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  • How to structure a Genetic Algorithm class hierarchy?

    - by MahlerFive
    I'm doing some work with Genetic Algorithms and want to write my own GA classes. Since a GA can have different ways of doing selection, mutation, cross-over, generating an initial population, calculating fitness, and terminating the algorithm, I need a way to plug in different combinations of these. My initial approach was to have an abstract class that had all of these methods defined as pure virtual, and any concrete class would have to implement them. If I want to try out two GAs that are the same but with different cross-over methods for example, I would have to make an abstract class that inherits from GeneticAlgorithm and implements all the methods except the cross-over method, then two concrete classes that inherit from this class and only implement the cross-over method. The downside to this is that every time I want to swap out a method or two to try out something new I have to make one or more new classes. Is there another approach that might apply better to this problem?

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  • add methods in subclasses within the super class constructor

    - by deamon
    I want to add methods (more specifically: method aliases) automatically to Python subclasses. If the subclass defines a method named 'get' I want to add a method alias 'GET' to the dictionary of the subclass. To not repeat myself I'd like to define this modifation routine in the base class. But if I check in the base class init method, there is no such method, since it is defined in the subclass. It will become more clear with some source code: class Base: def __init__(self): if hasattr(self, "get"): setattr(self, "GET", self.get) class Sub(Base): def get(): pass print(dir(Sub)) Output: ['__doc__', '__init__', '__module__', 'get'] It should also contain 'GET'. Is there a way to do it within the base class?

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  • Address calling class [Java]

    - by Samuel
    Hello! [I am rather new to Java and Object Oriented programming] I have an abstract class Moveable with the method abstract void move() which is extended by the class Bullet and the abstract class Character, and Character is extended by the class Survivor and the class Zombie. In Survivor and Bullet the move() method doesnt require any parameters while in the class Zombie the move() method depends on the actual position of the survivor. The survivor and multiple zombies are created in the class Gui. I wanted to access the survivor in Zombie - what's the best way of doing this? In Gui i wrote a method getSurvivor() but i don't see how to access this method in Zombie? I am aware that as a workaround i could just pass a [Survivor survivor] as parameter in move() and ignore it in Bullet and Survivor, but that feels so ... bad practice. Thank you for your time! Samuel [I am not sure what tags to set here, please correct me if i'm wrong]

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  • Perl: Exporting variables in a subclass

    - by Jonathan
    I have a base class like this: package MyClass; use vars qw/$ME list of vars/; use Exporter; @ISA = qw/Exporter/; @EXPORT_OK = qw/ many variables & functions/; %EXPORT_TAGS = (all => \@EXPORT_OK ); sub my_method { } sub other_methods etc { } --- more code--- I want to subclass MyClass, but only for one method. package MySubclass; use MyClass; use vars qw/@ISA/; @ISA = 'MyClass'; sub my_method { --- new method } And I want to call this MySubclass like I would the original MyClass, and still have access to all of the variables and functions from exporter. However I am having problems getting the Exporter variables from the original class, MyClass, to export correctly. Do I need to run exporter again inside the subclass? That seems redundant and unclear. Example file: #!/usr/bin/perl use MySubclass /$ME/; -- rest of code But I get compile errors when I try to import the $ME variable. Any suggestions?

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  • Using Include() with inherited entities problem

