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• Discover periodic patterns in a large data-set

  - by Miner

  I have a large sequence of tuples on disk in the form (t1, k1) (t2, k2) ... (tn, kn) ti is a monotonically increasing timestamp and ki is a key (assume a fixed length string if needed). Neither ti nor ki are guaranteed to be unique. However, the number of unique tis and kis is huge (millions). n itself is very large (100 Million+) and the size of k (approx 500 bytes) makes it impossible to store everything in memory. I would like to find out periodic occurrences of keys in this sequence. For example, if I have the sequence (1, a) (2, b) (3, c) (4, b) (5, a) (6, b) (7, d) (8, b) (9, a) (10, b) The algorithm should emit (a, 4) and (b, 2). That is a occurs with a period of 4 and b occurs with a period of 2. If I build a hash of all keys and store the average of the difference between consecutive timestamps of each key and a std deviation of the same, I might be able to make a pass, and report only the ones that have an acceptable std deviation(ideally, 0). However, it requires one bucket per unique key, whereas in practice, I might have very few really periodic patterns. Any better ways?

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• Constructor is being invoked twice

  - by Knowing me knowing you

  In code: LINT a = "12"; LINT b = 3; a = "3";//WHY THIS LINE INVOKES CTOR? std::string t = "1"; //LINT a = t;//Err NO SUITABLE CONV FROM STRING TO LINT. Shouldn't ctor do it? #pragma once #include "LINT_rep.h" class LINT { private: typedef LINT_rep value_type; const value_type* my_data_; template<class T> void init_(const T&); public: LINT(const char* = 0); LINT(const std::string&); LINT(const LINT&); LINT(const long_long&); LINT& operator=(const LINT&); virtual ~LINT(void); LINT operator+()const; //DONE LINT operator+(const LINT&)const;//DONE LINT operator-()const; //DONE LINT operator-(const LINT&)const;//DONE LINT operator*(const LINT&)const;//DONE LINT operator/(const LINT&)const;///WAITS FOR APPROVAL LINT& operator+=(const LINT&);//DONE LINT& operator-=(const LINT&);//DONE LINT& operator*=(const LINT&);//DONE LINT operator/=(const LINT&);///WAITS FOR APPROVAL }; in line number 3 instead of assignment optor ctor is invoked. Why? I'm willing to uppload entire solution on some server otherwise it's hard to put everything in here. I can also upload video file. Another thing is that when I implement this assignment optor I'm getting an error that this optor is already in obj file? What's going on?

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• Default template parameters with forward declaration

  - by Seth Johnson

  Is it possible to forward declare a class that uses default arguments without specifying or knowing those arguments? For example, I would like to declare a boost::ptr_list< TYPE > in a Traits class without dragging the entire Boost library into every file that includes the traits. I would like to declare namespace boost { template<class T> class ptr_list< T >; }, but that doesn't work because it doesn't exactly match the true class declaration: template < class T, class CloneAllocator = heap_clone_allocator, class Allocator = std::allocator<void*> > class ptr_list { ... }; Are my options only to live with it or to specify boost::ptr_list< TYPE, boost::heap_clone_allocator, std::allocator<void*> in my traits class? (If I use the latter, I'll also have to forward declare boost::heap_clone_allocator and include <memory>, I suppose.) I've looked through Stroustrup's book, SO, and the rest of the internet and haven't found a solution. Usually people are concerned about not including STL, and the solution is "just include the STL headers." However, Boost is a much more massive and compiler-intensive library, so I'd prefer to leave it out unless I absolutely have to.

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• Binder and variadic template ends up in a segmentation fault

  - by phlipsy

  I wrote the following program #include <iostream> template<typename C, typename Res, typename... Args> class bind_class_t { private: Res (C::*f)(Args...); C *c; public: bind_class_t(Res (C::*f)(Args...), C* c) : f(f), c(c) { } Res operator() (Args... args) { return (c->*f)(args...); } }; template<typename C, typename Res, typename... Args> bind_class_t<C, Res, Args...> bind_class(Res (C::*f)(Args...), C* c) { return bind_class<C, Res, Args...>(f, c); } class test { public: int add(int x, int y) { return x + y; } }; int main() { test t; // bind_class_t<test, int, int, int> b(&test::add, &t); bind_class_t<test, int, int, int> b = bind_class(&test::add, &t); std::cout << b(1, 2) << std::endl; return 0; } compiled it with gcc 4.3.3 and got a segmentation fault. After spending some time with gdb and this program it seems to me that the addresses of the function and the class are mixed up and a call of the data address of the class isn't allowed. Moreover if I use the commented line instead everything works fine. Can anyone else reproduce this behavior and/or explain me what's going wrong here?

