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• Laws of Computer Science and Programming

  - by Jonas

  We have Amdahl's law that basically states that if your program is 10% sequential you can get a maximum 10x performance boost by parallelizing your application. Another one is Wadler's law which states that In any language design, the total time spent discussing a feature in this list is proportional to two raised to the power of its position. 0. Semantics 1. Syntax 2. Lexical syntax 3. Lexical syntax of comments My question is this: What are the most important (or at least significant / funny but true / sad but true) laws of Computer Science and programming? I want named laws, and not random theorems, So an answer should look something like Surname's (law|theorem|conjecture|corollary...) Please state the law in your answer, and not only a link. Edit: The name of the law does not need to contain it's inventors surname. But I do want to know who stated (and perhaps proved) the law

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• QPluginLoader with PyQt modules as plugins: possible?

  - by Thorfin

  Hi! I have a C++ application that loads externals plugins thanks to QPluginloader. QPluginLoader provides access to a Qt plugin. A Qt plugin is stored in a shared library (a DLL). The plugins have to inherit from a pure virtual class ( and Q_DECLARE_INTERFACE ) and QObject. I would like to create plugins by using python and PyQt. Is this possible without too much work? For instance if I have to convert the modules to C++ with boost::python, this could be nonsense, as PyQt is already a conversion from C++ to Python... Thanks!

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• Is this a good code (came across while reading code of a colleague)

  - by eriks

  // file a.hpp class a; typedef boost::shared_ptr<a> aPtr class a{ public: static aPtr CreateImp(); virtual void Foo() = 0 ; .... }; //file aImp.hpp class aImp : public a{ virtual void Foo(); }; //file aImp.cpp aPtr a::CreateImp() { return aPtr(new aImp()); } void aImp::Foo(){} The client must use CreateImp to get ptr to 'a', and can't use 'a' other ways. What do you think about this implementation? what do you think about this king of implementation?

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• Virtual Earth Shape Rendering Performance

  - by Mike

  I am overlaying a transparent image on my VEMap control by rendering it as a single VEShape. The shape changes sizes dynamically depeding on the zoom level of my map and can be as large as 4000*4000px. In older browsers such as IE6 and early versions of Firefox 2.x, map control performance degrades rapidly when my shape gets larger than 1500*1500px. The mouse pointer moves slowly and the map responds very slowly to events. I don't see this issue at all in newer browsers (IE7+). Are there any workarounds to boost performance of rendering a large shape for IE6 users?

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• What are the pro and cons of statically linking a library?

  - by Mathieu Pagé

  Hi, I want to release an application I developed as a hobby both for Linux and Windows. This application depends on boost (and possibly other libraries). The norm for this kind of application (a chess engine) is to provide only an executable file and possibly some helper files. I tough it would be a good idea to statically link the libraries so the executable would not have any dependencies. So the end user can just put the executable in a directory and start using it. However, while doing some research online I found some negative comments about statically linking libraries, some even arguing that an application with statically linked libraries would be hardly portable, meaning that it would only run on my system of highly similar systems. So what are the pros and cons of statically linking library? I already know that the executable will be bigger. But I can't see why it would make my application less portable.

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• Custom iterator for a class based on two sets

  - by Dan Hook

  I have a class that contains two sets. They both contain the same key type but have different compare operations. I would like to provide an iterator for the class that iterates through the elements of both sets. I want to start with one set, then when I increment an iterator pointing to the last element of the first set, I want to go to the first element of the second set. How do I do this? I would like to preserve the bidirectional iterator semantics of std::set, but if it turns out that implementing a forward iterator is much easier, so be it. I'm willing to use the Boost Iterator library if that would help.

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• OpenCV, Python (ctypes), and OS X 10.5: dylib errors/confusion