    - by Peter Stegnar
    In EF eager loading related entities is easy. But I'm having difficulties including inherited entities when loading data using table-per-type model. This is my model: Entities: ArticleBase (base article entity) ArticleSpecial (inherited from ArticleBase) UserBase (base user entity) UserSpecial (inherited from UserBase) Image Relations are as shown on the image (omitting many columns): In reality my users are always of type UserSpecial, since UserBase is used in another application, thus we can share credentials. That's the only reason I have two separate tables. UserBase table can't be changed in any way shape or form, because the other app would break. Question How am I suppose to load ArticleSpecial with both CreatedBy and EditedBy set, so that both are of type UserSpecial (that defines Image relation)? I've tried (unsuccessfully though) these options: 1. Using lambda expressions: context.ArticleBases .OfType<ArticleSpecial>() .Include("UserCreated.Image") .Include("UserEdited.Image"); In this case the problem is that both CreatedBy and EditedBy are related to UserBase, that doesn't define Image navigation. So I should somehow cast these two to UserSpecial type like: context.ArticleBases .OfType<ArticleSpecial>() .Include("UserCreated<UserSpecial>.Image") .Include("UserEdited<UserSpecial>.Image"); But of course using generics in Include("UserCreated<UserSpecial>.Image") don't work. 2. I have tried using LINQ query var results = from articleSpecial in ctx.ArticleBase.OfType<ArticleSpecial>() join created in ctx.UserBase.OfType<UserSpecial>().Include("Image") on articleSpecial.UserCreated.Id equals created.Id join edited in ctx.UserBase.OfType<UserSpecial>().Include("Image") on articleSpecial.UserEdited.Id equals edited.Id select articleSpecial; In this case I'm only getting ArticleSpecial object instances without related properties being set. I know I should select those somehow, but I don't know how? Select part in my LINQ could be changed to something like select new { articleSpecial, articleSpecial.UserCreated, articleSpecial.UserEdited }; but images are still not loaded into my context. My joins in this case are barely used to filter out articleSpecial results, but they don't load entities into context (I suppose). Can anybody provide any help regarding this problem? I think it's not so uncommon.

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  • Vector of pointers to base class, odd behaviour calling virtual functions

    - by Ink-Jet
    I have the following code #include <iostream> #include <vector> class Entity { public: virtual void func() = 0; }; class Monster : public Entity { public: void func(); }; void Monster::func() { std::cout << "I AM A MONSTER" << std::endl; } class Buddha : public Entity { public: void func(); }; void Buddha::func() { std::cout << "OHMM" << std::endl; } int main() { const int num = 5; // How many of each to make std::vector<Entity*> t; for(int i = 0; i < num; i++) { Monster m; Entity * e; e = &m; t.push_back(e); } for(int i = 0; i < num; i++) { Buddha b; Entity * e; e = &b; t.push_back(e); } for(int i = 0; i < t.size(); i++) { t[i]->func(); } return 0; } However, when I run it, instead of each class printing out its own message, they all print the "Buddha" message. I want each object to print its own message: Monsters print the monster message, Buddhas print the Buddha message. What have I done wrong?

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  • Object hierarchy returned by WCF Service is different than expected

    - by robalot
    Good Day Everyone... My understanding may be wrong, but I thought once you applied the correct attributes the DataContractSerializer would render fully-qualified instances back to the caller. The code runs and the objects return. But oddly enough, once I look at the returned objects I noticed the namespacing disappeared and the object-hierarchy being exposed through the (web applications) service reference seems to become "flat" (somehow). Now, I expect this from a web-service…but not through WFC. Of course, my understanding of what WFC can do may be wrong. ...please keep in mind I'm still experimenting with all this. So my questions are… Q: Can I do something within the WFC Service to force the namespacing to render through the (service reference) data client proxy? Q: Or perhaps, am I (merely) consuming the service incorrectly? Q: Is this even possible? The service code looks like… [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)] public class DataService : IFishData { public C1FE GetC1FE(Int32 key) { //… more stuff here … } public Project GetProject(Int32 key) { //… more stuff here … } } [ServiceContract] [ServiceKnownType(typeof(wcfFISH.StateManagement.C1FE.New))] [ServiceKnownType(typeof(wcfFISH.StateManagement.Project.New))] public interface IFishData { [OperationContract] C1FE GetC1FE(Int32 key); [OperationContract] Project GetProject(Int32 key); } [DataContract] [KnownType(typeof(wcfFISH.StateManagement.ObjectState))] public class Project { [DataMember] public wcfFISH.StateManagement.ObjectState ObjectState { get; set; } //… more stuff here … } [DataContract] KnownType(typeof(wcfFISH.StateManagement.ObjectState))] public class C1FE { [DataMember] public wcfFISH.StateManagement.ObjectState ObjectState { get; set; } //… more stuff here … } [DataContract(Namespace = "wcfFISH.StateManagement")] [KnownType(typeof(wcfFISH.StateManagement.C1FE.New))] [KnownType(typeof(wcfFISH.StateManagement.Project.New))] public abstract class ObjectState { //… more stuff here … } [DataContract(Namespace = "wcfFISH.StateManagement.C1FE", Name="New")] [KnownType(typeof(wcfFISH.StateManagement.ObjectState))] public class New : ObjectState { //… more stuff here … } [DataContract(Namespace = "wcfFISH.StateManagement.Project", Name = "New")] [KnownType(typeof(wcfFISH.StateManagement.ObjectState))] public class New : ObjectState { //… more stuff here … } The web application code looks like… public partial class Fish_Invite : BaseForm { protected void btnTest_Click(object sender, EventArgs e) { Project project = new Project(); project.Get(base.ProjectKey, base.AsOf); mappers.Project mapProject = new mappers.Project(); srFish.Project fishProject = new srFish.Project(); srFish.FishDataClient fishService = new srFish.FishDataClient(); mapProject.MapTo(project, fishProject); fishProject = fishService.AddProject(fishProject, IUser.UserName); project = null; } } In case I’m not being clear… The issue arises as there is a difference in (the name spacing) that I expect to see (returned) is different from what is actually returned. fishProject.ObjectState should look like... srFish.StateManagement.Project.New fishC1FE.ObjectState should look like... srFish.StateManagement.C1FE.New fishProject.ObjectState looks like... srFish.New1 fishC1FE.ObjectState looks like... srFish.New …“Help me Obi-Wan Kenobi, you’re my only hope!”

