c++ - std::sort doing lots of unnecessary swaps -

-

i have following code try lexicographical comparison of 2 simple vectors

class expensiveclass { public:   static int increment;   std::vector<int> bigvector;    expensiveclass() {     (int j = 0; j < 1 + increment; ++j)       bigvector.emplace_back(j);     ++increment;   }   expensiveclass(const expensiveclass& other) {     std::cout << "expensive copy constructor called!" << std::endl;     this->bigvector = other.bigvector;   } };  int expensiveclass::increment = 0;  bool sortfunc(const expensiveclass& a, const expensiveclass& b) {   bool ret = a.bigvector < b.bigvector;   if (ret == false)     std::cout << "need swap" << std::endl;   return ret; }  int main() {   std::vector<expensiveclass> vectorofvectors;   vectorofvectors.reserve(100);   (int = 0; < 100; ++i)     vectorofvectors.emplace_back();    std::cout << "start sorting.." << std::endl;   std::sort(vectorofvectors.begin(), vectorofvectors.end(), sortfunc); }

and i'd know why std::sort doing lots of swaps on sorted vector?

i think we're getting dark corners of stl here. played code , discovered sortfunc() doesn't return true. because arguments being passed not in order. std::sort algorithm picks elements vectorofvectors out of order, i.e. index of a within vectorofvectors not guaranteed higher index of b. don't know how dig deeper created separate questions: https://stackoverflow.com/questions/30539898/counter-intuitive-behavior-of-stdsort

anyway, point made assumption on behavior of std::sort, that's implementation dependent.


Comments

Post a Comment

Popular posts from this blog

Java 3D LWJGL collision -

-
i making 3d java game lwjgl library, , wondering how add collision detection, player not go through models. i using obj models. here objloader class, loads models: package renderengine; import java.io.bufferedreader; import java.io.file; import java.io.filenotfoundexception; import java.io.filereader; import java.util.arraylist; import java.util.list; import models.rawmodel; import org.lwjgl.util.vector.vector2f; import org.lwjgl.util.vector.vector3f; public class objloader { public static rawmodel loadobjmodel(string filename, loader loader){ filereader fr = null; try { fr = new filereader(new file("res/"+filename+".obj")); } catch (filenotfoundexception e) { system.err.println("couldn't load file!"); e.printstacktrace(); } bufferedreader reader = new bufferedreader(fr); string line; list<vector3f> vertices = new arraylist<vector3f...
Read more

methods - python can't use function in submodule -

-
my folder structure this: pythonstuff/ program.py modulea/ __init__.py foo.py bar.py here's code bar.py : def hello(): print("hello, world!") here's code program.py : #!/usr/bin/env python3 modulea import bar bar.hello() i run $ python3 program.py somehow error: file "program.py", line 3, in <module> bar.hello() attributeerror: 'module' object has no attribute 'hello' edit: __init__.py file empty. edit2: after trying realized had bar.py in root directory contained hello() method. bar.py in modulea/ directory empty. add it import os __all__ = [] module in os.listdir(os.path.dirname(__file__)): if module != '__init__.py' , module[-3:] == '.py': __all__.append(module[:-3]) the init .py files required make python treat directories containing packages; done prevent directories common name, such string, unintentionally hiding v...
Read more

c# - ErrorThe type or namespace name 'AxWMPLib' could not be found (are you missing a using directive or an assembly reference?) -

-
what possible reference handler axwmplib._wmpocxevents_playstatechangeeventhandler ? when pass event in method, private void axwindowsmediaplayer1_playstatechange(object sender,axwmplib._wmpocxevents_playstatechangeevent e) compiling throws following error: the type or namespace name 'axwmplib' not found (are missing using directive or assembly reference?) if using visual studio make sure have axwmplib listed under references in solution explorer. if have embedded windows media player control in form, should have been added automatically along wmplib . an easy way add these libraries if missing add windows media player control form (an existing 1 do). right-click on toolbox , pick "choose items..." context menu, tick "windows media player" control on "com components" tab , click ok. should have little windows media player icon attached cursor; click on form add it. add axwmplib , wmplib references solution. can delete w...
Read more