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• To what extent should code try to explain fatal exceptions?

  - by Andrzej Doyle

  I suspect that all non-trivial software is likely to experience situations where it hits an external problem it cannot work around and thus needs to fail. This might be due to bad configuration, an external server being down, disk full, etc. In these situations, especially if the software is running in non-interactive mode, I expect that all one can really do is log an error and wait for the admin to read the logs and fix the problem. If someone happens to interact with the software in the meantime, e.g. a request comes in to a server that failed to initialize properly, then perhaps an appropriate hint can be given to check the logs and maybe even the error can be echoed (depending on whether you can tell if they're a technical guy as opposed to a business user). For the moment though let's not think too hard about this part. My question is, to what extent should the software be responsible for trying to explain the meaning of the fatal error? In general, how much competence/knowledge are you allowed to presume on administrators of the software, and how much should you include troubleshooting information and potential resolution steps when logging fatal errors? Of course if there's something that's unique to the runtime context this should definitely be logged; but lets assume your software needs to talk to Active Directory via LDAP and gets back an error "[LDAP: error code 49 - 80090308: LdapErr: DSID-0C090334, comment: AcceptSecurityContext error, data 525, vece]". Is it reasonable to assume that the maintainers will be able to Google the error code and work out what it means, or should the software try to parse the error code and log that this is caused by an incorrect user DN in the LDAP config? I don't know if there is a definitive best-practices answer for this, so I'm keen to hear a variety of views.

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• How to map a Dictionary<string, string> spanning several tables

  - by Kim Johansson

  I have four tables: CREATE TABLE [Languages] ( [Id] INTEGER IDENTITY(1,1) NOT NULL, [Code] NVARCHAR(10) NOT NULL, PRIMARY KEY ([Id]), UNIQUE INDEX ([Code]) ); CREATE TABLE [Words] ( [Id] INTEGER IDENTITY(1,1) NOT NULL, PRIMARY KEY ([Id]) ); CREATE TABLE [WordTranslations] ( [Id] INTEGER IDENTITY(1,1) NOT NULL, [Value] NVARCHAR(100) NOT NULL, [Word] INTEGER NOT NULL, [Language] INTEGER NOT NULL, PRIMARY KEY ([Id]), FOREIGN KEY ([Word]) REFERENCES [Words] ([Id]), FOREIGN KEY ([Language]) REFERENCES [Languages] ([Id]) ); CREATE TABLE [Categories] ( [Id] INTEGER IDENTITY(1,1) NOT NULL, [Word] INTEGER NOT NULL, PRIMARY KEY ([Id]), FOREIGN KEY ([Word]) REFERENCES [Words] ([Id]) ); So you get the name of a Category via the Word - WordTranslation - Language relations. Like this: SELECT TOP 1 wt.Value FROM [Categories] AS c LEFT JOIN [WordTranslations] AS wt ON c.Word = wt.Word WHERE wt.Language = ( SELECT TOP 1 l.Id FROM [Languages] WHERE l.[Code] = N'en-US' ) AND c.Id = 1; That would return the en-US translation of the Category with Id = 1. My question is how to map this using the following class: public class Category { public virtual int Id { get; set; } public virtual IDictionary<string, string> Translations { get; set; } } Getting the same as the SQL query above would be: Category category = session.Get<Category>(1); string name = category.Translations["en-US"]; And "name" would now contain the Category's name in en-US. Category is mapped against the Categories table. How would you do this and is it even possible?