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  • Inherited variables are not reading correctly when using bitwise comparisons

    - by Shawn B
    Hey, I have a few classes set up for a game, with XMapObject as the base, and XEntity, XEnviron, and XItem inheriting it. MapObjects have a number of flags, one of them being MAPOBJECT_SOLID. My problem is that XEntity is the only class that correctly detects MAPOBJECT_SOLID. Both Items are Environs are always considered solid by the game, regardless of the flag's state. What is important is that Environs and Item should almost never be solid. Here are the relevent code samples: XMapObject: class XMapObject : public XObject { public: Uint8 MapObjectType,Location[2],MapObjectFlags; XMapObject *NextMapObject,*PrevMapObject; XMapObject(); void CreateMapObject(Uint8 MapObjectType); void SpawnMapObject(Uint8 MapObjectLocation[2]); void RemoveMapObject(); void DeleteMapObject(); void MapObjectSetLocation(Uint8 Y,Uint8 X); void MapObjectMapLink(); void MapObjectMapUnlink(); }; XEntity: class XEntity : public XMapObject { public: Uint8 Health,EntityFlags; float Speed,Time; XEntity *NextEntity,*PrevEntity; XItem *IventoryList; XEntity(); void CreateEntity(Uint8 EntityType,Uint8 EntityLocation[2]); void DeleteEntity(); void EntityLink(); void EntityUnlink(); Uint8 MoveEntity(Uint8 YOffset,Uint8 XOffset); }; XEnviron: class XEnviron : public XMapObject { public: Uint8 Effect,TimeOut; void CreateEnviron(Uint8 Type,Uint8 Y,Uint8 X,Uint8 TimeOut); }; XItem: class XItem : public XMapObject { public: void CreateItem(Uint8 Type,Uint8 Y,Uint8 X); }; And lastly, the entity move code. Only entities are capable of moving themselves. Uint8 XEntity::MoveEntity(Uint8 YOffset,Uint8 XOffset) { Uint8 NewY = Location[0] + YOffset, NewX = Location[1] + XOffset; if((NewY >= 0 && NewY < MAPY) && (NewX >= 0 && NewX < MAPX)) { XTile *Tile = GetTile(NewY,NewX); if(Tile->MapList != NULL) { XMapObject *MapObject = Tile->MapList; while(MapObject != NULL) { if(MapObject->MapObjectFlags & MAPOBJECT_SOLID) { printf("solid\n"); return 0; } MapObject = MapObject->NextMapObject; } } if(Tile->Flags & TILE_SOLID && EntityFlags & ENTITY_CLIPPING) { return 0; } this->MapObjectSetLocation(NewY,NewX); return 1; } return 0; } What is wierd, is that the bitwise operator always returns true when the MapObject is an Environ or an Item, but it works correctly for Entities. For debug I am using the printf "Solid", and also a printf containing the value of the flag for both Environs and Items. Any help is greatly appreciated, as this is a major bug for the small game I am working on.

