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  • Generic applet style system for publishing mathematics demonstrations?

    - by Alex
    Anyone who's tried to study mathematics using online resources will have come across these Java applets that demonstrate a particular mathematical idea. Examples: http://www.math.ucla.edu/~tao/java/Mobius.html http://www.mathcs.org/java/programs/FFT/index.html I love the idea of this interactive approach because I believe it is very helpful in conveying mathematical principles. I'd like to create a system for visually designing and publishing these 'mathlets' such that they can be created by teachers with little programming experience. So in order to create this app, i'll need a GUI and a 'math engine'. I'll probably be working with .NET because thats what I know best and i'd like to start experimenting with F#. Silverlight appeals to me as a presentation framework for this project (im not worried about interoperability right now). So my questions are: does anything like this exist already in full form? are there any GUI frameworks for displaying mathematical objects such as graphs & equations? are there decent open source libraries that exposes a mathematical framework (Math.NET looks good, just wondering if there is anything else out there) is there any existing work on taking mathematical models/demos built with maple/matlab/octave/mathematica etc and publishing them to the web?

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  • Problem in mutiple :dependent=> :destroy when multiple polymorphic is true

    - by piemesons
    I have four models question, answer, comment and vote.Consider it same as stackoverflow. Question has_many comments Answers has_many comments Questions has_many votes answers has_many votes comments has_many votes Here are the models (only relevant things) class Comment < ActiveRecord::Base belongs_to :commentable, :polymorphic => true has_many :votes, :as => :votable, :dependent => :destroy end class Question < ActiveRecord::Base has_many :comments, :as => :commentable, :dependent => :destroy has_many :answers, :dependent => :destroy has_many :votes, :as => :votable, :dependent => :destroy end class Vote < ActiveRecord::Base belongs_to :votable, :polymorphic => true end class Answer < ActiveRecord::Base belongs_to :question, :counter_cache => true has_many :comments, :as => :commentable , :dependent => :destroy end Now the problem is whenever i am trying to delete any question/answer/comment its giving me an error NoMethodError in QuestionsController#destroy undefined method `each' for 0:Fixnum if i remove this line from any of the model (question/answer/comment) has_many :votes, :as => :votable, :dependent => :destroy then it works perfectly. It seems there is a problem while deleting the records active record is not able to find out the proper path because of multiple joins within the tables.

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  • Using Custom Generic Collection faster with objects than List

    - by Kaminari
    I'm iterating through a List<> to find a matching element. The problem is that object has only 2 significant values, Name and Link (both strings), but has some other values which I don't want to compare. I'm thinking about using something like HashSet (which is exactly what I'm searching for -- fast) from .NET 3.5 but target framework has to be 2.0. There is something called Power Collections here: http://powercollections.codeplex.com/, should I use that? But maybe there is other way? If not, can you suggest me a suitable custom collection?

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  • Find the class of the generic object

    - by Mgccl
    Suppose I have the following class. public class gen<T> { public gen(){ } Class<T> class(){ //something that figures out what T is. } } How can I implement the class() function without pass any additional information? What I did is here, but I have to pass a object of T into the object gen. public class gen<T> { public gen(){ } Class<T> class(T var){ return (Class<T>) var.getClass(); } }

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  • [VB.Net] Typecasting generic parameters.

    - by CFP
    Hello world! Using the following code: Function GetSetting(Of T)(ByVal SettingName As String, ByRef DefaultVal As T) As T Return If(Configuration.ContainsKey(SettingName), CType(Configuration(SettingName), T), DefaultVal) End Function Yields the following error: Value of type 'String' cannot be converted to 'T'. Any way I could specify that in all cases, the conversion will indeed be possible (I'm basically getting integers, booleans, doubles and strings). Thanks!

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  • Open Source - EER Modeling Tool

    - by Nick Fergis
    Is there a good open source or reasonably priced EER modeling tool for MySQL besides MySQL Workbench? I find the MySQL Workbench interface to be clunky. I would like to be able to manage my production schema beginning all design changes in the EER and propogating those out to my schema for created and altered tables. Is anyone use a tool they love to manage their environments in this way? Thanks. - Nick

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  • How to bind a List<DateTime> to a DataGridView/ListBox .NET C#

    - by nat
    Hi there I have a List that I want to bind to a DataGridView in a windows forms app. however, I cannot for the life of me find the DataPropertyName I need to show what I want. I have had similar problems with binding this list to a listbox. what i really want is to show .ToString("dddd, dd MMMM yyyy HH:mm"), or certainly more than the standard 'date' part of the datetime - do i need to add a property of my own? public partial class DateTime{ public string myFormat{ get{return this.ToString("dddd dd MMM yyyy HH:mm");} } } or some such? thanks nat

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  • removing items from a generic List<t>

    - by frosty
    I have the following method, I wish to remove items from my collection that match the product Id. Seems fairly straight forward, but i get an exception. Basically my collection is getting out of sync. So what is the best way to remove an item from a collection. public void RemoveOrderItem(Model.Order currentOrder, int productId) { foreach (var orderItem in currentOrder.OrderItems) { if (orderItem.Product.Id == productId) { currentOrder.OrderItems.Remove(orderItem); } } } Exception Details: System.InvalidOperationException: Collection was modified; enumeration operation may not execute

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  • MySQL - Find entries that refer to a specified index.

