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• Is the old vector get cleared? If yes, how and when?

  - by user180866

  I have the following code: void foo() { vector<double> v(100,1); // line 1 // some code v = vector<double>(200,2); // line 2 // some code } what happened to the vector of size 100 after the second line? Is it gets cleared by itself? If the answer is yes, how and when it is cleared? By the way, is there any other "easy and clean" ways to change the vector as in line 2? I don't want things like v.resize(200); for (int i=0; i<200; i++) v[i] = 2;

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• C++ vector<T>::iterator operator +

  - by Tom

  Hi, Im holding an iterator that points to an element of a vector, and I would like to compare it to the next element of the vector. Here is what I have Class Point{ public: float x,y; } //Somewhere in my code I do this vector<Point> points = line.getPoints(); foo (points.begin(),points.end()); where foo is: void foo (Vector<Point>::iterator begin,Vector<Point>::iterator end) { std::Vector<Point>::iterator current = begin; for(;current!=end-1;++current) { std::Vector<Point>::iterator next = current + 1; //Compare between current and next. } } I thought that this would work, but current + 1 is not giving me the next element of the vector. I though operator+ was the way to go, but doesnt seem so. Is there a workaround on this? THanks

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• fastest engine to convert PDF into PNG

  - by skyde

  I would like to know which of the opensource PDF engine can convert a pdf into a image the fastest. I don't care about the quality of the result (antialiasing ...) For my project it need to be very very fast. I would probably need to build my own but i dont wan't to start from scratch.

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• How to restrict the range of elements of C++ STL vector?

  - by cambr

  vector<int> l; for(int i=0;i<10;i++){ l.push_back(i); } I want the vector to only be able to store numbers from a specified range (or set). How can that be done, in general? In particular, I want to restrict the vector to beonly be able to store single digits. So, if I do a l[9]++, it should give me an error or warn me. (because 10 is not a single digit number). Similarly, l[0]-- should warn me. Is there a way to do this using C++ STL vector?

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• Attribute vector emptying itself

  - by ravloony

  Hello, I have two classes, derived from a common class. The common class has a pure virtual function called execute(), which is implemented in both derived classes. In the inherited class I have an attribute which is a vector. In both execute() methods I overwrite this vector with a result. I access both classes from a vector of pointers to their objects. The problem is when I try to access the result vector form outside the objects. In one case I can get the elements (which are simply pointers), in the other I cannot, the vector is empty. Code: class E; class A{ protected: vector<E*> _result; public: virtual void execute()=0; vector<E*> get_result(); }; vector<E*> A::get_result() { return _result; } class B : public A { public: virtual void execute(); }; B::execute() { //... _result = tempVec; return; } class C : public A { public: virtual void execute(); }; C::execute() { //different stuff to B _result = tempvec; return; } main() { B* b = new B(); C* c = new C(); b->execute(); c->execute(); b->get_result();//returns full vector c->get_result(); //returns empty vector!! } I have no idea what is going on here... I have tried filling _result by hand from a temp vector in the offending class, doing the same with vector::assign(), nothing works. And the other object works perfectly. I must be missing something.... Any help would be greatly appreciated.

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• which is better in general, map or vector in c++?

  - by tsubasa

  As I know that accessing an element in vector takes constant time while in map takes logarithmic time. However, storing a map takes less memory than storing a vector. Therefore, I want to ask which one is better in general? I'm considering using one of those two in my program, which has about 1000 elements. I plan to use 3 dimensional vector, which would take 1000x1000x1000 elements.

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• Does std::vector change its address? How to avoid

  - by kunigami

  Since vector elements are stored contiguously, I guess it may not have the same address after some push_back's , because the initial allocated space could not suffice. I'm working on a code where I need a reference to an element in a vector, like: int main(){ vector<int> v; v.push_back(1); int *ptr = &v[0]; for(int i=2; i<100; i++) v.push_back(i); cout << *ptr << endl; //? return 0; } But it's not necessarily true that ptr contains a reference to v[0], right? How would be a good way to guarantee it? My first idea would be to use a vector of pointers and dynamic allocation. I'm wondering if there's an easier way to do that? PS.: Actually I'm using a vector of a class instead of int, but I think the issues are the same.

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• How to initialize std::vector from C-style array?

  - by dehmann

  What is the cheapest way to initialize a std::vector from a C-style array? Example: In the following class, I have a vector, but due to outside restrictions, the data will be passed in as C-style array: class Foo { std::vector<double> w_; public: void set_data(double* w, int len){ // how to cheaply initialize the std::vector? } Obviously, I can call w_.resize() and then loop over the elements, or call std::copy(). Are there any better methods?

