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  • Can't declare an abstract method private....

    - by Zombies
    I want to do this, yet I can't. Here is my scenario and rational. I have an abstract class for test cases that has an abstract method called test(). The test() method is to be defined by the subclass; it is to be implemented with logic for a certain application, such as CRMAppTestCase extends CompanyTestCase. I don't want the test() method to be invoked directly, I want the super class to call the test() method while the sub class can call a method which calls this (and does other work too, such as setting a current date-time right before the test is executed for example). Example code: public abstract class CompanyTestCase { //I wish this would compile, but it cannot be declared private private abstract void test(); public TestCaseResult performTest() { //do some work which must be done and should be invoked whenever //this method is called (it would be improper to expect the caller // to perform initialization) TestCaseResult result = new TestCaseResult(); result.setBeginTime(new Date()); long time = System.currentTimeMillis(); test(); //invoke test logic result.setDuration(System.currentTimeMillis() - time); return result; } } Then to extend this.... public class CRMAppTestCase extends CompanyTestCase { public void test() { //test logic here } } Then to call it.... TestCaseResult result = new CRMAppTestCase().performTest();

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  • Querying using table-valued parameter

    - by antmx
    I need help please with writing a sproc, it takes a table-valued parameter @Locations, whose Type is defined as follows: CREATE TYPE [dbo].[tvpLocation] AS TABLE( [CountryId] [int] NULL, [ResortName] [nvarchar](100) NULL, [Ordinal] [int] NOT NULL, PRIMARY KEY CLUSTERED ( [Ordinal] ASC )WITH (IGNORE_DUP_KEY = OFF) ) @Locations will contain at least 1 row. Each row WILL have a non-null CountryId, and MAY have a non-null ResortName. Each row will have a unique Ordinal, the first being 0. The combinations of CountryId and ResortName in @Locations will be unique. The sproc needs to search against the following table structure. The image can be seen better by right-clicking it and View Image, or similar depending on your browser. Now this is where I'm stuck, the sproc should be able to find Tours where: The Tour's 1st TourHotel (Ordinal 0) has the same CountryId (and ResortName if specified) of the 1st row of @Locations (Ordinal 0). And also if @Locations has 1 row, the Tour must have additional TourHotels, ALL of which must be in the remaining CountryIds (and ResortNames if specified) of these remaining @Locations rows. Edit This is the code I finally used, based on Anthony Faull's suggestion. Thank you so much Anthony: select distinct T.Id from tblTour T join tblTourHotel TH on TH.TourId = T.Id join tblHotel H ON H.Id = TH.HotelId JOIN @Locations L ON ( ( L.Ordinal = 0 AND TH.Ordinal = 0 ) OR ( L.Ordinal > 0 AND TH.Ordinal > 0 ) ) AND L.CountryId = H.CountryId AND ( L.ResortName = H.ResortName OR L.ResortName IS NULL ) cross apply( select COUNT(TH2.Id) AS [Count] FROM tblTourHotel TH2 where TH2.TourId = TH.TourId ) TourHotelCount where TourHotelCount.[Count] = @LocationCount group by T.Id, T.TourRef, T.Description, T.DepartureDate, T.NumNights, T.DepartureAirportId, T.DestinationAirportId, T.AirlineId, T.FEPrice having COUNT(distinct TH.Id) = @LocationCount

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  • C++ STL question related to insert iterators and sets

    - by rshepherd
    #include #include #include #include using namespace std; class MyContainer { public: string value; MyContainer& operator=(const string& s) { this->value = s; return *this; } }; int main() { list<string> strings; strings.push_back("0"); strings.push_back("1"); strings.push_back("2"); set<MyContainer> containers; copy(strings.begin(), strings.end(), inserter(containers, containers.end())); } The preceeding code does not compile. In typical STL style the error output is verbose and difficult to understand. The key part seems to be this... /usr/include/c++/4.4/bits/stl_algobase.h:313: error: no match for ‘operator=’ in ‘__result.std::insert_iterator::operator* [with _Container = std::set, std::allocator ]() = __first.std::_List_iterator::operator* [with _Tp = std::basic_string, std::allocator ]()’ ...which I interpet to mean that the assignment operator needed is not defined. I took a look at the source code for insert_iterator and noted that it has overloaded the assignment operator. The copy algorithm must uses the insert iterators overloaded assignment operator to do its work(?). I guess that because my input iterator is on a container of strings and my output iterator is on a container of MyContainers that the overloaded insert_iterator assignment operator can no longer work. This is my best guess, but I am probably wrong. So, why exactly does this not work and how can I accomplish what I am trying to do?

