stdvector

问题 I have a std::vector<std::string> v; (initialized). How can I use the range-for loop for accessing all elements except the first one (on index zero). For all elements: for (const string & s: v) process(s); Instead of the v a range expression can be used. How can I write the range expression to skip the first element (or skip the first n elements) ? I know how to get the effect using v.begin() + 1 and using the classic loop. I am searching for the new, more readable, recommended alternative to

问题 How do we do the analysis of insertion at the back (push_back) in a std::vector? It's amortized time is O(1) per insertion. In particular in a video in channel9 by Stephan T Lavavej and in this ( 17:42 onwards ) he says that for optimal performance Microsoft's implementation of this method increases capacity of the vector by around 1.5. How is this constant determined? 回答1: Assuming you mean push_back and not insertion, I believe that the important part is the multiply by some constant (as

问题 I have a C# front end and a C++ backend for performance reasons. Now I would like to call a C++ function like for example: void findNeighbors(Point p, std::vector<Point> &neighbors, double maxDist); What I'd like to have is a C# wrapper function like: List<Point> FindNeigbors(Point p, double maxDist); I could pass a flat array like Point[] to the unmanaged C++ dll, but the problem is, that I don't know how much memory to allocate, because I don't know the number of elements the function will

问题 I have a number of callback functions with different signatures. Ideally, I would like to put these in a vector and call the appropriate one depending on certain conditions. e.g. void func1(const std::string& value); void func2(const std::string& value, int min, int max); const std::vector<std::function<void(std::string)>> functions { func1, func2, }; I realise the above isn't possible, but I wonder if there are any alternatives I should consider. I haven't been able to find any yet, and I've

问题 I like vectors a lot. They're nifty and fast. But I know this thing called a valarray exists. Why would I use a valarray instead of a vector? I know valarrays have some syntactic sugar, but other than that, when are they useful? 回答1: Valarrays (value arrays) are intended to bring some of the speed of Fortran to C++. You wouldn't make a valarray of pointers so the compiler can make assumptions about the code and optimise it better. (The main reason that Fortran is so fast is that there is no

问题 I have an array of integers that I need to remove duplicates from while maintaining the order of the first occurrence of each integer. I can see doing it like this, but imagine there is a better way that makes use of STL algorithms better? The insertion is out of my control, so I cannot check for duplicates before inserting. int unsortedRemoveDuplicates(std::vector<int> &numbers) { std::set<int> uniqueNumbers; std::vector<int>::iterator allItr = numbers.begin(); std::vector<int>::iterator

问题 E.g., if I have an Eigen::MatrixXd of size 10 columns and 3 rows, how can I alias that to a std::vector of 10 elements of Eigen::Vector3d ? when I say alias I mean using the same memory block without copying. I know that I can do the reversed mapping by something like: std::vector<Vector3d> v(10); ... Map<Matrix<double,3,Dynamic> > m(v.data().data(), 3, 10); but reversely, if I have an Eigen::Matrix, I tried to convert it to a vector of Eigen::vector but the following code line failed

问题 The struct struct Vanish { int iCount; int iRow; }; I defined a std::vector of Vanish as a member of my class, and want to initialise it in the constructor like this: class BigClass { public: BigClass(); private: std::vector<Vanish> tovanish; }; void BigClass::BigClass() { Vanish empty = {0,0}; tovanish.assign(MAX_VANISH, empty); } Is there a better way, or is this considered OK? 回答1: It is better to do that in the constructor's initializer list: BigClass::BigClass() : tovanish(MAX_VANISH) {

问题 The struct struct Vanish { int iCount; int iRow; }; I defined a std::vector of Vanish as a member of my class, and want to initialise it in the constructor like this: class BigClass { public: BigClass(); private: std::vector<Vanish> tovanish; }; void BigClass::BigClass() { Vanish empty = {0,0}; tovanish.assign(MAX_VANISH, empty); } Is there a better way, or is this considered OK? 回答1: It is better to do that in the constructor's initializer list: BigClass::BigClass() : tovanish(MAX_VANISH) {

问题 Similar Questions: STL vector reserve() and copy() std::vector reserve() and push_back() is faster than resize() and array index, why? std::vector::resize() vs. std::vector::reserve() #include <vector> #include <iostream> using namespace std; int main() { vector<vector<int> > vvi; vvi.resize(1); vvi[0].reserve(1); vvi[0][0] = 1; vector<int> vi = vvi[0]; cout << vi[0]; // cout << vvi[0][0]; works return 0; } This gives me a seg fault, and I can't tell why. 回答1: vvi[0].reserve(1); vvi[0][0] = 1