    - by Conor H
    Hi, So I have a booking system where I have a 'lesson_type' table with 'lesson_type_id' as PK. I have a constraint in place here so I can't delete a lesson_type if there are bookings made for that lesson_type. I would like to be able to determine if this lesson_type_id is being referred to by any entries in the bookings table (or any other table for that matter) so I can notify the user gracefully. i.e. not have a mysql error be thrown when they try and delete a record. What kind of query would I use for this? Thanks.

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  • C++ creating generic template function specialisations

    - by Fire Lancer
    I know how to specialise a template function, however what I want to do here is specialise a function for all types which have a given method, eg: template<typename T> void foo(){...} template<typename T, if_exists(T::bar)>void foo(){...}//always use this one if the method T::bar exists T::bar in my classes is static and has different return types. I tried doing this by having an empty base class ("class HasBar{};") for my classes to derive from and using boost::enable_if with boost::is_base_of on my "specialised" version. However the problem then is that for classes that do have bar, the compiler cant resolve which one to use :(. template<typename T> typename boost::enable_if<boost::is_base_of(HasBar, T>, void>::type f() {...} I know that I could use boost::disable_if on the "normal" version, however I do not control the normal version (its provided by a third party library and its expected for specialisations to be made, I just don't really want to make explicit specialisations for my 20 or so classes), nor do I have that much control over the code using these functions, just the classes implementing T::bar and the function that uses it. Is there some way to tell the compiler to "always use this version if possible no matter what" without altering the other versions?

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  • Logic: Best way to sample & count bytes of a 100MB+ file

    - by Jami
    Let's say I have this 170mb file (roughly 180 million bytes). What I need to do is to create a table that lists: all 4096 byte combinations found [column 'bytes'], and the number of times each byte combination appeared in it [column 'occurrences'] Assume two things: I can save data very fast, but I can update my saved data very slow. How should I sample the file and save the needed information? Here're some suggestions that are (extremely) slow: Go through each 4096 byte combinations in the file, save each data, but search the table first for existing combinations and update it's values. this is unbelievably slow Go through each 4096 byte combinations in the file, save until 1 million rows of data in a temporary table. Go through that table and fix the entries (combine repeating byte combinations), then copy to the big table. Repeat going through another 1 million rows of data and repeat the process. this is faster by a bit, but still unbelievably slow This is kind of like taking the statistics of the file. NOTE: I know that sampling the file can generate tons of data (around 22Gb from experience), and I know that any solution posted would take a bit of time to finish. I need the most efficient saving process

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  • What's does "cardinality of an relationship" mean in Core Data?

    - by dontWatchMyProfile
    From the docs: If all of a managed object's relationship delete rules are Nullify, then for that object at least there is no additional work to do (you may have to consider other objects that were at the destination of the relationship—if the inverse relationship was either mandatory or had a lower limit on cardinality, then the destination object or objects might be in an invalid state). Does someone have an example of this cardinality thing? What's this good for and what's important to know about this? (sounds very important...)

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  • How to add a new entry to a multiple has_many association?

    - by siulamvictor
    I am not sure am I doing these correct. I have 3 models, Account, User, and Event. Account contains a group of Users. Each User have its own username and password for login, but they can access the same Account data under the same Account. Events is create by a User, which other Users in the same Account can also read or edit it. I created the following migrations and models. User migration class CreateUsers < ActiveRecord::Migration def self.up create_table :users do |t| t.integer :account_id t.string :username t.string :password t.timestamps end end def self.down drop_table :users end end Account migration class CreateAccounts < ActiveRecord::Migration def self.up create_table :accounts do |t| t.string :name t.timestamps end end def self.down drop_table :accounts end end Event migration class CreateEvents < ActiveRecord::Migration def self.up create_table :events do |t| t.integer :account_id t.integer :user_id t.string :name t.string :location t.timestamps end end def self.down drop_table :events end end Account model class Account < ActiveRecord::Base has_many :users has_many :events end User model class User < ActiveRecord::Base belongs_to :account end Event model class Event < ActiveRecord::Base belongs_to :account belongs_to :user end so.... Is this setting correct? Every time when a user create a new account, the system will ask for the user information, e.g. username and password. How can I add them into correct tables? How can I add a new event? I am sorry for such a long question. I am not very understand the rails way in handling such data structure. Thank you guys for answering me. :)

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  • C++ Generic List Assignment

    - by S73417H
    I've clearly been stuck in Java land for too long... Is it possible to do the C++ equivalent of the following Java code: // Method List<Bar> getBars() { return new LinkedList<Bar>(); } // Assignment statement. List<Foo> stuff = getBars(); Where Foo is a sub-class of Bar. So in C++.... std::list<Bar> & getBars() { std::list<Bar> bars; return bars; } std::list<Foo> stuff = getBars(); Hope that makes sense....