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• Integrating legacy Ajax script in CakePHP

  - by octavian

  Hi, I have a legacy script that I would like to integrate in a cakephp app. The script makes use of $_POST and such and since I'm quite a noob I would need some help for the integration. Here is how the script looks like: THE JAVASCRIPT: prototype.js builder.js (these two are from the prototype fw) lib.js (makes a ajax requests to remote.php) THE PHP remote.php (contains FastJSON class and $_POST vars) if ($_POST['cmd'] == 'SAVETEAM' && $_POST['info']) { $INFO = json_decode(str_replace('\"', '"', $_POST['info'])); $nr = 1; $SORT = array($INFO->GK, $INFO->DEF, $INFO->MID, $INFO->FOR, $INFO->RZ); foreach ($SORT as $STD) foreach ($STD as $v) mysql_query("UPDATE players_teams SET fieldposition = ".$nr++." WHERE player_id = {$v->player_id} AND team_id = {$v->team_id}") or die(mysql_error()); // CAPTAION mysql_query("UPDATE `teams` SET captain = '{$_POST['captain']}' WHERE `user_id` = {$_POST['userid']}") or die(mysql_error()); } transfers.php (containts the form that uses the javascript and link to the JS) I have really no idea how to structure the files and calls in cakephp. Currently I have "Undefined index: cmd [APP/vendors/remote.php, line 230]" errors since I use $_POST['cmd'] (I placed remote.php in Vendors and included it, the JS was just included old fashion way, as a link and appears in the source code). How can I make this work? I'm sorry but I'm not familiar with AJAX and Cake... If you want a full look at the code, here it is: http://octavian.be/thecode.zip Thank you for reading and helping me out.

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• How to generate random numbers of lognormal distribution within specific range in Matlab

  - by Harpreet

  My grain sizes are defined as D=[1.19,1.00,0.84,0.71,0.59,0.50,0.42]. The problem is described below in steps. Grain sizes should follow lognormal distribution. The mean of the grain sizes is fixed as 0.84 and the standard deviation should be as low as possible but not zero. 90% of the grains (by weight %) fall in the size range of 1.19 to 0.59, and the rest 10% fall in size range of 0.50 to 0.42. Now I want to find the probabilities (weight percentage) of the grains falling in each grain size. It is allowable to split this grain size distribution into further small sizes but it must always be in the range of 1.19 and 0.42, i.e. 'D' can be continuous but 0.42 < D < 1.19. I need it fast. I tried on my own but I am not able to get the correct result. I am getting negative probabilities (weight percentages). Thanks to anyone who helps. I didn't incorporate the point 3 as I came to know about that condition later. Here are simple steps I tried: %% D=[1.19,1.00,0.84,0.71,0.59,0.50,0.42]; s=0.30; % std dev of the lognormal distribution m=0.84; % mean of the lognormal distribution mu=log(m^2/sqrt(s^2+m^2)); % mean of the associated normal dist. sigma=sqrt(log((s^2/m^2)+1)); % std dev of the associated normal dist. [r,c]=size(D); for i=1:c D(i)=mu+(sigma.*randn(1)); w(i)=(log(D(i))-mu)/sigma; % the probability or the wt. percentage of the grain sizes end grain_size=exp(D); %%

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• protecting COM interfaces from exceptions

  - by rmeador

  I have several dozen objects exposed through COM interfaces, each of which with many methods, totaling a few hundred methods. These interfaces expose business objects from my app to a scripting engine. I have been given the task of protecting every single one of these methods from exceptions being thrown (to catch them and return an error using COM's Error() function, which incidentally I can find no documentation on because it's impossible to google). To my understanding, this requires that I add a try/catch around the guts of each one of these methods. The catch blocks are going to be similar or identical for each and every one of these hundreds of methods, which strongly smells of a problem (massively violates the DRY principle), but I can't think of any way to avoid changing every method. As far as I can tell, these methods are invoked directly by COM, with no intervening code that I can hook into to catch the exceptions. My current best idea is to make a macro for the catch block, but that has it's own sort of code-smell. Can anyone come up with a better approach? BTW, my app's exceptions do not derive from std::exception, so if there is some way of COM automatically handling standard exceptions, it won't help. And I sadly cannot change the existing exceptions to derive from std::exception.