  - by Jeff

  I have tried two brands of ctypes (ctypes-opencv and another) wrappers for OpenCV (2.0 from svn head), can use Python and OpenCV in the Python console, but whenever I try to import the packages (from ctypes_opencv import *) from another Python file, I get the following error message: OSError: dlopen(/usr/local/lib/libcv.dylib, 6): Symbol not found: __ZN2cv10fastMallocEm Referenced from: /usr/local/lib/libcv.dylib Expected in: flat namespace And some more Python errors: module body in __init__.py at line 18 from ctypes_opencv.cxcore import * module body in cxcore.py at line 80 _cxDLL, _cvDLL, _hgDLL = detect_opencv() function detect_opencv in cxcore.py at line 59 cvDLL = cdll.LoadLibrary(find_lib('cv')) function LoadLibrary in __init__.py at line 431 return self._dlltype(name) function __init__ in __init__.py at line 353 self._handle = _dlopen(self._name, mode) I most recently built OpenCV using flags the ctypes_opencv wiki recommended: ENABLE_SSE(2) OFF and BUILD_NEW_PYTHON_SUPPORT OFF. I've tried a number of flag combinations though. I also tried using the MacPorts install of OpenCV, got the same messages, uninstalled it, and now it errors out as well. :P And if that wasn't enough, I tried using the latest ctypes wrapper, but get boost-jam errors. Any help would be very much appreciated.

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• Inverted index in a search engine

  - by Ayman

  Hello there, I'm trying to write some code to make a small application for searching text from files. Files should be crawled, and I need to put an inverted index to boost searches. My problem is that I kind of have ideas about how the parser would be, I'm willing to implement the AND, NOT, OR in the query. Whereas, I couldn't figure out how my index should be... I have never created an inverted index so if any body could suggest a feasable way to do it I would be very grateful... I do know in theory how it works but my problem is I absolutely have no idea to make happen in MySql I need to give keywords being indexed a weight too... Thank you so much.

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• Expression Tree

  - by nettguy

  My understanding of expression tree is : Expression trees are in-memory representation of expression like arithmetic or boolean expression.The expressions are stored into the parsed tree.so we can easily transalate into any other language. Linq to SQL uses expression tree.Normally in LINQ to SQL query the compiler translates it into parsed expression trees.These are passed to Sql Server as T-SQL Statements.The Sql server executes the T-SQL query and sends down the result back.That is why when you execute LINQ to SQL you gets IQueryable<T> not IEnumetrable<T>.Because IQuerybale contains public IQueryable:IEnumerable { Type Element {get;} Expression Expression {get;} IQueryaleProvider Provider {get;} } Questions : Microsoft uses Expression trees to play with LINQ-to-Sql.What are the different ways can i use expression trees to boost my code. Apart from LINQ to SQL,Linq to amazon ,who used expression trees in their applications? Linq to Object return IEnumerable,Linq to SQL return IQueryable ,What does LINQ to XML return?

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• console windows not close after stopping program

  - by robUK

  Hello, Visual Studio 2005 C++ Windows XP Pro I have a sample application that I am testing, that links with boost libraries. However, the program runs ok. However, when I try and stop the program by clicking the 'Stop Debugging' button. The program ends, but the console window remains open. So I have many of them, as during my testing I am starting and stopping the application. Even when I try and close it by clicking the close button it has no affect. And it doesn't seem to appear under task manager when the program ends. The only way I can close them if I reboot windows. I am thinking it might be a thread that has not closed, and maybe that is keeping the console windows open. Many thanks for any advice,

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• How can I refactor this to use an inline function or template instead of a macro?

  - by BillyONeal

  Hello, everyone :) I have a useful macro here: #define PATH_PREFIX_RESOLVE(path, prefix, environment) \ if (boost::algorithm::istarts_with(path, prefix)) { \ ExpandEnvironmentStringsW(environment, buffer, MAX_PATH); \ path.replace(0, (sizeof(prefix)/sizeof(wchar_t)) - 1, buffer); \ if (Exists(path)) return path; \ } It's used about 6 times within the scope of a single function (that's it), but macros seem to have "bad karma" :P Anyway, the problem here is the sizeof(prefix) part of the macro. If I just replace this with a function taking a const wchar_t[], then the sizeof() will fail to deliver expected results. Simply adding a size member doesn't really solve the problem either. Making the user supply the size of the constant literal also results in a mess of duplicated constants at the call site. Any ideas on this one?

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• FOX toolkit, file concatentation

  - by Robb

  I'm using the FOX Toolkit and C++ to develop a GUI. I'm Having an issue with the FXFile::concat(...) command. The method looks like: FXFile::concat(const FXString & srcfile1, const FXString & srcfile2, const FXString & dstfile, FXbool overwrite = false And when I call it with the srcfile1 and dstfile being the same thing (because I need to simply add to srcfile1), the method does not work. dstfile is empty. Anyone have experience with FOX and this concatentation problem? I'm open to other suggestions as well, including boost.