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• Resultant Vector Algorithm for 2D Collisions

  - by John

  I am making a Pong based game where a puck hits a paddle and bounces off. Both the puck and the paddles are Circles. I came up with an algorithm to calculate the resultant vector of the puck once it meets a paddle. The game seems to function correctly but I'm not entirely sure my algorithm is correct. Here are my variables for the algorithm: Given: velocity = the magnitude of the initial velocity of the puck before the collision x = the x coordinate of the puck y = the y coordinate of the puck moveX = the horizontal speed of the puck moveY = the vertical speed of the puck otherX = the x coordinate of the paddle otherY = the y coordinate of the paddle piece.horizontalMomentum = the horizontal speed of the paddle before it hits the puck piece.verticalMomentum = the vertical speed of the paddle before it hits the puck slope = the direction, in radians, of the puck's velocity distX = the horizontal distance between the center of the puck and the center of the paddle distY = the vertical distance between the center of the puck and the center of the paddle Algorithm solves for: impactAngle = the angle, in radians, of the angle of impact. newSpeedX = the speed of the resultant vector in the X direction newSpeedY = the speed of the resultant vector in the Y direction Here is the code for my algorithm: int otherX = piece.x; int otherY = piece.y; double velocity = Math.sqrt((moveX * moveX) + (moveY * moveY)); double slope = Math.atan(moveX / moveY); int distX = x - otherX; int distY = y - otherY; double impactAngle = Math.atan(distX / distY); double newAngle = impactAngle + slope; int newSpeedX = (int)(velocity * Math.sin(newAngle)) + piece.horizontalMomentum; int newSpeedY = (int)(velocity * Math.cos(newAngle)) + piece.verticalMomentum; for those who are not program savvy here is it simplified: velocity = v(moveX² + moveY²) slope = arctan(moveX / moveY) distX = x - otherX distY = y - otherY impactAngle = arctan(distX / distY) newAngle = impactAngle + slope newSpeedX = velocity * sin(newAngle) + piece.horizontalMomentum newSpeedY = velocity * cos(newAngle) + piece.verticalMomentum My Question: Is this algorithm correct? Is there an easier/simpler way to do what I'm trying to do?

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• How to print a rendered website to pdf or vector graphics?

  - by Lo Sauer

  This is a crucial question to many: Searching the web, I have found several command line tools that allow you to convert a HTML-document to a PDF-document, however they all seem to use their own, and rather incomplete rendering engine, resulting in poor quality How can you print the rendered output of a modern web-browser to pdf, (and/or svg) whilst retaining as much vector graphics as possible? There is a solution called: webkit-pdf (which renders everything to bitmap graphics) I am looking for options, alternatives, suggestions perhaps even a printer-driver or webservices? Thanks

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• C++ STL: How to iterate vector while requiring access to element and its index?

  - by Ashwin

  I frequently find myself requiring to iterate over STL vectors. While I am doing this I require access to both the vector element and its index. I used to do this as: typedef std::vector<Foo> FooVec; typedef FooVec::iterator FooVecIter; FooVec fooVec; int index = 0; for (FooVecIter i = fooVec.begin(); i != fooVec.end(); ++i, ++index) { Foo& foo = *i; if (foo.somethingIsTrue()) // True for most elements std::cout << index << ": " << foo << std::endl; } After discovering BOOST_FOREACH, I shortened this to: typedef std::vector<Foo> FooVec; FooVec fooVec; int index = -1; BOOST_FOREACH( Foo& foo, fooVec ) { ++index; if (foo.somethingIsTrue()) // True for most elements std::cout << index << ": " << foo << std::endl; } Is there a better or more elegant way to iterate over STL vectors when both reference to the vector element and its index is required? I am aware of the alternative: for (int i = 0; i < fooVec.size(); ++i) But I keep reading about how it is not a good practice to iterate over STL containers like this.

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• What is a truly empty std::vector in C++?

  - by RyanG

  I've got a two vectors in class A that contain other class objects B and C. I know exactly how many elements these vectors are supposed to hold at maximum. In the initializer list of class A's constructor, I initialize these vectors to their max sizes (constants). If I understand this correctly, I now have a vector of objects of class B that have been initialized using their default constructor. Right? When I wrote this code, I thought this was the only way to deal with things. However, I've since learned about std::vector.reserve() and I'd like to achieve something different. I'd like to allocate memory for these vectors to grow as large as possible because adding to them is controlled by user-input, so I don't want frequent resizings. However, I iterate through this vector many, many times per second and I only currently work on objects I've flagged as "active". To have to check a boolean member of class B/C on ever iteration is silly. I don't want these objects to even BE there for my iterators to see when I run through this list. Is reserving the max space ahead of time and using push_back to add a new object to the vector a solution to this?

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• C++: getting the address of the start of an std::vector ?