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• Generating lognormally distributed random number from mean, coeff of variation

  - by Richie Cotton

  Most functions for generating lognormally distributed random numbers take the mean and standard deviation of the associated normal distribution as parameters. My problem is that I only know the mean and the coefficient of variation of the lognormal distribution. It is reasonably straight forward to derive the parameters I need for the standard functions from what I have: If mu and sigma are the mean and standard deviation of the associated normal distribution, we know that coeffOfVar^2 = variance / mean^2 = (exp(sigma^2) - 1) * exp(2*mu + sigma^2) / exp(mu + sigma^2/2)^2 = exp(sigma^2) - 1 We can rearrange this to sigma = sqrt(log(coeffOfVar^2 + 1)) We also know that mean = exp(mu + sigma^2/2) This rearranges to mu = log(mean) - sigma^2/2 Here's my R implementation rlnorm0 <- function(mean, coeffOfVar, n = 1e6) { sigma <- sqrt(log(coeffOfVar^2 + 1)) mu <- log(mean) - sigma^2 / 2 rlnorm(n, mu, sigma) } It works okay for small coefficients of variation r1 <- rlnorm0(2, 0.5) mean(r1) # 2.000095 sd(r1) / mean(r1) # 0.4998437 But not for larger values r2 <- rlnorm0(2, 50) mean(r2) # 2.048509 sd(r2) / mean(r2) # 68.55871 To check that it wasn't an R-specific issue, I reimplemented it in MATLAB. (Uses stats toolbox.) function y = lognrnd0(mean, coeffOfVar, sizeOut) if nargin < 3 || isempty(sizeOut) sizeOut = [1e6 1]; end sigma = sqrt(log(coeffOfVar.^2 + 1)); mu = log(mean) - sigma.^2 ./ 2; y = lognrnd(mu, sigma, sizeOut); end r1 = lognrnd0(2, 0.5); mean(r1) % 2.0013 std(r1) ./ mean(r1) % 0.5008 r2 = lognrnd0(2, 50); mean(r2) % 1.9611 std(r2) ./ mean(r2) % 22.61 Same problem. The question is, why is this happening? Is it just that the standard deviation is not robust when the variation is that wide? Or have a screwed up somewhere?

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• Internal "Tee" setup

  - by RadlyEel

  I have inherited some really old VC6.0 code that I am upgrading to VS2008 for building a 64-bit app. One required feature that was implemented long, long ago is overriding std::cout so its output goes simultaneously to a console window and to a file. The implementation depended on the then-current VC98 library implementation of ostream and, of course, is now irretrievably broken with VS2008. It would be reasonable to accumulate all the output until program termination time and then dump it to a file. I got part of the way home by using freopen(), setvbuf(), and ios::sync_with_stdio(), but to my dismay, the internal library does not treat its buffer as a ring buffer; instead when it flushes to the output device it restarts at the beginning, so every flush wipes out all my accumulated output. Converting to a more standard logging function is not desirable, as there are over 1600 usages of "std::cout << " scattered throughout almost 60 files. I have considered overriding ostream's operator<< function, but I'm not sure if that will cover me, since there are global operator<< functions that can't be overridden. (Or can they?) Any ideas on how to accomplish this?