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• Algorithm - Pick the best combination of Armor (for a game)

  - by Chu

  I'm designing a piece of a game where the AI needs to determine which combination of armor will give the best overall stat bonus to the character. Each character will have about 10 stats, of which only 3-4 are important, and of those important ones, a few will be more important than the others. Armor will also give a boost to 1 or all stats. For example, a shirt might give +4 to the character's int and +2 stamina while at the same time, a pair of pants may have +7 strength and nothing else. So let's say that a character has a healthy choice of armor to use (5 pairs of pants, 5 pairs of gloves, etc.) We've designated that Int and Perception are the most important stats for this character. How could I write an algorithm that would determine which combination of armor and items would result in the highest of any given stat (say in this example Int and Perception)?

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• Random noise in Solr score

  - by Andrea Campi

  I am looking for a way of introducing random noise into my scoring function, and I'm at a loss on how to best proceed. Some background: We use Solr for a web application that manages large-ish sets of photos for agencies. One customer has an interesting requirement for scoring: 'quality' field, maintained by editors, from 1 (highest) to 3 (lowest); 'date' field, boosting more recent photos; I would probably use a logarithmic function; However, due to how the stock photo market works, this will likely result in many similar photos appearing together. Their request is to give 'quality' a large boost, but introduce some randomness so that photos will not appear in a strict date order. Any idea? EDITED: a key requirement is to have "stable" query results: if I search twice for "tropical island" I can get a slightly different result set, but if I ask for the first page, then the second, then the first, I'd better get the same results :)

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• C++ 'mutable' keyword

  - by Rob

  A while ago I came across some code that marked a member variable of a class with the 'mutable' keyword. As far as I can see it simply allows you to modify a variable in a 'const' method: class Foo { private: mutable bool done_; public: void doSomething() const { ...; done_ = true; } }; Is this the only use of this keyword or is there more to it than meets the eye? I have since used this technique in a class, marking a boost::mutex as mutable allowing const functions to lock it for thread-safety reasons, but, to be honest, it feels like a bit of a hack.

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• Toubleshooting mapkit performance

  - by brettr

  I'm plotting over 500 points on a map using mapkit. Zooming is a little jittery compared to the native google map app. I've discovered what is causing the slowness. I'm adding custom annotations so that I can later add different pin colors and buttons for detail views: - (MKAnnotationView *) mapView:(MKMapView *)mapView viewForAnnotation:(AddressNote *) annotation { MKPinAnnotationView *annView=[[MKPinAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"currentlocation"]; annView.pinColor = MKPinAnnotationColorGreen; annView.animatesDrop=TRUE; annView.canShowCallout = YES; annView.calloutOffset = CGPointMake(-5, 5); return annView; } If I comment out the above code, everything works fine. Very smooth with zooming in/out. Should I be adding annotations differently to boost performance?

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• .NET - downloading multiple pages from a website with a single DNS query

  - by lampak

  I'm using HttpRequest to download several pages from a website (in a loop). Simplifying it looks like this: HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create( "http://sub.domain.com/something/" + someString ); HttpWebResponse resp = (HttpWebResponse)req.GetResponse(); //do something I'm not quite sure actually but every request seems to resolve the address again (I don't know how to test if I'm right). I would like to boost it a little and resolve the address once and then reuse it for all requests. I can't work out how to force HttpRequest into using it, though. I have tried using Dns.GetHostAddresses, converting the result to a string and passing it as the address to HttpWebRequest.Create. Unfortunately, server returns error 404 then. I managed to google that's probably because the "Host" header of the http query doesn't match what the server expects. Is there a simple way to solve this?

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• The ctags command doesn't recurse saying "it is not a regular file".

  - by indiv

  When I run ctags -R *, I get errors saying that all directories are not regular files and it skips them instead of recursively generating tags for them. ctags: skipping arpa: it is not a regular file. ctags: skipping asm: it is not a regular file. ctags: skipping asm-generic: it is not a regular file. ctags: skipping bits: it is not a regular file. ctags: skipping blkid: it is not a regular file. ctags: skipping boost: it is not a regular file. What is the problem?