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• DotNetOpenAuth / WebSecurity Basic Info Exchange

  - by Jammer

  I've gotten a good number of OAuth logins working on my site now. My implementation is based on the WebSecurity classes with amends to the code to suit my needs (I pulled the WebSecurity source into mine). However I'm now facing a new set of problems. In my application I have opted to make the user email address the login identifier of choice. It's naturally unique and suits this use case. However, the OAuth "standards" strikes again. Some providers will return your email address as "username" (Google) some will return the display name (Facebook). As it stands I see to options given my particular scenario: Option 1 Pull even more framework source code into my solution until I can chase down where the OpenIdRelyingParty class is actually interacted with (via the DotNetOpenAuth.AspNet facade) and make addition information requests from the OpenID Providers. Option 2 When a user first logs in using an OpenID provider I can display a kind of "complete registration" form that requests missing info based on the provider selected.* Option 2 is the most immediate and probably the quickest to implement but also includes some code smells through having to do something different based on the provider selected. Option 1 will take longer but will ultimately make things more future proof. I will need to perform richer interactions down the line so this also has an edge in that regard. The more I get into the code it does seem that the WebSecurity class itself is actually very limiting as it hides lots of useful DotNetOpenAuth functionality in the name of making integration easier. Andrew (the author of DNOA) has said that the Attribute Exchange stuff happens in the OpenIdRelyingParty class but I cannot see from the DotNetOpenAuth.AspNet source code where this class is used so I'm unsure of what source would need to be pulled into my code in order to enable the functionality I need. Has anyone completely something similar?

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• Is there any algorithm that can solve ANY traditional sudoku puzzles, WITHOUT guessing (or similar techniques)?

  - by justin

  Is there any algorithm that solves ANY traditional sudoku puzzle, WITHOUT guessing? Here Guessing means trying an candidate and see how far it goes, if a contradiction is found with the guess, backtracking to the guessing step and try another candidate; when all candidates are exhausted without success, backtracking to the previous guessing step (if there is one; otherwise the puzzle proofs invalid.), etc. EDIT1: Thank you for your replies. traditional sudoku means 81-box sudoku, without any other constraints. Let us say the we know the solution is unique, is there any algorithm that can GUARANTEE to solve it without backtracking? Backtracking is a universal tool, I have nothing wrong with it but, using a universal tool to solve sudoku decreases the value and fun in deciphering (manually, or by computer) sudoku puzzles. How can a human being solve the so called "the hardest sudoku in the world", does he need to guess? I heard some researcher accidentally found that their algorithm for some data analysis can solve all sudoku. Is that true, do they have to guess too?

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• Consolidate multiple site files into single location

  - by seengee

  We have a custom PHP/MySQL CMS running on Linux/Apache thats rolled out to multiple sites (20+) on the same server. Each site uses exactly the same CMS files with a few files for each site being customised. The customised files for each site are: /library/mysql_connect.php /public_html/css/* /public_html/ftparea/* /public_html/images/* There's also a couple of other random files inside /public_html/includes/ that are unique to each site. Other than this each site on the server uses the exact same files. Each site sitting within /home/username/. There is obviously a massive amount of replication here as each time we want to deploy a system update we need to update to each user account. Given the common site files are all stored in SVN it would make far more sense if we were able to simply commit to SVN and deploy to a single location direct from there. Unfortunately, making a major architecture change at this stage could be problematic. In my mind the ideal scenario would mean creating an account like /home/commonfiles/ and each site using these common files unless an account specific file exists, for example a request is made to /home/user/public_html/index.php but as this file doesnt exist the request is then redirected to /home/commonfiles/public_html/index.php. I know that generally this approach is possible, similar to how Zend Framework (and probably others) redirect all requests that dont match a specific file to index.php. I'm just not sure about how exactly to go about implementing it and whether its actually advisable. Would really welcome any input/ideas people have got. Thanks.

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• How would I find all sets of N single-digit, non-repeating numbers that add up to a given sum in PHP

  - by TerranRich

  Let's say I want to find all sets of 5 single-digit, non-repeating numbers that add up to 30... I'd end up with [9,8,7,5,1], [9,8,7,4,2], [9,8,6,4,3], [9,8,6,5,2], [9,7,6,5,3], and [8,7,6,5,4]. Each of those sets contains 5 non-repeating digits that add up to 30, the given sum. Any help would be greatly appreciated. Even just a starting point for me to use would be awesome. I came up with one method, which seems like a long way of going about it: get all unique 5-digit numbers (12345, 12346, 12347, etc.), add up the digits, and see if it equals the given sum (e.g. 30). If it does, add it to the list of possible matching sets. I'm doing this for a personal project, which will help me in solving Kakuro puzzles without actually solving the whole thing at once. Yeah, it may be cheating, but it's... it's not THAT bad... :P

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• How to have multiple instances of jQuery plugin on single page?