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  • C++ STL question related to insert iterators and overloaded operators

    - by rshepherd
    #include <list> #include <set> #include <iterator> #include <algorithm> using namespace std; class MyContainer { public: string value; MyContainer& operator=(const string& s) { this->value = s; return *this; } }; int main() { list<string> strings; strings.push_back("0"); strings.push_back("1"); strings.push_back("2"); set<MyContainer> containers; copy(strings.begin(), strings.end(), inserter(containers, containers.end())); } The preceeding code does not compile. In standard C++ fashion the error output is verbose and difficult to understand. The key part seems to be this... /usr/include/c++/4.4/bits/stl_algobase.h:313: error: no match for ‘operator=’ in ‘__result.std::insert_iterator::operator* [with _Container = std::set, std::allocator ]() = __first.std::_List_iterator::operator* [with _Tp = std::basic_string, std::allocator ]()’ ...which I interpet to mean that the assignment operator needed is not defined. I took a look at the source code for insert_iterator and noted that it has overloaded the assignment operator. The copy algorithm must uses the insert iterators overloaded assignment operator to do its work(?). I guess that because my input iterator is on a container of strings and my output iterator is on a container of MyContainers that the overloaded insert_iterator assignment operator can no longer work. This is my best guess, but I am probably wrong. So, why exactly does this not work and how can I accomplish what I am trying to do?

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  • Python script not working when run from browser directly

    - by splatterdash
    I'm trying to run this script: import re, os def build_pool(cwd): global xtn_pool, file_pool xtn, xtn_pool = re.compile('\\.[0-9a-zA-Z]{1,4}$'), [] file_pool = [files for files in os.listdir(cwd) if os.path.isfile(files) and xtn.search(files)] # Lists all the file extension in the folder for file in file_pool: if not xtn_pool.__contains__(xtn.search(file).group()): xtn_pool.append(xtn.search(file).group()) return xtn_pool.sort(), file_pool if __name__ == '__main__': import sys #if path is given, change working directory to path if len(sys.argv) >= 2: os.chdir(sys.argv[1]) build_pool(os.getcwd()) #if no path is given when running, do renaming in current folder else: build_pool(os.getcwd()) print('The folder contains the following extensions: ') for i in range(0, len(xtn_pool)): print(repr(i+1) + '. ' + xtn_pool[i][1:]) opt = int(input('Which one would you like to replace? ')) xtn_pick = xtn_pool[opt-1] # Lists all the file with the chosen extension xtn_file_pool = [file for file in file_pool if file.endswith(xtn_pick)] print('There are {0} files with the {1} extension.'.format(len(xtn_file_pool), xtn_pick)) xtn_new = input('Input replacement extension: ') # The actual renaming process for file in xtn_file_pool: os.rename(file, file[:-len(xtn_pick)+1] + xtn_new) directly from my file browser (Nautilus), but for some reason it's not working. When I run it from terminal (python3 scriptname.py) it works fine as intended. But when I just click the script file in Nautilus, choose 'Run in Terminal', it always stops after asking 'Input replacement extension: '. How can I make this script run without using the terminal?

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  • Java JRE vs GCJ

    - by Martijn Courteaux
    Hi, I have this results from a speed test I wrote in Java: Java real 0m20.626s user 0m20.257s sys 0m0.244s GCJ real 3m10.567s user 3m5.168s sys 0m0.676s So, what is the but of GCJ then? With this results I'm sure I'm not going to compile it with GCJ! I tested this on Linux, are the results in Windows maybe better than that? This was the code from the application: public static void main(String[] args) { String str = ""; System.out.println("Start!!!"); for (long i = 0; i < 5000000L; i++) { Math.sqrt((double) i); Math.pow((double) i, 2.56); long j = i * 745L; String string = new String(String.valueOf(i)); string = string.concat(" kaka pipi"); // "Kaka pipi" is a kind of childly call in Dutch. string = new String(string.toUpperCase()); if (i % 300 == 0) { str = ""; } else { str += Long.toHexString(i); } } System.out.println("Stop!!!"); }

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  • Google App Engine/GWT/Eclipse Plugin Newbie Question- how to autobuild client side resources?

