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• C++ vector that *doesn't* initialize its members?

  - by Mehrdad

  I'm making a C++ wrapper for a piece of C code that returns a large array, and so I've tried to return the data in a vector<unsigned char>. Now the problem is, the data is on the order of megabytes, and vector unnecessarily initializes its storage, which essentially turns out to cut down my speed by half. How do I prevent this? Or, if it's not possible -- is there some other STL container that would avoid such needless work? Or must I end up making my own container? (Pre-C++11) Note: I'm passing the vector as my output buffer. I'm not copying the data from elsewhere.

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• Perturb vector by some angle

  - by Myx

  Hello: I have a unit vector in 3D space whose direction I wish to perturb by some angle within the range 0 to theta, with the position of the vector remaining the same. What is a way I can accomplish this? Thanks.

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• Find which numbers appears most in a vector

  - by VaioIsBorn

  I have some numbers stored in a vector . I want to find which number appears most in the vector. Is there any easy/fast algorithm (STL or whatever) that does this ?

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• initializing a vector of custom class in c++

  - by Flamewires

  Hey basically Im trying to store a "solution" and create a vector of these. The problem I'm having is with initialization. Heres my class for reference class Solution { private: // boost::thread m_Thread; int itt_found; int dim; pfn_fitness f; double value; std::vector<double> x; public: Solution(size_t size, int funcNo) : itt_found(0), x(size, 0.0), value(0.0), dim(30), f(Eval_Functions[funcNo]) { for (int i = 1; i < (int) size; i++) { x[i] = ((double)rand()/((double)RAND_MAX))*maxs[funcNo]; } } Solution() : itt_found(0), x(31, 0.0), value(0.0), dim(30), f(Eval_Functions[1]) { for (int i = 1; i < 31; i++) { x[i] = ((double)rand()/((double)RAND_MAX))*maxs[1]; } } Solution operator= (Solution S) { x = S.GetX(); itt_found = S.GetIttFound(); dim = S.GetDim(); f = S.GetFunc(); value = S.GetValue(); return *this; } void start() { value = f (dim, x); } /* plus additional getter/setter methods*/ } Solution S(30, 1) or Solution(2, 5) work and initalizes everything, but I need X of these solution objects. std::vector<Solution> Parents(X) will create X solutions with the default constructor and i want to construct using the (int, int) constructor. Is there any easy(one liner?) way to do this? Or would i have to do something like: size_t numparents = 10; vector<Solution> Parents; Parents.reserve(numparents); for (int i = 0; i<(int)numparents; i++) { Solution S(31, 0); Parents.push_back(S); }

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• Algorithms for finding closest vector

  - by joel

  I have a set of vectors. For a vector in that set I like to find the sub set that is closeest to this vector. What algorithm can do this.

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• Initializing a C++ vector to random values... fast

  - by Flamewires

  Hey, id like to make this as fast as possible because it gets called A LOT in a program i'm writing, so is there any faster way to initialize a C++ vector to random values than: double range;//set to the range of a particular function i want to evaluate. std::vector<double> x(30, 0.0); for (int i=0;i<x.size();i++) { x.at(i) = (rand()/(double)RAND_MAX)*range; } EDIT:Fixed x's initializer.

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• C++ Vector vs Array (Time)

  - by vsha041

  I have got here two programs with me, both are doing exactly the same task. They are just setting an boolean array / vector to the value true. The program using vector takes 27 seconds to run whereas the program involving array with 5 times greater size takes less than 1 s. I would like to know the exact reason as to why there is such a major difference ? Are vectors really that inefficient ? Program using vectors #include <iostream> #include <vector> #include <ctime> using namespace std; int main(){ const int size = 2000; time_t start, end; time(&start); vector<bool> v(size); for(int i = 0; i < size; i++){ for(int j = 0; j < size; j++){ v[i] = true; } } time(&end); cout<<difftime(end, start)<<" seconds."<<endl; } Runtime - 27 seconds Program using Array #include <iostream> #include <ctime> using namespace std; int main(){ const int size = 10000; // 5 times more size time_t start, end; time(&start); bool v[size]; for(int i = 0; i < size; i++){ for(int j = 0; j < size; j++){ v[i] = true; } } time(&end); cout<<difftime(end, start)<<" seconds."<<endl; } Runtime - < 1 seconds Platform - Visual Studio 2008 OS - Windows Vista 32 bit SP 1 Processor Intel(R) Pentium(R) Dual CPU T2370 @ 1.73GHz Memory (RAM) 1.00 GB Thanks Amare