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  • SQL database self interaction entity

    - by Ricardo Costa
    I've been working on a database, wich is referent to an Aeroport management. I'm having a problem that it's freaking me out.. What i'm trying to do is, assuming that a client wants to know the distance between 2 locations, in miles or kms. As an example, if the user wants to know the distance between London and Amsterdam, should that distance be calculated by a formule or should it be already stored on the database? (1,N) ____________ ____________|__ | | | | | City/Airport |<---------| |______________| How can i show to user the distance between his 2 choices? RicardoCosta

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  • Why can I derived from a templated/generic class based on that type in C# / C++

    - by stusmith
    Title probably doesn't make a lot of sense, so I'll start with some code: class Foo : public std::vector<Foo> { }; ... Foo f; f.push_back( Foo() ); Why is this allowed by the compiler? My brain is melting at this stage, so can anyone explain whether there are any reasons you would want to do this? Unfortunately I've just seen a similar pattern in some production C# code and wondered why anyone would use this pattern.

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  • C#, Generic Lists and Inheritance

    - by Andy
    I have a class called Foo that defines a list of objects of type A: class Foo { List<A> Items = new List<A>(); } I have a class called Bar that can save and load lists of objects of type B: class Bar { void Save(List<B> ComplexItems); List<B> Load(); } B is a child of A. Foo, Bar, A and B are in a library and the user can create children of any of the classes. What I would like to do is something like the following: Foo MyFoo = new Foo(); Bar MyBar = new Bar(); MyFoo.Items = MyBar.Load(); MyBar.Save(MyFoo.Items); Obviously this won't work. Is there a clever way to do this that avoids creating intermediate lists? thanks, Andy

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  • Example of user-defined integrity rule in database systems?

    - by Pavel
    Hey everyone. I'm currently preparing for my exams and would like to know some examples of user-defined integrity rule in database systems. As far as I understand, it means that I can set up certain conditions for the columns and when data is inserted it needs to fulfill these conditions. For example: if I set up a rule that an ID needs to consist of 5 integers ONLY then when I insert a row with ID which is made up of integers and some chars then it won't accept it and return an error. Could someone confirm and give me some opinion on that? Thank you very much in advance!

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  • WPF Converter to look up in generic list

    - by user310046
    I have a list of data where I keep e.g. countries with code, name and some other data. List<Country> countries = <deserialized objects from file> which consist of objects like this: public class Country { public string Code { get; set;} public string Name { get; set;} } The object which use as a DataContext may look like this: public class Address { public string StreetName{ get; set;} public string CountryCode { get; set;} } Then in my XAML I want to do something like this to show the name of the country <TextBlock Text="{Binding Path=CountryCode, Converter={StaticResource CountryNameLookupConverter}}"/> But how can I make the CountryNameLookupConverter use the countries list I read from the xml file?

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  • Is the method that I am using for retrieval in my generic list optimized

    - by fishhead
    At some time there will be a large amount of records, about 50,000. with that in mind is the method GetEquipmentRecord up to the task. thanks for you opinions. public enum EquipShift { day, night }; public class EquipStatusList : List<EquipStatus> { string SerialFormat = "yyyyMMdd"; int _EquipmentID; string _DateSerial; EquipShift _Shift; public EquipStatus GetEquipmentRecord(int equipmentID, EquipShift shift, DateTime date) { _DateSerial = date.ToString(SerialFormat); _Shift = shift; _EquipmentID = equipmentID; return this.Find(checkforEquipRecord); } bool checkforEquipRecord(EquipStatus equip) { if ((equip.EquipmentID == _EquipmentID) && (equip.Shift == _Shift) && (equip.Date.ToString(SerialFormat) == _DateSerial)) return true; else return false; } }

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  • ASP.NET MVC Generic Partial

    - by gnome
    Is is possible to have a partial view inherit more than one model? I have three models (Contacts, Clients, Vendors) that all have address information (Address). In the interest of being DRY I pulled the address info into it's own model, Addresses. I created a partial create / update view of addresses and what to render this in other other three model's create / update views.

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  • Reflection for Class of generic parameter in Java?

    - by hatboysam
    Imagine the following scenario: class MyClass extends OtherClass<String>{ String myName; //Whatever } class OtherClass<T> { T myfield; } And I am analyzing MyClass using reflection specifically (MyClass.class).getDeclaredFields(), in this case I will get the following fields (and Types, using getType() of the Field): myName --> String myField --> T I want to get the actual Type for T, which is known at runtime due to the explicit "String" in the extends notation, how do I go about getting the non-genetic type of myField?

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  • Invoking static methods containing Generic Parameters using Reflection.

    - by AJP
    While executing the following code i gets this error "Late bound operations cannot be performed on types or methods for which ContainsGenericParameters is true." class Program { static void Main(string[] args) { MethodInfo MI = typeof(MyClass).GetMethod("TestProc"); MI.MakeGenericMethod(new [] {typeof(string)}); MI.Invoke(null, new [] {"Hello"}); } } class MyClass { public static void TestProc<T>(T prefix) { Console.WriteLine("Hello"); } } Please help.

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