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• Template operator linker error

  - by Dani

  I have a linker error I've reduced to a simple example. The build output is: debug/main.o: In function main': C:\Users\Dani\Documents\Projects\Test1/main.cpp:5: undefined reference tolog& log::operator<< (char const (&) [6])' collect2: ld returned 1 exit status It looks like the linker ignores the definition in log.cpp. I also cant put the definition in log.h because I include the file alot of times and it complains about redefinitions. main.cpp: #include "log.h" int main() { log() << "hello"; return 0; } log.h: #ifndef LOG_H #define LOG_H class log { public: log(); template<typename T> log &operator <<(T &t); }; #endif // LOG_H log.cpp: #include "log.h" #include <iostream> log::log() { } template<typename T> log &log::operator <<(T &t) { std::cout << t << std::endl; return *this; }

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• Insert MANY key value pairs fast into berkeley db with hash access

  - by Kungi

  Hi, i'm trying to build a hash with berkeley db, which shall contain many tuples (approx 18GB of key value pairs), but in all my tests the performance of the insert operations degrades drastically over time. I've written this script to test the performance: #include<iostream> #include<db_cxx.h> #include<ctime> #define MILLION 1000000 int main () { long long a = 0; long long b = 0; int passes = 0; int i = 0; u_int32_t flags = DB_CREATE; Db* dbp = new Db(NULL,0); dbp->set_cachesize( 0, 1024 * 1024 * 1024, 1 ); int ret = dbp->open( NULL, "test.db", NULL, DB_HASH, flags, 0); time_t time1 = time(NULL); while ( passes < 100 ) { while( i < MILLION ) { Dbt key( &a, sizeof(long long) ); Dbt data( &b, sizeof(long long) ); dbp->put( NULL, &key, &data, 0); a++; b++; i++; } DbEnv* dbep = dbp->get_env(); int tmp; dbep->memp_trickle( 50, &tmp ); i=0; passes++; std::cout << "Inserted one million --> pass: " << passes << " took: " << time(NULL) - time1 << "sec" << std::endl; time1 = time(NULL); } } Perhaps you can tell me why after some time the "put" operation takes increasingly longer and maybe how to fix this. Thanks for your help, Andreas

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• Declare module name of classes for logging

  - by Space_C0wb0y

  I currently am adding some features to our logging-library. One of these is the possibility to declare a module-name for a class that automatically gets preprended to any log-messages writing from within that class. However, if no module-name is provided, nothing is prepended. Currently I am using a trait-class that has a static function that returns the name. template< class T > struct ModuleNameTrait { static std::string Value() { return ""; } }; template< > struct ModuleNameTrait< Foo > { static std::string Value() { return "Foo"; } }; This class can be defined using a helper-macro. The drawback is, that the module-name has to be declared outside of the class. I would like this to be possible within the class. Also, I want to be able to remove all logging-code using a preprocessor directive. I know that using SFINAE one can check if a template argument has a certain member, but since other people, that are not as friendly with templates as I am, will have to maintain the code, I am looking for a much simpler solution. If there is none, I will stick with the traits approach. Thanks in advance!

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• function objects versus function pointers

  - by kumar_m_kiran

  Hi All, I have two questions related to function objects and function pointers, Question : 1 When I read the different uses sort algorithm of STL, I see that the third parameter can be a function objects, below is an example class State { public: //... int population() const; float aveTempF() const; //... }; struct PopLess : public std::binary_function<State,State,bool> { bool operator ()( const State &a, const State &b ) const { return popLess( a, b ); } }; sort( union, union+50, PopLess() ); Question : Now, How does the statement, sort(union, union+50,PopLess()) work? PopLess() must be resolved into something like PopLess tempObject.operator() which would be same as executing the operator () function on a temporary object. I see this as, passing the return value of overloaded operation i.e bool (as in my example) to sort algorithm. So then, How does sort function resolve the third parameter in this case? Question : 2 Question Do we derive any particular advantage of using function objects versus function pointer? If we use below function pointer will it derive any disavantage? inline bool popLess( const State &a, const State &b ) { return a.population() < b.population(); } std::sort( union, union+50, popLess ); // sort by population PS : Both the above references(including example) are from book "C++ Common Knowledge: Essential Intermediate Programming" by "Stephen C. Dewhurst". I was unable to decode the topic content, thus have posted for help. Thanks in advance for your help.