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• variable scope when adding a value to a vector in class constructor

  - by TheFuzz

  I have a level class and a Enemy_control class that is based off an vector that takes in Enemys as values. in my level constructor I have: Enemy tmp( 1200 ); enemys.Add_enemy( tmp ); // this adds tmp to the vector in Enemy_control enemys being a variable of type Enemy_control. My program crashes after these statements complaining about some destructor problem in level and enemy_control and enemy. Any ideas?

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• Euler rotation of direction vector

  - by Tom Savage

  I have defined an object in 3D space with position, rotation and scale values (all defined as 3D vectors). It also has upwards and forwards direction vectors. When I rotate the object, I need these direction vectors to rotate with it. Assuming my up vector is (0, 1, 0) and my forwards vector is (0, 0, 1) at zero rotation, how might I achieve this?

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• C++ std::vector capacity

  - by aaa

  hi. does vector::operator= change vector capacity? if so, how? does copy constructor copy capacity? I looked through documentation but could not find specific answer. is it implementation dependent? Thanks

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• Copy vector of values to vector of pairs in one line

  - by Kirill V. Lyadvinsky

  I have the following types: struct X { int x; X( int val ) : x(val) {} }; struct X2 { int x2; X2() : x2() {} }; typedef std::pair<X, X2> pair_t; typedef std::vector<pair_t> pairs_vec_t; typedef std::vector<X> X_vec_t; I need to initialize instance of pairs_vec_t with values from X_vec_t. I use the following code and it works as expected: int main() { pairs_vec_t ps; X_vec_t xs; // this is not empty in the production code ps.reserve( xs.size() ); { // I want to change this block to one line code. struct get_pair { pair_t operator()( const X& value ) { return std::make_pair( value, X2() ); } }; std::transform( xs.begin(), xs.end(), back_inserter(ps), get_pair() ); } return 0; } What I'm trying to do is to reduce my copying block to one line with using boost::bind. This code is not working: for_each( xs.begin(), xs.end(), boost::bind( &pairs_vec_t::push_back, ps, boost::bind( &std::make_pair, _1, X2() ) ) ); I know why it is not working, but I want to know how to make it working without declaring extra functions and structs?

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• how to determine if a character vector is a valid numeric or integer vector

  - by Andrew Barr

  I am trying to turn a nested list structure into a dataframe. The list looks similar to the following (it is serialized data from parsed JSON read in using the httr package). myList <- list(object1 = list(w=1, x=list(y=0.1, z="cat")), object2 = list(w=2, x=list(y=0.2, z="dog"))) unlist(myList) does a great job of recursively flattening the list, and I can then use lapply to flatten all the objects nicely. flatList <- lapply(myList, FUN= function(object) {return(as.data.frame(rbind(unlist(object))))}) And finally, I can button it up using plyr::rbind.fill myDF <- do.call(plyr::rbind.fill, flatList) str(myDF) #'data.frame': 2 obs. of 3 variables: #$ w : Factor w/ 2 levels "1","2": 1 2 #$ x.y: Factor w/ 2 levels "0.1","0.2": 1 2 #$ x.z: Factor w/ 2 levels "cat","dog": 1 2 The problem is that w and x.y are now being interpreted as character vectors, which by default get parsed as factors in the dataframe. I believe that unlist() is the culprit, but I can't figure out another way to recursively flatten the list structure. A workaround would be to post-process the dataframe, and assign data types then. What is the best way to determine if a vector is a valid numeric or integer vector?

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• Split vector vs matrix notation for transformation

  - by seahorse

  Some rendering engines like Ogre prefer to use a individual vector based notation for transformations like the following Split vector notation: Net Transformation is represented by Scale vector = sx, sy, sz Transformation vector = tx, ty, tz Rotation Quaternion Vector = w,x,y,z Matrix notation: There are other engines which simply use a net combined transformation matrix. What are the advantages of the first notation over the second? Also for animation interpolation does it work in the first notation that we interpolate across the individual components and use the interpolated parts to get the net transformation? Is this another advantage?