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  • Accessing super class function using subclass object

    - by bdhar
    I have an object for a subclass who is of it's superclass type. There is a overridden function in subclass which gets executed when called using the object. How to call the parent's function using the same object? Is there any way to achieve this? package supercall; public class Main { public static void main(String[] args) { SomeClass obj = new SubClass(); obj.go(); } } class SomeClass { SomeClass() { } public void go() { System.out.println("Someclass go"); } } class SubClass extends SomeClass { SubClass() { } @Override public void go() { System.out.println("Subclass go"); } } Consider the code above. Here it prints Subclass go . Instead I have to print Superclass go .

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  • ASP: Extend control (ASCX) and access base markup file from subclass code

    - by Pathoschild
    Hello, I'm building form validation controls for our C# ASP application. The bulk of the work is handled by a BaseValidator control (subclassing System.Web.UI.UserControl), which also has the markup for the validation output. This is then extended by subcontrols like PasswordValidator, that provides the Validate method and any extra fields needed by that validator control. (The end goal is to have controls like <uc1:PasswordValidator ControlId="txtPassword" /> which we can plop into any form with minimum duplication.) However, PasswordValidator.ascx.cs cannot access the form elements defined in BaseValidator.ascx; the only way I've found to do so is to duplicate the markup in each subcontrol's *.ascx file. How can I extend BaseValidator.ascx.cs and access BaseValidator.ascx's markup in the subclass?

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  • How can I share variables between a base class and subclass in Perl?

    - by Jonathan
    I have a base class like this: package MyClass; use vars qw/$ME list of vars/; use Exporter; @ISA = qw/Exporter/; @EXPORT_OK = qw/ many variables & functions/; %EXPORT_TAGS = (all => \@EXPORT_OK ); sub my_method { } sub other_methods etc { } --- more code--- I want to subclass MyClass, but only for one method. package MySubclass; use MyClass; use vars qw/@ISA/; @ISA = 'MyClass'; sub my_method { --- new method } And I want to call this MySubclass like I would the original MyClass, and still have access to all of the variables and functions from Exporter. However I am having problems getting the Exporter variables from the original class, MyClass, to export correctly. Do I need to run Exporter again inside the subclass? That seems redundant and unclear. Example file: #!/usr/bin/perl use MySubclass /$ME/; -- rest of code But I get compile errors when I try to import the $ME variable. Any suggestions?

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  • How to determine which inheriting class is using an abstract class' methods.

    - by Kin
    In my console application have an abstract Factory class "Listener" which contains code for listening and accepting connections, and spawning client classes. This class is inherited by two more classes (WorldListener, and MasterListener) that contain more protocol specific overrides and functions. I also have a helper class (ConsoleWrapper) which encapsulates and extends System.Console, containing methods for writing to console info on what is happening to instances of the WorldListener and MasterListener. I need a way to determine in the abstract ListenerClass which Inheriting class is calling its methods. Any help with this problem would be greatly appreciated! I am stumped :X Simplified example of what I am trying to do. abstract class Listener { public void DoSomething() { if(inheriting class == WorldListener) ConsoleWrapper.WorldWrite("Did something!"); if(inheriting class == MasterListener) ConsoleWrapper.MasterWrite("Did something!"); } } public static ConsoleWrapper { public void WorldWrite(string input) { System.Console.WriteLine("[World] {0}", input); } } public class WorldListener : Listener { public void DoSomethingSpecific() { ConsoleWrapper.WorldWrite("I did something specific!"); } } public void Main() { new WorldListener(); new MasterListener(); } Expected output [World] Did something! [World] I did something specific! [Master] Did something! [World] I did something specific!

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  • Class Self-Reference

    - by Free Bullets
    I have the following code. The angle function needs some information from the class it was called from. What's the best way to do this? class MyScannedRobotEvent extends robocode.ScannedRobotEvent { public int angle(robocode.Robot myRobot) { return (int) Math.toRadians((myRobot.getHeading() + getBearing()) % 360); } } public class MyRobot extends robocode.Robot { int a = angle(**WHATDOIPUTHERE?**) }

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  • ASP.NET UserControl Inheritence

    - by Craig
    I have a UserControl that is working fine. It is declared like this. public partial class DynamicList : System.Web.UI.UserControl { protected static BaseListController m_GenericListController = null; public DynamicList() { m_GenericListController = new GenericListController(this); } } Now I want to override this control so I can change some of the properties. I have created a class like this. public partial class JobRunningList : DynamicList { public JobRunningList() { m_GenericListController = new JobListController(this); (m_GenericListController as GenericListController).ModuleId = 14; } } It appears that the controls in the DynamicList are not getting created though when I use the JobRunningList control now causing predictably bad results. The DynamicList UserControl has a ListView on it and a few other controls. It appears these are not created when using the JobRunningList. Is there any secret to this?