  - by James Skidmore

  I'm writing a simple jQuery plugin, but I'm having trouble being able to use multiple instances on a page. For instance, here is a sample plugin to illustrate my point: (function($) { $.fn.samplePlugin = function(options) { if (typeof foo != 'undefined') { alert('Already defined!'); } else { var foo = 'bar'; } }; })(jQuery); And then if I do this: $(document).ready(function(){ $('#myDiv').samplePlugin({}); // does nothing $('#myDiv2').samplePlugion({}); // alerts "Already defined!" }); This is obviously an over-simplified example to get across the point. So my question is, how do I have two separate instances of the plugin? I'd like to be able to use it across multiple instances on the same page. I'm guessing that part of the problem might be with defining the variables in a global scope. How can I define them unique to that instance of the plugin then? Thank you for your guidance!

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• Forum achievements on a phpBB3 board?

  - by VIVA LA NWO

  I recently started a new community. The forum software is phpBB3, and so far so good. In an attempt to make my community more unique and interesting, I had to idea of having user achievements. Let me give you a quick run-down. Each user has achievements that they can earn (these will probably be across all users), for example an achievement for when a user hits 1,000 posts, when they upload an avatar, when one of their topics gets 1,000 views and so on. Each achievement has points, for example an achievement like uploading an avatar will be 10 points and reaching 10,000 points will grant 50 achievement points. If anyone here plays World of Warcraft you may be seeing where I'm getting the ideas from. :) What I'm struggling to get my head around though is how exactly to code this... I could keep a record of all users activity and add it to a special database table possibly, and then check via cron every minute or so if any user has met achievement criteria... but then I also want it controllable through the ACP so I can easily add new achievements and change their points etc. My mind is pretty blank when it comes to anything but the most simple things. What I really posted here for was feedback on the idea and how you all think I should go about doing this. The coding part should be pretty simple for me once I get my head around how phpBBB MODs need to be written. Thanks for reading, and I look forward to your replies. :)

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• Good data structure for efficient insert/querying on arbitrary properties

  - by Juliet

  I'm working on a project where Arrays are the default data structure for everything, and every query is a linear search in the form of: Need a customer with a particular name? customer.Find(x => x.Name == name) Need a customer with a particular unique id? customer.Find(x => x.Id == id) Need a customer of a particular type and age? customer.Find(x => x is PreferredCustomer && x.Age >= age) Need a customer of a particular name and age? customer.Find(x => x.Name == name && x.Age == age) In almost all instances, the criteria for lookups is well-defined. For example, we only search for customers by one or more of the properties Id, Type, Name, or Age. We rarely search by anything else. Is a good data structure to support arbitrary queries of these types with lookup better than O(n)? Any out-of-the-box implementations for .NET?

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• How do I establish table association in JPA / Hibernate with existing database?

  - by Paperino

  Currently I have two tables in my database Encounters and Referrals: There is a one to many relationship between these two tables. Currently they are linked together with foreign keys. Right now I have public class Encounter extends JPASupport implements java.io.Serializable { @Column(name="referralid", unique=false, nullable=true, insertable=true, updatable=true) public Integer referralid; } But what I really want is public class Encounter extends JPASupport implements java.io.Serializable { .......... @OneToMany(cascade=CascadeType.PERSIST) public Set<Referrals> referral; ............ } So that I can eventually do a query like this: List<Encounter> cases = Encounter.find( "select distinct p from Encounter p join p.referrals as t where t.caseid =103" ).fetch(); How do I tell JPA that even though I have non-standard column names for my foreign keys and primary keys that its the object models that I want linked, not simply the integer value for the keys? Does this make sense? I hope so. Thanks in advanced!