    - by Dieter Hanover
    Hi there, I'm tinkering with the default GWT application generated by the Google Eclipse plugin when I click the Google "New Web Application Project" button in Eclipse 3.5. This will no doubt be familiar to many of you.. basically there is an h1 title stating "Web Application Starter Project," a text field, and a Send button. What I've found is that whenever I make changes to the client side resources, e.g. change the text on the Send button to "Submit" in the .java file, Eclipse does not appear to autobuild these resources. In fact I have to rebuild the entire project in order for these changes to be reflected in my browser. I do have "build automatically" selected in eclipse. I should state that this is my second GWT project, the first was almost entirely server side (restlet on GAE) and everything built automatically nicely. When I first tried this new project with updated client resources, on refreshing my browser, the browser stated "you may need to (re)compile your project." I'm not sure if this is relevant but I thought I'd mention it all the same. So what's going on? How do I get Eclipse/GWT to autobuild these client side resources? Cheers for any help you can offer! :-)

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  • Pass the return type as a parameter in java?

    - by jonderry
    I have some files that contain logs of objects. Each file can store objects of a different type, but a single file is homogeneous -- it only stores objects of a single type. I would like to write a method that returns an array of these objects, and have the array be of a specified type (the type of objects in a file is known and can be passed as a parameter). Roughly, what I want is something like the following: public static <T> T[] parseLog(File log, Class<T> cls) throws Exception { ArrayList<T> objList = new ArrayList<T>(); FileInputStream fis = new FileInputStream(log); ObjectInputStream in = new ObjectInputStream(fis); try { Object obj; while (!((obj = in.readObject()) instanceof EOFObject)) { T tobj = (T) obj; objList.add(tobj); } } finally { in.close(); } return objList.toArray(new T[0]); } The above code doesn't compile (there's an error on the return statement, and a warning on the cast), but it should give you the idea of what I'm trying to do. Any suggestions for the best way to do this?

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  • C++ CRTP question

    - by aaa
    following piece of code does not compile, the problem is in T::rank not be inaccessible (I think) or uninitialized in parent template. Can you tell me exactly what the problem is? is passing rank explicitly the only way? or is there a way to query tensor class directly? Thank you #include <boost/utility/enable_if.hpp> template<class T, // size_t N, class enable = void> struct tensor_operator; // template<class T, size_t N> template<class T> struct tensor_operator<T, typename boost::enable_if_c< T::rank == 4>::type > { tensor_operator(T &tensor) : tensor_(tensor) {} T& operator()(int i,int j,int k,int l) { return tensor_.layout.element_at(i, j, k, l); } T &tensor_; }; template<size_t N, typename T = double> // struct tensor : tensor_operator<tensor<N,T>, N> { struct tensor : tensor_operator<tensor<N,T> > { static const size_t rank = N; }; I know the workaround, however am interested in mechanics of template instantiation for self-education

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  • const pod and std::vector

    - by Baz
    To get this code to compile: std::vector<Foo> factory() { std::vector<Foo> data; return data; } I have to define my POD like this: struct Foo { const int i; const int j; Foo(const int _i, const int _j): i(_i), j(_j) {} Foo(Foo& foo): i(foo.i), j(foo.j){} Foo operator=(Foo& foo) { Foo f(foo.i, foo.j); return f; } }; Is this the correct approach for defining a pod where I'm not interested in changing the pod members after creation? Why am I forced to define a copy constructor and overload the assignment operator? Is this compatible for different platform implementations of std::vector? Is it wrong in your opinion to have const PODS like this? Should I just leave them as non-const?

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  • g++ fails mysteriously only if a .h is in a certain directory

    - by ggambett
    I'm experiencing an extremely weird problem in a fresh OSX 10.4.11 + Xcode 2.5 installation. I've reduced it to a minimal test case. Here's test.cpp: #include "macros.h" int main (void) { return 1; } And here's macros.h: #ifndef __JUST_TESTING__ #define __JUST_TESTING__ template<typename T> void swap (T& pT1, T& pT2) { T pTmp = pT1; pT1 = pT2; pT2 = pTmp; } #endif //__JUST_TESTING__ This compiles and works just fine if both files are in the same directory. HOWEVER, if I put macros.h in /usr/include/gfc2 (it's part of a custom library I use) and change the #include in test.cpp, compilation fails with this error : /usr/include/gfc2/macros.h:4: error: template with C linkage I researched that error and most of the comments point to a "dangling extern C", which doesn't seem to be the case at all. I'm at a complete loss here. Is g++ for some reason assuming everything in /usr/include/gfc2 is C even though it's included from a .cpp file that doesn't say extern "C" anywhere? Any ideas? EDIT : It does compile if I use the full path in the #include, ie #include "/usr/include/gfc2/macros.h" EDIT2 : It's not including the wrong header. I've verified this using cpp, g++ -E, and renaming macros.h to foobarmacros.h