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• How to use void*

  - by Rondogiannis Aristophanes

  I am imlementing a simple merge function and I have got stuck, as the compiler gives me errors that I cannot explain. Here is my merge function: void merge(void *a, int beg, int middle, int end, int (*cmp)(const void*, const void* { std::stack<void*> first; std::stack<void*> second; for(int i = beg; i < middle; i++) { first.push(a+i); } for(int i = middle; i < end; i++) { second.push(a+i); } for(int i = beg; i < end; i++) { if(first.empty()) { void *tmp = second.top(); second.pop(); a+i = tmp; } else if(second.empty()) { void *tmp = first.top(); first.pop(); a+i = tmp; } else if(cmp(first.top(), second.top())) { void *tmp = first.top(); first.pop(); a+i = tmp; } else { void *tmp = second.top(); second.pop(); a+i = tmp; } } } And here is the error: sort.h: In function `void merge(void*, int, int, int, int (*)(const void*, const void*))': sort.h:9: error: pointer of type `void *' used in arithmetic sort.h:12: error: pointer of type `void *' used in arithmetic sort.h:19: error: pointer of type `void *' used in arithmetic sort.h:19: error: non-lvalue in assignment sort.h:23: error: pointer of type `void *' used in arithmetic sort.h:23: error: non-lvalue in assignment sort.h:27: error: pointer of type `void *' used in arithmetic sort.h:27: error: non-lvalue in assignment sort.h:31: error: pointer of type `void *' used in arithmetic sort.h:31: error: non-lvalue in assignment Can anyone help me? TIA.

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• Potential g++ template bug?

  - by Evan Teran

  I've encountered some code which I think should compile, but doesn't. So I'm hoping some of the local standards experts here at SO can help :-). I basically have some code which resembles this: #include <iostream> template <class T = int> class A { public: class U { }; public: U f() const { return U(); } }; // test either the work around or the code I want... #ifndef USE_FIX template <class T> bool operator==(const typename A<T>::U &x, int y) { return true; } #else typedef A<int> AI; bool operator==(const AI::U &x, int y) { return true; } #endif int main() { A<int> a; std::cout << (a.f() == 1) << std::endl; } So, to describe what is going on here. I have a class template (A) which has an internal class (U) and at least one member function which can return an instance of that internal class (f()). Then I am attempting to create an operator== function which compares this internal type to some other type (in this case an int, but it doesn't seem to matter). When USE_FIX is not defined I get the following error: test.cc: In function 'int main()': test.cc:27:25: error: no match for 'operator==' in 'a.A<T>::f [with T = int]() == 1' Which seems odd, because I am clearly (I think) defining a templated operator== which should cover this, in fact if I just do a little of the work for the compiler (enable USE_FIX), then I no longer get an error. Unfortunately, the "fix" doesn't work generically, only for a specific instantiation of the template. Is this supposed to work as I expected? Or is this simply not allowed? BTW: if it matters I am using gcc 4.5.2.

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• Throwing a C++ exception after an inline-asm jump

  - by SoapBox

  I have some odd self modifying code, but at the root of it is a pretty simple problem: I want to be able to execute a jmp (or a call) and then from that arbitrary point throw an exception and have it caught by the try/catch block that contained the jmp/call. But when I do this (in gcc 4.4.1 x86_64) the exception results in a terminate() as it would if the exception was thrown from outside of a try/catch. I don't really see how this is different than throwing an exception from inside of some far-flung library, yet it obviously is because it just doesn't work. How can I execute a jmp or call but still throw an exception back to the original try/catch? Why doesn't this try/catch continue to handle these exceptions as it would if the function was called normally? The code: #include <iostream> #include <stdexcept> using namespace std; void thrower() { cout << "Inside thrower" << endl; throw runtime_error("some exception"); } int main() { cout << "Top of main" << endl; try { asm volatile ( "jmp *%0" // same thing happens with a call instead of a jmp : : "r"((long)thrower) : ); } catch (exception &e) { cout << "Caught : " << e.what() << endl; } cout << "Bottom of main" << endl << endl; } The expected output: Top of main Inside thrower Caught : some exception Bottom of main The actual output: Top of main Inside thrower terminate called after throwing an instance of 'std::runtime_error' what(): some exception Aborted

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• Cann Boost Program_options separate comma separated argument values