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• MVVM: where is the best place to integrate an id counter, in ViewModel or Repository or... ?

  - by msfanboy

  Hello, I am using http://loungerepo.codeplex.com/ this library needs a unique id when I persist my entities to the repository. I decided for integer not Guid. The question is now where do I retrieve a new integer and how do I do it? This is my current approach in the SchoolclassAdministrationViewModel.cs: public SchoolclassAdministrationViewModel() { _schoolclassRepo = new SchoolclassRepository(); _pupilRepo = new PupilRepository(); _subjectRepo = new SubjectRepository(); _currentSchoolclass = new SchoolclassModel(); _currentPupil = new PupilModel(); _currentSubject = new SubjectModel(); ... } private void AddSchoolclass() { // get the last free id for a schoolclass entity _currentSchoolclass.SchoolclassID = _schoolclassRepo.LastID; // add the new schoolclass entity to the repository _schoolclassRepo.Add(SchoolclassModel.SchoolclassModelToSchoolclass(_currentSchoolclass)); // add the new schoolclass entity to the ObservableCollection bound to the View Schoolclasses.Add(_currentSchoolclass); // Create a new schoolclass entity and the bound UI controls content gets cleaned CurrentSchoolclass = new SchoolclassModel(); } public class SchoolclassRepository : IRepository<Schoolclass> { private int _lastID; public SchoolclassRepository() { _lastID = FetchLastId(); } public void Add(Schoolclass entity) { //repo.Store(entity); } private int FetchLastId() { return // repo.GetIDOfLastEntryAndDoInc++ } public int LastID { get { return _lastID; } } } Explanation: Every time the user switches to the SchoolclassAdministrationViewModel which is datatemplated with a UserControl the saVM Ctor is called and the schoolclass repository is created wherein the FetchLastId() is called and I am up to date with the last ID doing a inc++ on it to get the free one... Do you have any better ideas? What I do not like about my current apporach: -Having a private method in repositry class because a repositry is to fetch data only not "entity logic" like the counter - Having to access from the ViewModel a public property - located in the repository -, actually its not the ViewModel concern to get a entity id and assign it. Actually the ViewModel should ask for a schoolclass POCO and get a SchoolclassModel to bind to the UI. But then I have again to re-read the Schoolclass properties into the SchoolclassModel properties what I want to avoid.

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• Determining polygon intersection and containment

  - by Victor Liu

  I have a set of simple (no holes, no self-intersections) polygons, and I need to check that they don't intersect each other (one can be entirely contained in another; that is okay). I can check this by simply checking the per-vertex inside-ness of one polygon versus other polygons. I also need to determine the containment tree, which is the set of relationships that say which polygon contains any given polygon. Since no polygon can intersect any other, then any contained polygon has a unique container; the "next-bigger" one. In other words, if A contains B contains C, then A is the parent of B, and B is the parent of C, and we don't consider A the parent of C. The question: How do I efficiently determine the containment relationships and check the non-intersection criterion? I ask this as one question because maybe a combined algorithm is more efficient than solving each problem separately. The algorithm should take as input a list of polygons, given by a list of their vertices. It should produce a boolean B indicating if none of the polygons intersect any other polygon, and also if B = true, a list of pairs (P, C) where polygon P is the parent of child C. This is not homework. This is for a hobby project I am working on.

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• Django - foreignkey problem