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  • Instantiating a class within a class

    - by Ink-Jet
    Hello. I'm trying to instantiate a class within a class, so that the outer class contains the inner class. This is my code: #include <iostream> #include <string> class Inner { private: std::string message; public: Inner(std::string m); void print() const; }; Inner::Inner(std::string m) { message = m; } void Inner::print() const { std::cout << message << std::endl; std::cout << message << std::endl; } class Outer { private: std::string message; Inner in; public: Outer(std::string m); void print() const; }; Outer::Outer(std::string m) { message = m; } void Outer::print() const { std::cout << message << std::endl; } int main() { Outer out("Hello world."); out.print(); return 0; } "Inner in", is my attempt at containing the inner within the outer, however, when I compile, i get an error that there is no matching function for call to Inner::Inner(). What have I done wrong? Thanks.

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  • Make conversion to a native type explicit in C++

    - by Tal Pressman
    I'm trying to write a class that implements 64-bit ints for a compiler that doesn't support long long, to be used in existing code. Basically, I should be able to have a typedef somewhere that selects whether I want to use long long or my class, and everything else should compile and work. So, I obviously need conversion constructors from int, long, etc., and the respective conversion operators (casts) to those types. This seems to cause errors with arithmetic operators. With native types, the compiler "knows" that when operator*(int, char) is called, it should promote the char to int and call operator*(int, int) (rather than casting the int to char, for example). In my case it gets confused between the various built-in operators and the ones I created. It seems to me like if I could flag the conversion operators as explicit somehow, that it would solve the issue, but as far as I can tell the explicit keyword is only for constructors (and I can't make constructors for built-in types). So is there any way of marking the casts as explicit? Or am I barking up the wrong tree here and there's another way of solving this? Or maybe I'm just doing something else wrong...

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  • Fortran severe (40) Error... Help?!

    - by Taka
    I can compile but when I run I get this error "forrtl: severe (40): recursive I/O operation, unit -1, file unknown" if I set n = 29 or more... Can anyone help with where I might have gone wrong? Thanks. PROGRAM SOLUTION IMPLICIT NONE ! Variable Declaration INTEGER :: i REAL :: dt DOUBLE PRECISION :: st(0:9) DOUBLE PRECISION :: stmean(0:9) DOUBLE PRECISION :: first_argument DOUBLE PRECISION :: second_argument DOUBLE PRECISION :: lci, uci, mean REAL :: exp1, n REAL :: r, segma ! Get inputs WRITE(*,*) 'Please enter number of trials: ' READ(*,*) n WRITE(*,*) dt=1.0 segma=0.2 r=0.1 ! For n Trials st(0)=35.0 stmean(0)=35.0 mean = stmean(0) PRINT *, 'For ', n ,' Trials' PRINT *,' 1 ',st(0) ! Calculate results DO i=0, n-2 first_argument = r-(1/2*(segma*segma))*dt exp1 = -(1/2)*(i*i) second_argument = segma*sqrt(dt)*((1/sqrt(2*3.1416))*exp(exp1)) st(i+1) = st(i) * exp(first_argument+second_argument) IF(st(i+1)<=20) THEN stmean(i+1) = 0.0 st(i+1) = st(i) else stmean(i+1) = st(i+1) ENDIF PRINT *,i+2,' ',stmean(i+1) mean = mean+stmean(i+1) END DO ! Output results uci = mean+(1.96*(segma/sqrt(n))) lci = mean-(1.96*(segma/sqrt(n))) PRINT *,'95% Confidence Interval for ', n, ' trials is between ', lci, ' and ', uci PRINT *,'' END PROGRAM SOLUTION

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  • How to construct objects based on XML code?