  - by shoosh

  Sometimes it is useful to use the starting address of an std::vector and temporarily treat that address as the address of a regularly allocated buffer. For instance replace this: char* buf = new char[size]; fillTheBuffer(buf, size); useTheBuffer(buf, size); delete[] buf; With This: vector<char> buf(size); fillTheBuffer(&buf[0], size); useTheBuffer(&buf[0], size); The advantage of this is of course that the buffer is deallocated automatically and I don't have to worry about the delete[]. The problem I'm having with this is when size == 0. In that case the first version works ok. An empty buffer is "allocated" and the subsequent functions do nothing size they get size == 0. The second version however fails if size == 0 since calling buf[0] may rightly contain an assertion that 0 < size. So is there an alternative to the idiom &buf[0] that returns the address of the start of the vector even if the vector is empty? I've also considered using buf.begin() but according to the standard it isn't even guaranteed to return a pointer.

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• Adding cards to a vector for computer card game

  - by Tucker Morgan

  I am writing a Card game that has a deck size of 30 cards, each one of them has to be a unique, or at least a another (new XXXX) statement in a .push_back function, into a vector. Right now my plan is to add them to a vector one at a time with four separate, depending on what deck type you choose, collections of thirty .push_back functions. If the collection of card is not up for customization, other than what one of the four suits you pick, is there a quicker way of doing this, seems kinda tedious, and something that someone would have found a better way of doing.

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• How to convert poor quality bitmap image to vector?

  - by Macha

  I'm designing a website for a group which has lost the original digital image for their logo. The only file they have of it is a jpg which was embedded into a word document. The image has everything possible wrong with it: Anti-aliased onto a white background where it should be transparent Image artefacts Resized downwards poorly. Lines that should be straight and solid aren't. I've currently used the wand tool to get rid of the white background, and stuck it on the website, but it's poor quality makes it stick out like a sore thumb. I need a few different sizes of it to use, so how would I go about creating a vector image based on it?

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• Rotation matrix for a 3D vector

  - by Shashwat

  I have a direction vector on which I have to apply some rotation to align it to positive z-axis. To use Matrix.CreateRotationX(angle) of XNA, I need the angle for which I'd have to compute cos or tan inverse. I think this is a complex task to do. Also, eventually those are also converted to sin(angle) and cos(angle) in the matrix. Is there any inbuilt way to create rotation matrix from a 3D vector? However, I can write the function but still asking if there is one already there.

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• infer half vector length in BRDF

  - by cician

  it's my first question on stack. Is it possible to infer length of the half angle vector for specular lighting from N·L and N·V without the whole view and light vectors? I may be completely off-track, but I have this gut feeling it's possible... Why? I'm working on a skin shader and I'm already doing one texture lookup with N·L+N·E and one texture lookup for specular with N·H+N·V. The latter one can be transformed into N·L+N·E lookup if only I had the half vector length. Doing so could simplify the shader a bit and move some operations into the pre-computed lookup texture. It would make a huge difference since I'm trying to squeeze as much functionality as possible to a single pass mobile version so instruction count matters. Thanks.

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• A "quick" vector editor (SVG) for Linux (for annotating images?)

  - by sdaau

  I often need to take a bitmap (.png) image, and draw some lines or text on top of it, and possibly export a new, thusly "annotated" image. I know I can basically do all this in inkscape - but inkscape is a complex program, and it needs almost a minute to start up properly on my PCs. So I was thinking - is there something like a "mini" vector editor for Linux, which would start up fast, and allow me to: Right-click, open an image in this editor program The program scales the active "document"/"window size" to the size of the image I can zoom in/zoom out (and possibly crop) the image I can add at least lines, boxes and text in different colors? A bonus for me would be to have the overlay graphics saved as SVG format, say with the same filename as the image - as in, "image.png.svg" being saved in the same directory where the original "image.png" is located (thus allowing opening and editing these "annotations" further, either in this editor, or possibly in inkscape). And another bonus would be the export of the annotated image to a bitmap. Anyone know about anything like this?

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• no match for operator= using a std::vector

  - by Max

  I've got a class declared like this: class Level { private: std::vector<mapObject::MapObject> features; (...) }; and in one of its member functions I try to iterate through that vector like this: vector<mapObject::MapObject::iterator it; for(it=features.begin(); it<features.end(); it++) { /* loop code */ } This seems straightforward to me, but g++ gives me this error: src/Level.cpp:402: error: no match for ‘operator=’ in ‘it = ((const yarl::level::Level*)this)-yarl::level::Level::features.std::vector<_Tp, _Alloc::begin [with _Tp = yarl::mapObject::MapObject, _Alloc = std::allocator<yarl::mapObject::MapObject>]()’ /usr/include/c++/4.4/bits/stl_iterator.h:669: note: candidates are: __gnu_cxx::__normal_iterator<yarl::mapObject::MapObject*,std::vector & __gnu_cxx::__normal_iterator<yarl::mapObject::MapObject*,std::vector >::operator=(const __gnu_cxx::__normal_iterator<yarl::mapObject::MapObject*, ``std::vector<yarl::mapObject::MapObject, std::allocator<yarl::mapObject::MapObject> > >&) Anyone know why this is happening?