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• When to use an Array vs When to use a Vector, when dealing with GameObjects?

  - by user32465

  I understand that from other answers, Arrays and Vectors are the best choices. Many on SE claim that Linked Lists and Maps are bad for video game programming. I understand that for the most part, I can use Arrays. However, I don't really understand exactly when to use Vectors over Arrays. Why even use Vectors? Wouldn't it be best if I simply always used an Array, that way I know how much memory my game needs? Specifically my game would only ever load a single "Map" area of tiles, such as Map[100][100], so I could very easily have an array of GameObjectContainer GameObjects[100][100], which would reserve an entire map's worth of possible gameobjects, correct? So why use a Vector instead? Memory is quite large on modern hardware.

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• What would be a good game making engine supporting Vector images?

  - by Qqwy

  I want to create a simple platforming game, in which you are a square in a wonderful world. I would like this game to be able to be played in browsers. Basically I am searching for something similar to "Flixel", but with the following features: Support Vector Graphics Allow zooming/rotating objects without producing huge amounts of lag as soon as you are using more objects. (Because I want to rotate the map around the player) So in other words, preferably zoom the viewport/camera instead of the objects themselves. Does an engine like that exist?

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• Check if the vector is behind another or maybe opposite directions?

  - by Gilson

  I'm doing a network game and on the client side, i interpolate the client position with the server sent extrapolated position. The client has its own physics simulation wich is corrected by the server in steps. The problem is when it laggs and i 'kick' the ball, the server gets a delayed message and sends me the position backwards of the client position wich makes the ball goes back and forth. I want to ignore those and maybe compensate that on the server, not sure though. The problem is the clock difference on those case are 0.07ms or 0.10 ms wich isn't that high to ignore the message i guess. When i get the server position, i extrapolate with the clock interval * serverBallVelocity Can i check if my new ball server position is behind my actual ball vector position? I tried to use the dot product after normalized the two vectors to check if they are opposite but it ain't working properly. Any suggestions on checking that?

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• Convert a Unit Vector to a Quaternion

  - by Hmm

  So I'm very new to quaternions, but I understand the basics of how to manipulate stuff with them. What I'm currently trying to do is compare a known quaternion to two absolute points in space. I'm hoping what I can do is simply convert the points into a second quaternion, giving me an easy way to compare the two. What I've done so far is to turn the two points into a unit vector. From there I was hoping I could directly plug in the i j k into the imaginary portion of the quaternion with a scalar of zero. From there I could multiply one quaternion by the other's conjugate, resulting in a third quaternion. This third quaternion could be converted into an axis angle, giving me the degree by which the original two quaternions differ by. Is this thought process correct? So it should just be [ 0 i j k ]. I may need to normalize the quaternion afterwards, but I'm not sure about that. I have a bad feeling that it's not a direct mapping from a vector to a quaternion. I tried looking at converting the unit vector to an axis angle, but I'm not sure this would work, since I don't know what angle to give as an input.

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• Use a vector to index a matrix without linear index

  - by David_G

  G'day, I'm trying to find a way to use a vector of [x,y] points to index from a large matrix in MATLAB. Usually, I would convert the subscript points to the linear index of the matrix.(for eg. Use a vector as an index to a matrix in MATLab) However, the matrix is 4-dimensional, and I want to take all of the elements of the 3rd and 4th dimensions that have the same 1st and 2nd dimension. Let me hopefully demonstrate with an example: Matrix = nan(4,4,2,2); % where the dimensions are (x,y,depth,time) Matrix(1,2,:,:) = 999; % note that this value could change in depth (3rd dim) and time (4th time) Matrix(3,4,:,:) = 888; % note that this value could change in depth (3rd dim) and time (4th time) Matrix(4,4,:,:) = 124; Now, I want to be able to index with the subscripts (1,2) and (3,4), etc and return not only the 999 and 888 which exist in Matrix(:,:,1,1) but the contents which exist at Matrix(:,:,1,2),Matrix(:,:,2,1) and Matrix(:,:,2,2), and so on (IRL, the dimensions of Matrix might be more like size(Matrix) = (300 250 30 200) I don't want to use linear indices because I would like the results to be in a similar vector fashion. For example, I would like a result which is something like: ans(time=1) 999 888 124 999 888 124 ans(time=2) etc etc etc etc etc etc I'd also like to add that due to the size of the matrix I'm dealing with, speed is an issue here - thus why I'd like to use subscript indices to index to the data. I should also mention that (unlike this question: Accessing values using subscripts without using sub2ind) since I want all the information stored in the extra dimensions, 3 and 4, of the i and jth indices, I don't think that a slightly faster version of sub2ind still would not cut it..