    - by the_drow
    I have XML files that are representation of a portion of HTML code. Those XML files also have widget declarations. Example XML file: <message id="msg"> <p> <Widget name="foo" type="SomeComplexWidget" attribute="value"> inner text here, sets another attribute or inserts another widget to the tree if needed... </Widget> </p> </message> I have a main Widget class that all of my widgets inherit from. The question is how would I create it? Here are my options: Create a compile time tool that will parse the XML file and create the necessary code to bind the widgets to the needed objects. Advantages: No extra run-time overhead induced to the system. It's easy to bind setters. Disadvantages: Adds another step to the build chain. Hard to maintain as every widget in the system should be added to the parser. Use of macros to bind the widgets. Complex code Find a method to register all widgets into a factory automatically. Advantages: All of the binding is done completely automatically. Easier to maintain then option 1 as every new widget will only need to call a WidgetFactory method that registers it. Disadvantages: No idea how to bind setters without introducing a maintainability nightmare. Adds memory and run-time overhead. Complex code What do you think is better? Can you guys suggest a better solution?

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  • login/logout problem in PHP

    - by user356170
    i have a question. I have a website in which i am giving security like login id and password( as usual). No what i want is that, 1) I don't want to allow a single user to login in different machine at the same time. 2) For this i am using a column in database which is keeping the current status of user(i.e. loging/logout). I am allowing user to login only when has session has not closed and status is login. 3) So my problem is that when i am logging out manually. it is closing the session as well as updating the database with status "logout". 4) but when i am closing the window from Cross buttonat top right corner. it is closing the ssion but table data is still "login". so later on i can't be able to login into the same user. 5) So how could i solve this problem. Please help me!

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  • Calling member functions dynamically

    - by user652511
    I'm pretty sure it's possible to call a class and its member function dynamically in Delphi, but I can't quite seem to make it work. What am I missing? // Here's a list of classes (some code removed for clarity) moClassList : TList; moClassList.Add( TClassA ); moClassList.Add( TClassB ); // Here is where I want to call an object's member function if the // object's class is in the list: for i := 0 to moClassList.Count - 1 do if oObject is TClass(moClassList[i]) then with oObject as TClass(moClassList[i]) do Foo(); I get an undeclared identifier for Foo() at compile. Clarification/Additional Information: What I'm trying to accomplish is to create a Change Notification system between business classes. Class A registers to be notified of changes in Class B, and the system stores a mapping of Class A - Class B. Then, when a Class B object changes, the system will call a A.Foo() to process the change. I'd like the notification system to not require any hard-coded classes if possible. There will always be a Foo() for any class that registers for notification. Maybe this can't be done or there's a completely different and better approach to my problem. By the way, this is not exactly an "Observer" design pattern because it's not dealing with objects in memory. Managing changes between related persistent data seems like a standard problem to be solved, but I've not found very much discussion about it. Again, any assistance would be greatly appreciated. Jeff

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  • C++ template type deduction problem

    - by hamishmcn
    motivation: I would like to create a utility class so that instead of having to write: if( someVal == val1 || someVal == val2 || someVal == val3 ) I could instead write: if( is(someVal).in(val1, val2, val3) ) which is much closer to the mathematical 'a is an element of (b,c,d)' and also would save on a lot of typing when the variable name 'someVal' is long. Here is the code I have so far (for 2 and 3 values): template<class T> class is { private: T t_; public: is(T t) : t_(t) { } bool in(const T& v1, const T& v2) { return t_ == v1 || t_ == v2; } bool in(const T& v1, const T& v2, const T& v3) { return t_ == v1 || t_ == v2 || t_ == v3; } }; However it fails to compile if I write: is(1).in(3,4,5); instead I have to write is<int>(1).in(3,4,5); Which isn't too bad, but it would be better if somehow the compiler could figure out that the type is int with out me having to explicitly specify it. Is there anyway to do this or I am stuck with specifying it explicitly?

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  • C++: Maybe you know this pitfall?