  - by realshadow

  Hey, Imagine you have this model: class Category(models.Model): node_id = models.IntegerField(primary_key = True) type_id = models.IntegerField(max_length = 20) parent_id = models.IntegerField(max_length = 20) sort_order = models.IntegerField(max_length = 20) name = models.CharField(max_length = 45) lft = models.IntegerField(max_length = 20) rgt = models.IntegerField(max_length = 20) depth = models.IntegerField(max_length = 20) added_on = models.DateTimeField(auto_now = True) updated_on = models.DateTimeField(auto_now = True) status = models.IntegerField(max_length = 20) node = models.ForeignKey(Category_info, verbose_name = 'Category_info', to_field = 'node_id' The important part is the foreignkey. When I try: Category.objects.filter(type_id = type_g.type_id, parent_id = offset, status = 1) I get an error that get returned more than category, which is fine, because it is supposed to return more than one. But I want to filter the results trough another field, which would be type id (from the second Model) Here it is: class Category_info(models.Model): objtree_label_id = models.AutoField(primary_key = True) node_id = models.IntegerField(unique = True) language_id = models.IntegerField() label = models.CharField(max_length = 255) type_id = models.IntegerField() The type_id can be any number from 1 - 5. I am desparately trying to get only one result where the type_id would be number 1. Here is what I want in sql: SELECT n.*, l.* FROM objtree_nodes n JOIN objtree_labels l ON (n.node_id = l.node_id) WHERE n.type_id = 15 AND n.parent_id = 50 AND l.type_id = 1 Any help is GREATLY appreciated. Regards

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• Why is my django bulk database population so slow and frequently failing?

  - by bryn

  I decided I'd like to use django's model system rather than coding raw SQL to interface with my database, but I am having a problem that surely is avoidable. My models.py contains: class Student(models.Model): student_id = models.IntegerField(unique = True) form = models.CharField(max_length = 10) preferred = models.CharField(max_length = 70) surname = models.CharField(max_length = 70) and I'm populating it by looping through a list as follows: from models import Student for id, frm, pref, sname in large_list_of_data: s = Student(student_id = id, form = frm, preferred = pref, surname = sname) s.save() I don't really want to be saving this to the database each time but I don't know another way to get django to not forget about it (I'd rather add all the rows and then do a single commit). There are two problems with the code as it stands. It's slow -- about 20 students get updated each second. It doesn't even make it through large_list_of_data, instead throwing a DatabaseError saying "unable to open database file". (Possibly because I'm using sqlite3.) My question is: How can I stop these two things from happening? I'm guessing that the root of both problems is that I've got the s.save() but I don't see a way of easily batching the students up and then saving them in one commit to the database.

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• keyUp event heard?: Overridden NSView method

  - by Old McStopher

  UPDATED: I'm now overriding the NSView keyUp method from a NSView subclass set to first responder like below, but am still not seeing evidence that it is being called. @implementation svsView - (BOOL)acceptsFirstResponder { return YES; } - (void)keyUp:(NSEvent *)event { //--do key up stuff-- NSLog(@"key up'd!"); } @end --ORIGINAL POST-- I'm new to Cocoa and Obj-C and am trying to do a (void)keyUp: from within the implementation of my controller class (which itself is of type NSController). I'm not sure if this is the right place to put it, though. I have a series of like buttons each set to a unique key equivalent (IB button attribute) and each calls my (IBAction)keyInput method which then passes the identity of each key onto another object. This runs just fine, but I also want to track when each key is released. --ORIGINAL [bad] EXAMPLE-- @implementation svsController //init //IBActions - (IBAction)keyInput:(id)sender { //--do key down stuff-- } - (void)keyUp:(NSEvent *)event { //--do key up stuff-- } @end Upon fail, I also tried the keyUp as an IBAction (instead of void), like the user-defined keyInput is, and hooked it up to the appropriate buttons in Interface Builder, but then keyUp was only called when the keys were down and not when released. (Which I kind of figured would happen.) Pardon my noobery, but should I be putting this method in another class or doing something differently? Wherever it is, though, I need it be able to access objects owned by the controller class. Thanks for any insight you may have.