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• If-elseif-else Logic Question

  - by Changeling

  I have a set of three values, call them x, y, and z. If value A happens to match only one in the set x, y, and z, then that means we have a proper match and we stop searching for a match, even if it is at y. It can match any one in that set. These values x, y, and z are non-constant so I cannot use a switch-case statement. How do I do this with an if-elseif-else statements without having to use GOTO. I am using C++ (no boost or any of that other fancy stuff). Now, I am trying to do this in code and it is racking my brain this morning (not enough coffee?)

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• Using * in SELECT Query

  - by libregeek

  I am currently porting an application written in MySQL3 and PHP4 to MySQL5 and PHP5. On analysis I found several SQL queries which uses "select * from tablename" even if only one column(field) is processed in PHP. The table has almost 60 columns and it has a primary key. In most cases, the only column used is id which is the primary key. Will there be any performance boost if I use queries in which the column names are explicitly mentioned instead of * ? (In this application there is only one method which we need all the columns and all other methods return only a subset of the columns)

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• Code reading: where can i read great, modern, and well-documented C++ code?

  - by baol

  Reading code is one of the best ways to learn new idioms, tricks, and techniques. Sadly it's very common to find badly written C++ code. Some use C++ as it was C, others as if it was Java, some just shoot in their feet. I believe gtkmm is a good example of C++ design, but a binding could not be the better code to read (you need to know the C library behind that). Boost libraries (at least the one I read) tend to be less readable than I'd like. Can you mention open source projects (or other projects which source is freely readable) that are good example of readable, modern, well-documented, and auto-contained, C++ code to learn from? (I believe that one project per answer will be better, and I'd include the motivation that led you to selecting that one.)

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• Expression Tree : C#

  - by nettguy

  My understanding of expression tree is : Expression trees are in-memory representation of expression like arithmetic or boolean expression.The expressions are stored into the parsed tree.so we can easily transalate into any other language. Linq to SQL uses expression tree.Normally when our LINQ to SQL query compiler translates it to parsed expression trees.These are passed to Sql Server as T-SQL Statements.The Sql server executes the T-SQL query and sends down the result back.That is why when you execute LINQ to SQL you gets IQueryable<T> not IEnumetrable<T>.Because IQuerybale contains public IQueryable:IEnumerable { Type Element {get;} Expression Expression {get;} IQueryaleProvider Provider {get;} } Questions : Microsoft uses Expression trees to play with LINQ-to-Sql.What are the different ways can i use expression trees to boost my code. Apart from LINQ to SQL,Linq to amazon ,who used expression trees in their applications? Linq to Object return IEnumerable,Linq to SQL return IQueryable ,What does LINQ to XML return?

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• Container of shared_ptr's but iterate with raw pointers

  - by Sean Lynch

  I have a class that holds a list containing boost::shared_ptrs to objects of another class. The class member functions that give access to the elemets in the list return raw pointers. For consistency I'd also like to be able to iterate with raw pointers instead of shared_ptrs. So when I dereference the list iterator, I'd like to get raw pointer, not a shared_ptr. I assume I need to write a custom iterator for this. Is this correct? If so can someone point me in the right direction - I've never done this before.

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• Why would the assignment operator ever do something different than its matching constructor?

  - by Neil G

  I was reading some boost code, and came across this: inline sparse_vector &assign_temporary(sparse_vector &v) { swap(v); return *this; } template<class AE> inline sparse_vector &operator=(const sparse_vector<AE> &ae) { self_type temporary(ae); return assign_temporary(temporary); } It seems to be mapping all of the constructors to assignment operators. Great. But why did C++ ever opt to make them do different things? All I can think of is scoped_ptr?

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• Is there a standard Cyclic Iterator in C++

  - by Hippicoder

  Based on the following question: Check if one string is a rotation of other string I was thinking of making a cyclic iterator type that takes a range, and would be able to solve the above problem like so: std::string s1 = "abc" ; std::string s2 = "bca" ; std::size_t n = 2; // number of cycles cyclic_iterator it(s2.begin(),s2.end(),n); cyclic_iterator end; if (std::search(it, end, s1.begin(),s1.end()) != end) { std::cout << "s1 is a rotation of s2" << std::endl; } My question, Is there already something like this available? I've check Boost and STL doesn't have one. I've got a simple hand-written one but would rather use an already made/tested implementation.

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