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• Need to calculate rotation-vector from Sensor.TYPE_ORIENTATION data

  - by Sponge

  I need to calculate a rotation vector out of the data i get from Sensor.TYPE_ORIENTATION. The sensor data is defined like this: the values have to be recalculated to become a correct 3d position: values[0]: Azimuth, angle between the magnetic north direction and the Y axis, around the Z axis (0 to 359). 0=North, 90=East, 180=South, 270=West values[1]: Pitch, rotation around X axis (-180 to 180), with positive values when the z-axis moves toward the y-axis. values[2]: Roll, rotation around Y axis (-90 to 90), with positive values when the x-axis moves away from the z-axis I need all three values like the Z axis value (from 0 to 360 degree). I tried a lot but cant figure out how to do this :/ i also tried to use Sensor.TYPE_ACCELEROMETER and Sensor.TYPE_MAGNETIC_FIELD to calculate this 3d vector on my own. here is the code: final float[] inR = new float[16]; // load inR matrix from current sensor data: SensorManager.getRotationMatrix(inR, null, gravityValues, geomagneticValues); float[] orientation = new float[3]; SensorManager.getOrientation(inR, orientation); mapMagAndAcclDataToVector(orientation); //here i do some *360 stuff orientetionChanged(orientation); //then the correct values are passed (in theorie) But this didn't work and i think it is much to complicated. So i bet there is a simple solution how to recalc the values of ensor.TYPE_ORIENTATION to make them a 3d rotation vector, but i just dont know how to do it. If you know the answer please tell me.

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• vector <T *> destructor

  - by Daniel.Z

  I have a class defined like: Class A { public: int num; A *parent; vector<A *> children; ... // constructor without parameters A(void) { this->num = 3; this->parent = 0; for (int i=0;i<num;++i) children.push_back(new A(this,num-1)); } // constructor with parameters A(A *a,int n) { this->num = n; this->children->parent = a; for (int i=0;i<num;++i) this->children.push_back(new A(this,this->num-1)); } }; now, the constructor works fine. there are some problem with destructor. currently, the destructor is defined as: A::~A(void) { if (this->parent!=0) this->parent = 0; for (int i=0;i<(int)children.size();++i) this->children[i]->~A(); vector <A *> ().swap(this->children); } but every time when I debug it, it will break at: void deallocate(pointer _Ptr, size_type) { // deallocate object at _Ptr, ignore size ::operator delete(_Ptr); } it looks like I cannot delete the pointer in the vector of this-children, is there any way that I can de-construct the class successfully?

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• STL vector performance

  - by iAdam

  STL vector class stores a copy of the object using copy constructor each time I call push_back. Wouldn't it slow down the program? I can have a custom linkedlist kind of class which deals with pointers to objects. Though it would not have some benefits of STL but still should be faster. See this code below: #include <vector> #include <iostream> #include <cstring> using namespace std; class myclass { public: char* text; myclass(const char* val) { text = new char[10]; strcpy(text, val); } myclass(const myclass& v) { cout << "copy\n"; //copy data } }; int main() { vector<myclass> list; myclass m1("first"); myclass m2("second"); cout << "adding first..."; list.push_back(m1); cout << "adding second..."; list.push_back(m2); cout << "returning..."; myclass& ret1 = list.at(0); cout << ret1.text << endl; return 0; } its output comes out as: adding first...copy adding second...copy copy The output shows the copy constructor is called both times when adding and when retrieving the value even then. Does it have any effect on performance esp when we have larger objects?