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• Plotting an Arc in Discrete Steps

  - by phobos51594

  Good afternoon, Background My question relates to the plotting of an arbitrary arc in space using discrete steps. It is unique, however, in that I am not drawing to a canvas in the typical sense. The firmware I am designing is for a gcode interpreter for a CNC mill that will translate commands into stepper motor movements. Now, I have already found a similar question on this very site, but the methodology suggested (Bresenham's Algorithm) appears to be incompatable for moving an object in space, as it only relies on the calculation of one octant of a circle which is then mirrored about the remaining axes of symmetry. Furthermore, the prescribed method of calculation an arc between two arbitrary angles relies on trigonometry (I am implementing on a microcontroller and would like to avoid costly trig functions, if possible) and simply not taking the steps that are out of the range. Finally, the algorithm only is designed to work in one rotational direction (e.g. counterclockwise). Question So, on to the actual question: Does anyone know of a general-purpose algorithm that can be used to "draw" an arbitrary arc in discrete steps while still giving respect to angular direction (CW / CCW)? The final implementation will be done in C, but the language for the purpose of the question is irrelevant. Thank you in advance. References S.O post on drawing a simple circle using Bresenham's Algorithm: "Drawing" an arc in discrete x-y steps Wiki page describing Bresenham's Algorithm for a circle http://en.wikipedia.org/wiki/Midpoint_circle_algorithm Gcode instructions to be implemented (see. G2 and G3) http://linuxcnc.org/docs/html/gcode.html

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• A GUID as the MySQL table's Primary Key or as a separate column

  - by Ben

  I have a multi-process program that performs, in a 2 hour period, 5-10 million inserts to a 34GB table within a single Master/Slave MySQL setup (plus an equal number of reads in that period). The table in question has only 5 fields and 3 (single field) indexes. The primary key is auto-incrementing. I am far from a DBA, but the database appears to be crippled during this two hour period. So, I have a couple of general questions. 1) How much bang will I get out of batching these writes into units of 10? Currently, I am writing each insert serially because, after writing, I immediately need to know, in my program, the resulting primary key of each insert. The PK is the only unique field presently and approximating the order of insertion with something like a Datetime field or a multi-column value is not acceptable. If I perform a bulk insert, I won't know these IDs, which is a problem. So, I've been thinking about turning the auto-increment primary key into a GUID and enforcing uniqueness. I've also been kicking around the idea of creating a new column just for the purposes of the GUID. I don't really see the what that achieves though, that the PK approach doesn't already offer. As far as I can tell, the big downside to making the PK a randomly generated number is that the index would take a long time to update on each insert (since insertion order would not be sequential). Is that an acceptable approach for a table that is taking this number of writes? Thanks, Ben

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• Proper status codes for JSON responses to Ajax calls?

  - by anonymous coward

  My project is returning JSON to Ajax calls from the browser. I'm wondering what the proper status code is for sending back with responses to invalid (but successfully handled) data submissions. For example, jQuery has the following two particular callbacks when making Ajax requests: success: Fired when a 200/2xx status code is delivered along with the response. error: Fired when 4xx, 5xx, etc, status codes come back with the response. If a user attempts to create a new "Person" object, I send back a JSON representation of the newly created object upon success, thus giving javascript access to the necessary unique ID's for the new object, etc. This, of course, is sent with a 200 status code. If a user submits malformed or invalid data (say, an invalid/incomplete "name" field), I would like to send back the validation error messages via JSON. (I don't see why this would be a bad thing). My question is: in doing so, should I send a 200 status code, because I successfully handled their invalid data? Therefore, I'd be using the jQuery success callback, but simply check for errors... Or, should I use a 4xx status code, perhaps 'Bad Request', because the data they sent me is invalid? (and thus, use the error callback to do the necessary client-side notifications).

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• Initialize Static Array of Structs in C

  - by russell_h

  I implementing a card game in C. There are lots of types of cards and each has a bunch of information, including some actions that will need to be individually scripted associated with it. Given a struct like this (and I'm not certain I have the syntax right for the function pointer) struct CARD { int value; int cost; // This is a pointer to a function that carries out actions unique // to this card int (*do_actions) (struct GAME_STATE *state, int choice1, int choice2); }; I would like to initialize a static array of these, one for each card. I'm guessing this would look something like this int do_card0(struct GAME_STATE *state, int choice1, int choice2) { // Operate on state here } int do_card1(struct GAME_STATE *state, int choice1, int choice2) { // Operate on state here } extern static struct cardDefinitions[] = { {0, 1, do_card0}, {1, 3, do_card1} }; Will this work, and am I going about this the right way at all? I'm trying to avoid huge numbers of switch statements. Do I need to define the 'do_cardN' functions ahead of time, or is there some way to define them inline in the initialization of the struct (something like a lambda function in python)? I'll need read-only access to cardDefinitions from a different file - is 'extern static' correct for that? I know this is a lot of questions rolled into one but I'm really a bit vague about how to go about this. Thanks.