  - by lrm

  If my command line is: > prog --mylist=a,b,c Can Boost's program_options be setup to see three distinct argument values for the mylist argument? I have configured program_options as: namespace po = boost::program_options; po::options_description opts("blah") opts.add_options() ("mylist", std::vector<std::string>>()->multitoken, "description"); po::variables_map vm; po::store(po::parse_command_line(argc, argv, opts), vm); po::notify(vm); When I check the value of the mylist argument, I see one value as a,b,c. I'd like to see three distinct values, split on comma. This works fine if I specify the command line as: > prog --mylist=a b c or > prog --mylist=a --mylist=b --mylist=c Is there a way to configure program_options so that it sees a,b,c as three values that should each be inserted into the vector, rather than one? I am using boost 1.41, g++ 4.5.0 20100520, and have enabled c++0x experimental extensions.

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• Is private members hacking a defined behaviour ?

  - by ereOn

  Hi, Lets say I have the following class: class BritneySpears { public: int getValue() { return m_value; }; private: int m_value; }; Which is an external library (that I can't change). I obviously can't change the value of m_value, only read it. Even subclassing BritneySpears won't work. What if I define the following class: class AshtonKutcher { public: int getValue() { return m_value; }; public: int m_value; }; And then do: BritneySpears b; // Here comes the ugly hack AshtonKutcher* a = reinterpret_cast<AshtonKutcher*>(&b); a->m_value = 17; // Print out the value std::cout << b.getValue() << std::endl; I know this is a bad practice. But just for curiosity: is this guaranted to work ? Is it a defined behaviour ? Bonus question: Have you ever had to use such an ugly hack ? Thanks !

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• C++ find largest BST in a binary tree

  - by fonjibe

  what is your approach to have the largest BST in a binary tree? I refer to this post where a very good implementation for finding if a tree is BST or not is bool isBinarySearchTree(BinaryTree * n, int min=std::numeric_limits<int>::min(), int max=std::numeric_limits<int>::max()) { return !n || (min < n->value && n->value < max && isBinarySearchTree(n->l, min, n->value) && isBinarySearchTree(n->r, n->value, max)); } It is quite easy to implement a solution to find whether a tree contains a binary search tree. i think that the following method makes it: bool includeSomeBST(BinaryTree* n) { if(!isBinarySearchTree(n)) { if(!isBinarySearchTree(n->left)) return isBinarySearchTree(n->right); } else return true; else return true; } but what if i want the largest BST? this is my first idea, BinaryTree largestBST(BinaryTree* n) { if(isBinarySearchTree(n)) return n; if(!isBinarySearchTree(n->left)) { if(!isBinarySearchTree(n->right)) if(includeSomeBST(n->right)) return largestBST(n->right); else if(includeSomeBST(n->left)) return largestBST(n->left); else return NULL; else return n->right; } else return n->left; } but its not telling the largest actually. i struggle to make the comparison. how should it take place? thanks

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• How to use boost normal distribution classes?

  - by David Alfonso

  Hi all, I'm trying to use boost::normal_distribution in order to generate a normal distribution with mean 0 and sigma 1. The following code uses boost normal classes. Am I using them correctly? #include <boost/random.hpp> #include <boost/random/normal_distribution.hpp> int main() { boost::mt19937 rng; // I don't seed it on purpouse (it's not relevant) boost::normal_distribution<> nd(0.0, 1.0); boost::variate_generator<boost::mt19937&, boost::normal_distribution<> > var_nor(rng, nd); int i = 0; for (; i < 10; ++i) { double d = var_nor(); std::cout << d << std::endl; } } The result on my machine is: 0.213436 -0.49558 1.57538 -1.0592 1.83927 1.88577 0.604675 -0.365983 -0.578264 -0.634376 As you can see all values are not between -1 and 1. Thank you all in advance!