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  • Inheriting from class that inherits from DynamicObject

    - by SeveQ
    Hello there, I'm experimenting with C# 4.0's dynamic object model. I've created an abstract class named "Block" that inherits from DynamicObject. It overrides TryGetMember and TrySetMember. Furthermore I've created a usable class named "Brush" that inherits from "Block". I want it to be usable dynamically. But when I create a dynamic object from it and try to access a runtime bound member, it pelts me with an exception telling me that the member doesn't exist. The overridden TryGetMember or TrySetMember methods of the abstract parent class aren't getting called. Is this behaviour to be expected? Thank you!

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  • sti and polymorphic's

    - by Alexey Poimtsev
    Hi, I have problem with my code class Post < ActiveRecord::Base end class NewsArticle < Post has_many :comments, :as => :commentable, :dependent => :destroy, :order => 'created_at' end class Comment < ActiveRecord::Base belongs_to :commentable, :polymorphic => true, :counter_cache => true end And on attempt go get comments for some NewsArticle i see in logs something like Comment Load (0.9ms) SELECT "comments".* FROM "comments" WHERE ("comments"."commentable_id" = 1 and "comments"."commentable_type" = 'Post') ORDER BY created_at Strange that "commentable_type" = 'Post'. Whats wrong? PS: Rails 2.3.5 && ruby 1.8.7 (2010-01-10 patchlevel 249) [i686-darwin10]

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  • How can I make datagridview can only select the cells in the same column at a time?

    - by MemoryLeak
    I am using winforms to develop my application. And I set my datagridview control's selectionmode to "CellSelect", and this allow the user to select as many cells as he want which spread over several columns; but I want to constraint my user can only select cells in single column at a time, and there isn't any such kind of selectionmode for me. So If I want to implement this, how can I extend the datagridview class ? I also think that I can check in eventhandler whenever the selection cells are changed, through which I might make the user can not select cells spread over multiple columns, but this is not that good, I think. Can any other people help me to find out a better solution ?

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  • ASP.NET Model Binder and base type

    - by user137348
    My model inherits from an interface: public interface IGrid { ISearchExpression Search { get; set; } . . } public interface ISearchExpression { IRelationPredicateBucket Get(); } The model: public class Project : IGrid { public ISearchExpression Search { get; set; } public Project() { this.Search = new ProjectSearch(); } } The ProjectSearch: public class ProjectSearch: ISearchExpression { public string Name { get; set; } public string Number { get; set; } public IRelationPredicateBucket Get() {...} } And the strong typed partialview in the main view: <%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<ProjectSearch>" %> <%= Html.TextBoxFor(x=>x.Name)%> <%= Html.TextBoxFor(x => x.Number)%> .... When I submit the form, the Search property don't get bound properly. Everything is empty.The action takes an argument of ProjectSearch type. Why the Search don't get bound as supposed ?

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  • STI and polymorphs

    - by Alexey Poimtsev
    Hi, I have problem with my code class Post < ActiveRecord::Base end class NewsArticle < Post has_many :comments, :as => :commentable, :dependent => :destroy, :order => 'created_at' end class Comment < ActiveRecord::Base belongs_to :commentable, :polymorphic => true, :counter_cache => true end And on attempt go get comments for some NewsArticle i see in logs something like Comment Load (0.9ms) SELECT "comments".* FROM "comments" WHERE ("comments"."commentable_id" = 1 and "comments"."commentable_type" = 'Post') ORDER BY created_at Strange that "commentable_type" = 'Post'. What's wrong? PS: Rails 2.3.5 && ruby 1.8.7 (2010-01-10 patchlevel 249) [i686-darwin10]

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  • Calling function using 'new' is less expensive than without it?