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• SQL Duplicates Issue SQL SERVER 2000

  - by jeff

  I have two tables : Product and ProductRateDetail. The parent table is Product. I have duplicate records in the product table which need to be unique. There are entries in the ProductRateDetail table which correspond to duplicate records in the product table. Somehow I need to update the ProductRateDetail table to match the original (older) ID from the Product table and then remove the duplicates from the product table. I would do this manually but there are 100's of records. i.e. something like UPDATE tbl_productRateDetail SET productID = (originalID from tbl_product) then something like DELETE from tbl_product WHERE duplicate ID and only delete the recently added ID data example: (sorry can't work out this formatting thing) tbl_Product select * from dbo.Product where ProductCode = '10003' ProductID ProductTypeID ProductDescription ProductCode ProductSize 365 1 BEND DOUBLE FLANGED 10003 80mmX90deg 1354 1 BEND DOUBLE FLANGED 10003 80mmX90deg tbl_ProductRateDetail SELECT * FROM [MSTS2].[dbo].[ProductRateDetail] WHERE ProductID in (365,1354) ProductRateDetailID ProductRateID ProductID UnitRate 365 1 365 16.87 1032 5 365 16.87 2187 10 365 16.87 2689 11 365 16.87 3191 12 365 16.87 7354 21 1354 21.30 7917 22 1354 21.30 8480 23 1354 21.30 9328 25 1354 21.30 9890 26 1354 21.30 10452 27 1354 21.30 Please help!

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• Create new or update existing entity at one go with JPA

  - by Alex R

  A have a JPA entity that has timestamp field and is distinguished by a complex identifier field. What I need is to update timestamp in an entity that has already been stored, otherwise create and store new entity with the current timestamp. As it turns out the task is not as simple as it seems from the first sight. The problem is that in concurrent environment I get nasty "Unique index or primary key violation" exception. Here's my code: // Load existing entity, if any. Entity e = entityManager.find(Entity.class, id); if (e == null) { // Could not find entity with the specified id in the database, so create new one. e = entityManager.merge(new Entity(id)); } // Set current time... e.setTimestamp(new Date()); // ...and finally save entity. entityManager.flush(); Please note that in this example entity identifier is not generated on insert, it is known in advance. When two or more of threads run this block of code in parallel, they may simultaneously get null from entityManager.find(Entity.class, id) method call, so they will attempt to save two or more entities at the same time, with the same identifier resulting in error. I think that there are few solutions to the problem. Sure I could synchronize this code block with a global lock to prevent concurrent access to the database, but would it be the most efficient way? Some databases support very handy MERGE statement that updates existing or creates new row if none exists. But I doubt that OpenJPA (JPA implementation of my choice) supports it. Event if JPA does not support SQL MERGE, I can always fall back to plain old JDBC and do whatever I want with the database. But I don't want to leave comfortable API and mess with hairy JDBC+SQL combination. There is a magic trick to fix it using standard JPA API only, but I don't know it yet. Please help.

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• What exactly is a reentrant function?