    - by Martijn Courteaux
    Hi, I'm developing a game. I have a header GameSystem (just methods like the game loop, no class) with two variables: int mouseX and int mouseY. These are updated in my game loop. Now I want to access them from Game.cpp file (a class built by a header-file and the source-file). So, I #include "GameSystem.h" in Game.h. After doing this I get a lot of compile errors. When I remove the include he says of course: Game.cpp:33: error: ‘mouseX’ was not declared in this scope Game.cpp:34: error: ‘mouseY’ was not declared in this scope Where I want to access mouseX and mouseY. All my .h files have Header Guards, generated by Eclipse. I'm using SDL and if I remove the lines that wants to access the variables, everything compiles and run perfectly (*). I hope you can help me... This is the error-log when I #include "GameSystem.h" (All the code he is refering to works, like explained by the (*)): In file included from ../trunk/source/domein/Game.h:14, from ../trunk/source/domein/Game.cpp:8: ../trunk/source/domein/GameSystem.h:30: error: expected constructor, destructor, or type conversion before ‘*’ token ../trunk/source/domein/GameSystem.h:46: error: variable or field ‘InitGame’ declared void ../trunk/source/domein/GameSystem.h:46: error: ‘Game’ was not declared in this scope ../trunk/source/domein/GameSystem.h:46: error: ‘g’ was not declared in this scope ../trunk/source/domein/GameSystem.h:46: error: expected primary-expression before ‘char’ ../trunk/source/domein/GameSystem.h:46: error: expected primary-expression before ‘bool’ ../trunk/source/domein/FPS.h:46: warning: ‘void FPS_SleepMilliseconds(int)’ defined but not used This is the code which try to access the two variables: SDL_Rect pointer; pointer.x = mouseX; pointer.y = mouseY; pointer.w = 3; pointer.h = 3; SDL_FillRect(buffer, &pointer, 0xFF0000);

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  • Which Android hardware devices should I test on? [closed]

    - by Tchami
    Possible Duplicate: What hardware devices do you test your Android apps on? I'm trying to compile a list of Android hardware devices that it would make sense to buy and test against if you want to target an as broad audience as possible, while still not buying every single Android device out there. I know there's a lot of information regarding screen sizes and Android versions available elsewhere, but: when developing for Android it's not terribly useful to know if the screen size of a device is 480x800 or 320x240, unless you feel like doing the math to convert that into Android "units" (i.e. small, normal, large or xlarge screens, and ldpi, mdpi, hdpi or xhdpi densities). Even knowing the dimensions of a device, you cannot be sure of the actual Android units as there's some overlap, see Range of screens supported in the Android documentation Taking into account the distribution of Platform versions and Screen Sizes and Densities, below is my current list based on information from the Wikipedia article on Comparison of Android devices. I'm fairly sure the information in this list is correct, but I'd welcome any suggestions/changes. Phones | Model | Android Version | Screen Size | Density | | HTC Wildfire | 2.1/2.2 | Normal | mdpi | | HTC Tattoo | 1.6 | Normal | mdpi | | HTC Hero | 2.1 | Normal | mdpi | | HTC Legend | 2.1 | Normal | mdpi | | Sony Ericsson Xperia X8 | 1.6/2.1 | Normal | mdpi | | Motorola Droid | 2.0-2.2 | Normal | hdpi | | Samsung Galaxy S II | 2.3 | Normal | hdpi | | Samsung Galaxy Nexus | 4.0 | Normal | xhdpi | | Samsung Galaxy S III | 4.0 | Normal | xhdpi | **Tablets** | Model | Android Version | Screen Size | Density | | Samsung Galaxy Tab 7" | 2.2 | Large | hdpi | | Samsung Galaxy Tab 10" | 3.0 | X-Large | mdpi | | Asus Transformer Prime | 4.0 | X-Large | mdpi | | Motorola Xoom | 3.1/4.0 | X-Large | mdpi | N.B.: I have seen (and read) other posts on SO on this subject, e.g. Which Android devices should I test against? and What hardware devices do you test your Android apps on? but they don't seem very canonical. Maybe this should be marked community wiki?

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  • in haskell, why do I need to specify type constraints, why can't the compiler figure them out?

    - by Steve
    Consider the function, add a b = a + b This works: *Main> add 1 2 3 However, if I add a type signature specifying that I want to add things of the same type: add :: a -> a -> a add a b = a + b I get an error: test.hs:3:10: Could not deduce (Num a) from the context () arising from a use of `+' at test.hs:3:10-14 Possible fix: add (Num a) to the context of the type signature for `add' In the expression: a + b In the definition of `add': add a b = a + b So GHC clearly can deduce that I need the Num type constraint, since it just told me: add :: Num a => a -> a -> a add a b = a + b Works. Why does GHC require me to add the type constraint? If I'm doing generic programming, why can't it just work for anything that knows how to use the + operator? In C++ template programming, you can do this easily: #include <string> #include <cstdio> using namespace std; template<typename T> T add(T a, T b) { return a + b; } int main() { printf("%d, %f, %s\n", add(1, 2), add(1.0, 3.4), add(string("foo"), string("bar")).c_str()); return 0; } The compiler figures out the types of the arguments to add and generates a version of the function for that type. There seems to be a fundamental difference in Haskell's approach, can you describe it, and discuss the trade-offs? It seems to me like it would be resolved if GHC simply filled in the type constraint for me, since it obviously decided it was needed. Still, why the type constraint at all? Why not just compile successfully as long as the function is only used in a valid context where the arguments are in Num? Thank you.