    - by Matthew Taylor
    Given this very familiar model of prototypal construction: function Rectangle(w,h) { this.width = w; this.height = h; } Rectangle.prototype.area = function() { return this.width * this.height; }; Can anyone explain why calling "new Rectangle(2,3)" is consistently 10x FASTER than calling "Rectangle(2,3)" without the 'new' keyword? I would have assumed that because new adds more complexity to the execution of a function by getting prototypes involved, it would be slower. Example: var myTime; function startTrack() { myTime = new Date(); } function stopTrack(str) { var diff = new Date().getTime() - myTime.getTime(); println(str + ' time in ms: ' + diff); } function trackFunction(desc, func, times) { var i; if (!times) times = 1; startTrack(); for (i=0; i<times; i++) { func(); } stopTrack('(' + times + ' times) ' + desc); } var TIMES = 1000000; trackFunction('new rect classic', function() { new Rectangle(2,3); }, TIMES); trackFunction('rect classic (without new)', function() { Rectangle(2,3); }, TIMES); Yields (in Chrome): (1000000 times) new rect classic time in ms: 33 (1000000 times) rect classic (without new) time in ms: 368 (1000000 times) new rect classic time in ms: 35 (1000000 times) rect classic (without new) time in ms: 374 (1000000 times) new rect classic time in ms: 31 (1000000 times) rect classic (without new) time in ms: 368

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  • "Undefined Symbols" when inheriting from stdexcept classes

    - by Austin Hyde
    Here is an exception defined in <stdexcept>: class length_error : public logic_error { public: explicit length_error(const string& __arg); }; Here is my exception: class rpn_expression_error : public logic_error { public: explicit rpn_expression_error(const string& __arg); }; Why do I get this error when <stdexcept> does not? Undefined symbols: rpn_expression_error::rpn_expression_error(/*string*/ const&), referenced from: ... ld: symbol(s) not found

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  • Getting "value is not defined" while inheriting a type

    - by James Black
    I can't see what I am doing wrong, as the files are in the correct order. In this case it is: BaseDAO.fs CreateDatabase.fs They are in the same namespace, but even when I had them in different modules, and opened the module in CreateDatabase the same error. The error is: Error 1 The value or constructor 'execNonQuery' is not defined I am trying to inherit BaseDAO and use a member that will be common to several files, and I don't see why I get the error above. namespace RestaurantServiceDAO open MySql.Data.MySqlClient type BaseDAO() = let connString = @"Server=localhost;Database=mysql;Uid=root;Pwd=$$$$;" let conn = new MySqlConnection(connString) member self.execNonQuery(sqlStr) = conn.Open() let comm = new MySqlCommand(sqlStr, conn, CommandTimeout = 10) comm.ExecuteNonQuery |> ignore comm.Dispose |> ignore The type that does inherit is here, and execNonQuery is not defined. namespace RestaurantServiceDAO open MySql.Data.MySqlClient type CreateDatabase() = inherit BaseDAO() let createRestaurantTable conn = execNonQuery "CREATE TABLE restaurant(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, name VARCHAR(100), cur_timestamp TIMESTAMP(8))"

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  • Refactoring a C# derived class with method dependancies

    - by drelihan
    Hi Folks, I want to get your opinion on this. I have a class which is derived from a base class. I don't have control over the code in the base class and it is critical to the system that I derive from it. In my class I inherite two methods that are critical to the system and are used in pretty much every function, many times. I intend to refactor this derived class and extract some classes from it - this won't be a problem. What I'm not sure about is, is it worth extracting class if I have to constantly make call backs to my main class to access the two methods (or public wrappers to the methods)??? Thanks

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  • Custom array class with assotiated objects

    - by FancyDancy
    I have a simple structure, it's just an array of model's objects. For example, it's a users with books. @books = Book.find(:all, :include = :users) I need to check, does user have a book? I have written a helper method: def has_book?(user_id) @books.select{|b| b.user_id == user_id}.any? end Then, i need to get only books from selected library def in_library(n) @books.select{|b| b.library == n} end I have tried to make custom Array class: class BooksList < Array def initialize(books) self << books end # its my custom methods def has_book?(user_id) self.select{|b| b.user_id == user_id}.any? end def in_library(n) self.select{|b| b.library == n} end end It works, but i have only one problem. I can't access Book's assotiated object (user). So i can't write: @books = BookList.new(Book.find(:all, :include => :users)) @books.first.user.id # it says undefined method `user' for #<Array:0x104b43e08>

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