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• sum of square of each elements in the vector using for_each

  - by pierr

  Hi, As the function accepted by for_each take only one parameter (the element of the vector), I have to define a static int sum = 0 somewhere so that It can be accessed after calling the for_each . I think this is awkward. Any better way to do this (still use for_each) ? #include <algorithm> #include <vector> #include <iostream> using namespace std; static int sum = 0; void add_f(int i ) { sum += i * i; } void test_using_for_each() { int arr[] = {1,2,3,4}; vector<int> a (arr ,arr + sizeof(arr)/sizeof(arr[0])); for_each( a.begin(),a.end(), add_f); cout << "sum of the square of the element is " << sum << endl; } In Ruby, We can do it this way: sum = 0 [1,2,3,4].each { |i| sum += i*i} #local variable can be used in the callback function puts sum #=> 30 Would you please show more examples how for_each is typically used in practical programming (not just print out each element)? Is it possible use for_each simulate 'programming pattern' like map and inject in Ruby (or map /fold in Haskell). #map in ruby >> [1,2,3,4].map {|i| i*i} => [1, 4, 9, 16] #inject in ruby [1, 4, 9, 16].inject(0) {|aac ,i| aac +=i} #=> 30 EDIT: Thank you all. I have learned so much from your replies. We have so many ways to do the same single thing in C++ , which makes it a little bit difficult to learn. But it's interesting :)

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• Vector Usage in MPI(C++)

  - by lsk1985

  I am new to MPI programming,stiil learning , i was successful till creating the Derived data-types by defining the structures . Now i want to include Vector in my structure and want to send the data across the Process. for ex: struct Structure{ //Constructor Structure(): X(nodes),mass(nodes),ac(nodes) { //code to calculate the mass and accelerations } //Destructor Structure() {} //Variables double radius; double volume; vector<double> mass; vector<double> area; //and some other variables //Methods to calculate some physical properties Now using MPI i want to sent the data in the structure across the processes. Is it possible for me to create the MPI_type_struct vectors included and send the data? I tried reading through forums, but i am not able to get the clear picture from the responses given there. Hope i would be able to get a clear idea or approach to send the data PS: i can send the data individually , but its an overhead of sending the data using may MPI_Send/Recieve if we consider the domain very large(say 10000*10000)

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• Reversing strings in a vector using for_each and bind

  - by fmuecke

  Hi! I was wandering how it's possible to reverese strings that are contained in a vector using a single for_each command just in one "simple" line. Yea, I know it is easy with a custom functor, but I can't accept, that it can't be done using bind (at least I couldn't do it). #include <vector> #include <string> #include <algorithm> std::vector<std::string> v; v.push_back("abc"); v.push_back("12345"); std::for_each(v.begin(), v.end(), /*call std::reverse for each element*/); Edit: Thanks a lot for those funtastic solutions. However, the solution for me was not to use the tr1::bind that comes with the Visual Studio 2008 feature pack/SP1. I don't know why it does not work like expected but that's the way it is (even MS admits that it's buggy). Maybe some hotfixes will help. With boost::bind everything works like desired and is so easy (but sometimes relly messy:)). I really should have tried boost::bind in the first place...

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• (C++) Loading a file into a vector

  - by Alden

  This is probably a simple question, however I am new to C++ and I cannot figure this out. I am trying to load a binary file and load each byte to a vector. This works fine with a small file, but when I try to read larger than 410 bytes the program crashes and says: This application has requested the Runtime to terminate it in an unusual way. Please contact the application's support team for more information. I am using code::blocks on windows. This is the code: #include <iostream> #include <fstream> #include <vector> using namespace std; int main() { std::vector<char> vec; std::ifstream file; file.exceptions( std::ifstream::badbit | std::ifstream::failbit | std::ifstream::eofbit); file.open("file.bin"); file.seekg(0, std::ios::end); std::streampos length(file.tellg()); if (length) { file.seekg(0, std::ios::beg); vec.resize(static_cast<std::size_t>(length)); file.read(&vec.front(), static_cast<std::size_t>(length)); } int firstChar = static_cast<unsigned char>(vec[0]); cout << firstChar <<endl; return 0; } Thank you for your help!

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