this out there and see what folks came up with.

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• Getting up to speed on modern architecture

  - by Matt Thrower

  Hi, I don't have any formal qualifications in computer science, rather I taught myself classic ASP back in the days of the dotcom boom and managed to get myself a job and my career developed from there. I was a confident and, I think, pretty good programmer in ASP 3 but as others have observed one of the problems with classic ASP was that it did a very good job of hiding the nitty-gritty of http so you could become quite competent as a programmer on the basis of relatively poor understanding of the technology you were working with. When I changed on to .NET at first I treated it like classic ASP, developing stand-alone applications as individual websites simply because I didn't know any better at the time. I moved jobs at this point and spent the next several years working on a single site whose architecture relied heavily on custom objects: in other words I gained a lot of experience working with .NET as a middle-tier development tool using a quite old-fashioned approach to OO design along the lines of the classic "car" class example that's so often used to teach OO. Breaking down programs into blocks of functionality and basing your classes and methods around that. Although we worked under an Agile approach to manage the work the whole setup was classic client/server stuff. That suited me and I gradually got to grips with .NET and started using it far more in the manner that it should be, and I began to see the power inherent in the technology and precisely why it was so much better than good old ASP 3. In my latest job I have found myself suddenly dropped in at the deep end with two quite young, skilled and very cutting-edge programmers. They've built a site architecture which is modelling along a lot of stuff which is new to me and which, in truth I'm having a lot of trouble understanding. The application is built on a cloud computing model with multi-tenancy and the architecture is all loosely coupled using a lot of interfaces, factories and the like. They use nHibernate a lot too. Shortly after I joined, both these guys left and I'm now supposedly the senior developer on a system whose technology and architecture I don't really understand and I have no-one to ask questions of. Except you, the internet. Frankly I feel like I've been pitched in at the deep end and I'm sinking. I'm not sure if this is because I lack the educational background to understand this stuff, if I'm simply not mathematically minded enough for modern computing (my maths was never great - my approach to design is often to simply debug until it works, then refactor until it looks neat), or whether I've simply been presented with too much of too radical a nature at once. But the only way to find out which it is is to try and learn it. So can anyone suggest some good places to start? Good books, tutorials or blogs? I've found a lot of internet material simply presupposes a level of understanding that I just don't have. Your advice is much appreciated. Help a middle-aged, stuck in the mud developer get enthusastic again! Please!

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• Simplifying const Overloading?

  - by templatetypedef

  Hello all- I've been teaching a C++ programming class for many years now and one of the trickiest things to explain to students is const overloading. I commonly use the example of a vector-like class and its operator[] function: template <typename T> class Vector { public: T& operator[] (size_t index); const T& operator[] (size_t index) const; }; I have little to no trouble explaining why it is that two versions of the operator[] function are needed, but in trying to explain how to unify the two implementations together I often find myself wasting a lot of time with language arcana. The problem is that the only good, reliable way that I know how to implement one of these functions in terms of the other is with the const_cast/static_cast trick: template <typename T> const T& Vector<T>::operator[] (size_t index) const { /* ... your implementation here ... */ } template <typename T> T& Vector<T>::operator[] (size_t index) { return const_cast<T&>(static_cast<const Vector&>(*this)[index]); } The problem with this setup is that it's extremely tricky to explain and not at all intuitively obvious. When you explain it as "cast to const, then call the const version, then strip off constness" it's a little easier to understand, but the actual syntax is frightening,. Explaining what const_cast is, why it's appropriate here, and why it's almost universally inappropriate elsewhere usually takes me five to ten minutes of lecture time, and making sense of this whole expression often requires more effort than the difference between const T* and T* const. I feel that students need to know about const-overloading and how to do it without needlessly duplicating the code in the two functions, but this trick seems a bit excessive in an introductory C++ programming course. My question is this - is there a simpler way to implement const-overloaded functions in terms of one another? Or is there a simpler way of explaining this existing trick to students? Thanks so much!

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