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• Basic question about std::vector instantiation

  - by recipriversexclusion

  This looks simple but I am confused: The way I create a vector of hundred, say, ints is std::vector<int> *pVect = new std::vector<int>(100); However, looking at std::vector's documentation I see that its constructor is of the form explicit vector ( size_type n, const T& value= T(), const Allocator& = Allocator() ); So, how does the previous one work? Does new call the constructor with an initialization value obtained from the default constructor? If that is the case, would std::vector<int, my_allocator> *pVect = new std::vector<int>(100, my_allocator); where I pass my own allocator, also work?

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• How can I eliminate an element in a vector if a condition is met

  - by michael

  Hi, I have a vector of Rect: vector<Rect> myRecVec; I would like to remove the ones which are overlapping in the vector: So I have 2 nested loop like this: vector<Rect>::iterator iter1 = myRecVec.begin(); vector<Rect>::iterator iter2 = myRecVec.begin(); while( iter1 != myRecVec.end() ) { Rectangle r1 = *iter1; while( iter2 != myRecVec.end() ) { Rectangle r2 = *iter1; if (r1 != r2) { if (r1.intersects(r2)) { // remove r2 from myRectVec } } } } My question is how can I remove r2 from the myRectVect without screwing up both my iterators? Since I am iterating a vector and modifying the vector at the same time? I have thought about putting r2 in a temp rectVect and then remove them from the rectVect later (after the iteration). But how can I skip the ones in this temp rectVect during iteration?

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• Why can't I access a const vector with iterator?

  - by tsubasa

  My example is as below. I found out the problem is with "const" in function void test's parameter. I don't know why the compiler does not allow. Could anybody tell me? Thanks. vector<int> p; void test(const vector<int> &blah) { vector<int>::iterator it; for (it=blah.begin(); it!=blah.end(); it++) { cout<<*it<<" "; } } int main() { p.push_back(1); p.push_back(2); p.push_back(3); test(p); return 0; }

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• Reusing a vector in C++

  - by Bobby

  I have a vector declared as a global variable that I need to be able to reuse. For example, I am reading multiple files of data, parsing the data to create objects that are then stored in a vector. vector<Object> objVector(100); void main() { while(THERE_ARE_MORE_FILES_TO_READ) { // Pseudocode ReadFile(); ParseFileIntoVector(); ProcessObjectsInVector(); /* Here I want to 'reset' the vector to 100 empty objects again */ } } Can I reset the vector to be "vector objVector(100)" since it was initially allocated on the stack? If I do "objVector.clear()", it removes all 100 objects and I would have a vector with a size of 0. I need it to be a size of 100 at the start of every loop.

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• Deleting a element from a vector of pointers in C++.

  - by Kranar

  I remember hearing that the following code is not C++ compliant and was hoping someone with much more C++ legalese than me would be able to confirm or deny it. std::vector<int*> intList; intList.push_back(new int(2)); intList.push_back(new int(10)); intList.push_back(new int(17)); for(std::vector<int*>::iterator i = intList.begin(); i != intList.end(); ++i) { delete *i; } intList.clear() The rationale was that it is illegal for a vector to contain pointers to invalid memory. Now obviously my example will compile and it will even work on all compilers I know of, but is it standard compliant C++ or am I supposed to do the following, which I was told is in fact the standard compliant approach: while(!intList.empty()) { int* element = intList.back(); intList.pop_back(); delete element; }

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• hash tables and 2d vectors

  - by Sunil

  I want to push a 2d vector into a hash table row by row and later search for a row (vector) in the hash table and want to be able to find it. I want to do something like #include <iostream> #include <set> #include <vector> using namespace std; int main(){ std::set < vector<int> > myset; vector< vector<int> > v; int k = 0; for ( int i = 0; i < 5; i++ ) { v.push_back ( vector<int>() ); for ( int j = 0; j < 5; j++ ) v[i].push_back ( k++ ); } for ( int i = 0; i < 5; i++ ) { std::copy(v[i].begin(),v[i].end(),std::inserter(myset)); // This is not correct but what is the right way ? // and also here, I want to search for a particular vector if it exists in the table. for ex. the second row of vector v. } return 0; } I'm not sure how to insert and look up a vector in a set. So if nybody could guide me, it will be helpful. Thanks

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