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• Having trouble creating vectors of System::String^

  - by Justen

  So I have a regex expression to parse certain parts of a file name. I'm trying to store each part in its own vector until I use it later, but it won't let me. One error I get when I try making a vector of System::String^ is that error C3698: 'System::String ^' : cannot use this type as argument of 'new' Then, when I try just making a vector of std::string, it can't implicitly convert to type System::String^, and casting won't work either. void parseData() { System::String^ pattern = "(\\D+)(\\d+)(\\w{1})(\\d+)\\.(\\D{3})"; std::vector < System::String^ > x, y, filename, separator; Regex re( pattern ); for ( int i = 0; i < openFileDialog1->FileNames->Length; i++ ) { Match^ m = re.Match( openFileDialog1->FileNames[i] ); filename.push_back( m->Groups[0]->Value );/* x.push_back( m->Groups[1]->Value ); separator.push_back( m->Groups[2]->Value ); y.push_back( m->Groups[3]->Value );*/ } }

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• C++ Exception Handling

  - by user1413793

  So I was writing some code and I noticed that apart from syntactical, type, and other compile-time errors, C++ does not throw any other exceptions. So I decided to test this out with a very trivial program: #include<iostream> int main() { std::count<<5/0<<std::endl; return 1 } When I compiled it using g++, g++ gave me a warning saying I was dividing by 0. But it still compiled the code. Then when I ran it, it printed some really large arbitrary number. When I want to know is, how does C++ deal with exceptions? Integer division by 0 should be a very trivial example of when an exception should be thrown and the program should terminate. Do I have to essentially enclose my entire program in a huge try block and then catch certain exceptions? I know in Python when an exception is thrown, the program will immediately terminate and print out the error. What does C++ do? Are there even runtime exceptions which stop execution and kill the program?

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• C++ overide global operator comma gives error

  - by uray

  the second function gives error C2803 http://msdn.microsoft.com/en-us/library/zy7kx46x%28VS.80%29.aspx : 'operator ,' must have at least one formal parameter of class type. any clue? template<class T,class A = std::allocator<T>> class Sequence : public std::vector<T,A> { public: Sequence<T,A>& operator,(const T& a) { this->push_back(a); return *this; } Sequence<T,A>& operator,(const Sequence<T,A>& a) { for(Sequence<T,A>::size_type i=0 ; i<a.size() ; i++) { this->push_back(a.at(i)); } return *this; } }; //this works! template<typename T> Sequence<T> operator,(const T& a, const T&b) { Sequence<T> seq; seq.push_back(a); seq.push_back(b); return seq; } //this gives error C2803! Sequence<double> operator,(const double& a, const double& b) { Sequence<double> seq; seq.push_back(a); seq.push_back(b); return seq; }

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• Launch a QWidget from a QMain window

  - by Opera

  Hello everyone ! I'm doing my first C++ - Qt4 application and I'm having some trouble "connecting" my different uis. I have a main window with several buttons and when I click on one, I want another window to open. The MyMainWindowClass inherits from QMainWindow and the other from QWidget. Here is the code I have written so far : #include <iostream #include "MyWidgetClass.hpp" #include "MyMainWindowClass.hpp" #include "ui_MyMainWindowClassUi.h" MyMainWindowClass::MyMainWindowClass(QWidget *parent) : QMainWindow(parent), m_ui(new Ui::MyMainWindowClassUi) { m_ui-setupUi(this); initConnect(); } void MyMainWindowClass::initConnect() { QObject::connect(m_ui-SomeBtn, SIGNAL(clicked()), this, SLOT(SomeBtnClicked())); // Some other QObject::connect calls return; } void MyMainWindowClass::SomeBtnClicked() { std::cout << "Some Btn has been clicked" << std::endl; this-setEnabled(false); MyWidgetClass mwc(this); mwc.show(); return; } This calls the Ctor and the Dtor from MyWidgetClass, disables the MyMainWindowClassUi, but doesn't show my other GUI. What am I missing to have the window showed when I click on the button ?

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• C++ explicit template specialization of templated constructor of templated class

  - by Victor Liu

  I have a class like template <class T> struct A{ template <class U> A(U u); }; I would like to write an explicit specialization of this for a declaration like A<int>::A(float); In the following test code, if I comment out the specialization, it compiles with g++. Otherwise, it says I have the wrong number of template parameters: #include <iostream> template <class T> struct A{ template <class U> A(T t, U *u){ *u += U(t); } }; template <> template <> A<int>::A<int,float>(int t, float *u){ *u += U(2*t); } int main(){ float f = 0; int i = 1; A<int>(i, &f); std::cout << f << std::endl; return 0; }

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