  - by eSKay

  Most of the times, the definition of reentrance is quoted from Wikipedia: A computer program or routine is described as reentrant if it can be safely called again before its previous invocation has been completed (i.e it can be safely executed concurrently). To be reentrant, a computer program or routine: Must hold no static (or global) non-constant data. Must not return the address to static (or global) non-constant data. Must work only on the data provided to it by the caller. Must not rely on locks to singleton resources. Must not modify its own code (unless executing in its own unique thread storage) Must not call non-reentrant computer programs or routines. How is safely defined? If a program can be safely executed concurrently, does it always mean that it is reentrant? What exactly is the common thread between the six points mentioned that I should keep in mind while checking my code for reentrant capabilities? Also, Are all recursive functions reentrant? Are all thread-safe functions reentrant? Are all recursive and thread-safe functions reentrant? While writing this question, one thing comes to mind: Are the terms like reentrance and thread safety absolute at all i.e. do they have fixed concrete definations? For, if they are not, this question is not very meaningful. Thanks!

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• how to Solve the "Digg" problem in MongoDB

  - by user193116

  A while back,a Digg developer had posted this blog ,"http://about.digg.com/blog/looking-future-cassandra", where the he described one of the issues that were not optimally solved in MySQL. This was cited as one of the reasons for their move to Cassandra. I have been playing with MongoDB and I would like to understand how to implement the MongoDB collections for this problem From the article, the schema for this information in MySQL : CREATE TABLE Diggs ( id INT(11), itemid INT(11), userid INT(11), digdate DATETIME, PRIMARY KEY (id), KEY user (userid), KEY item (itemid) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; CREATE TABLE Friends ( id INT(10) AUTO_INCREMENT, userid INT(10), username VARCHAR(15), friendid INT(10), friendname VARCHAR(15), mutual TINYINT(1), date_created DATETIME, PRIMARY KEY (id), UNIQUE KEY Friend_unique (userid,friendid), KEY Friend_friend (friendid) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; This problem is ubiquitous in social networking scenario implementation. People befriend a lot of people and they in turn digg a lot of things. Quickly showing a user what his/her friends are up to is very critical. I understand that several blogs have since then provided a pure RDBMs solution with indexes for this issue; however I am curious as to how this could be solved in MongoDB.

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• Design Philosophy Question - When to create new functions

  - by Eclyps19

  This is a general design question not relating to any language. I'm a bit torn between going for minimum code or optimum organization. I'll use my current project as an example. I have a bunch of tabs on a form that perform different functions. Lets say Tab 1 reads in a file with a specific layout, tab 2 exports a file to a specific location, etc. The problem I'm running into now is that I need these tabs to do something slightly different based on the contents of a variable. If it contains a 1 I may need to use Layout A and perform some extra concatenation, if it contains a 2 I may need to use Layout B and do no concatenation but add two integer fields, etc. There could be 10+ codes that I will be looking at. Is it more preferable to create an individual path for each code early on, or attempt to create a single path that branches out only when absolutely required. Creating an individual path for each code would allow my code to be extremely easy to follow at a glance, which in turn will help me out later on down the road when debugging or making changes. The downside to this is that I will increase the amount of code written by calling some of the same functions in multiple places (for example, steps 3, 5, and 9 for every single code may be exactly the same. Creating a single path that would branch out only when required will be a bit messier and more difficult to follow at a glance, but I would create less code by placing conditionals only at steps that are unique. I realize that this may be a case-by-case decision, but in general, if you were handed a previously built program to work on, which would you prefer?

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• Modifying CSS class property values on the fly with JavaScript / jQuery

  - by JPN

  all. I've run into a unique situation that I have so far been unable to find a solution for: dynamically assigning a value to a CSS style. I know how to use jQuery to assign width, height, etc. to an element, but what I'm trying to do is actually change the value defined in the stylesheet so that the dynamically-created value can be assigned to multiple elements. What I'm building is a slideshow of images that occupy the full viewport, recalculating the image's width, height, and left properties on resize so that the image is always centered, favors width over height, except when the viewport is taller than it is wide (resizing does not reload the page, just fires a function to resize the image). I have successfully been able to get it to work on one image, and now I'm trying to determine the best way to assign those property values to all images in the slideshow without having to specify those three things individually for every image. Can the values of properties in a class be modified on the fly? I'm sure the answer is out there, I'm probably just not using the correct terminology in my searches. Hope I did a good job of describing the problem. TIA.

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