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  • How to align all fields as label width grows

    - by TheCloudlessSky
    I have a form where the labels are on the left and the fields on the right. This layout works great when the labels have small amounts of text. I can easily set a min-width on the labels to ensure they all have the same distance to the fields. In the first picture below, this works as expected. A problem arises when the label's text becomes too long, it'll either overflow to the next line or push the field on the same line over to the left (as seen in picture 2). This doesn't push the other labels so it is left with a "jagged" look. Ideally, it should like to style it as picture 3 with something like the following markup: <fieldset> <label>Name</label><input type="text" /><br /> <label>Username</label><input type="text" /> </fieldset> I created a jsFiddle to show the issue. Of course, the easy cross-browser way to solve this would be to use tables. That just creates tag-hell for something that should be so simple. Note: this does not need to support IE6.

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  • Class lookup structure array in C++

    - by wyatt
    I'm trying to create a structure array which links input strings to classes as follows: struct {string command; CommandPath cPath;} cPathLookup[] = { {"set an alarm", AlarmCommandPath}, {"send an email", EmailCommandPath}, {"", NULL} }; which will be used as follows: CommandPath *cPath = NULL; string input; getline(cin, input); for(int i = 0; cPathLookup[i] != ""; i++) { if(cPathLookup[i].command == input) cPath = new cPathLookup[i].cPath; } Obviously, this code is meaningless, but I think my intention is apparent - depending on input, I'd like cPath to be initialized as either a new AlarmCommandPath or a new EmailCommandPath. I could handle it with a function returning an instance depending on input, but a whole sequence of ifs just seems inelegant. I should also note that, in case it's not apparent and important, that AlarmCommandPath and EmailCommandPath are derived from CommandPath, and CommandPath is an abstract class. Thanks for any help you can offer. EDIT: I just noticed that, in spite of CommandPath being abstract, I have a declaration: CommandPath *cPath = NULL; in working code. Why does that compile?

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  • Strange overloading rules in C++

    - by bucels
    I'm trying to compile this code with GCC 4.5.0: #include <algorithm> #include <vector> template <typename T> void sort(T, T) {} int main() { std::vector<int> v; sort(v.begin(), v.end()); } But it doesn't seem to work: $ g++ -c nm.cpp nm.cpp: In function ‘int main()’: nm.cpp:9:28: error: call of overloaded ‘sort(std::vector<int>::iterator, std::vector<int>::iterator)’ is ambiguous nm.cpp:4:28: note: candidates are: void sort(T, T) [with T = __gnu_cxx::__normal_iterator<int*, std::vector<int> >] /usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/bits/stl_algo.h:5199:69: note: void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<int*, std::vector<int> >] Comeau compiles this code without errors. (4.3.10.1 Beta2, strict C++03, no C++0x) Is this valid C++?

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  • Web service failing when installed in seperate project from the website...

    - by Adam
    I am totally new to web services and cannot get mine to work. My setup is on VS 2008 using IIS. I have one solution file with 3 projects in it: website, code, and services. If I put my webservice into my website and call it locally then it will work fine (it's just a hello world web service). I want to put the service into a different location for use from multiple sites. I don't know what I'm doing wrong - i've read so much conflicting info regarding disco files, access files, silverlight, flash, java, etc. I just looking for quick simple steps to create a web service that I can access from javascript and deploy to a seperate website. End goal is to have functionality in webservices so that website will call via JS and run much smoother in the loading time and async calls. Do I need to create a disco file? Do I need to configure security? -- I know this is prob best, i'm just looking to get it working at all. Do I need to allow cross browser access on IIS or on my hosted server? Are there any quick reference websites that you can recommend? Should I be using WCF as new technology? - I saw this on MSDN but seems to be more for windows apps then web apps. I'm not getting any specific error codes. I have installed the firefox debugging tools (firebug) and I can see what the headers are but I don't know how to interpret them and there is no response being passed back. Any help is appreciated!!!

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