stl

来源： https://stackoverflow.com/questions/56483606/cannot-emplace-back-a-braced-initializer-on-a-vector-of-vectors

来源： https://stackoverflow.com/questions/56483606/cannot-emplace-back-a-braced-initializer-on-a-vector-of-vectors

问题 For this code: #include<unordered_map> #include<iostream> using namespace std; struct myhash { unsigned operator()(const unsigned&v)const { cout<<"Hash function is called:"<<v<<endl; return v; } }; unordered_map<unsigned,unsigned,myhash>mp; int main() { for (unsigned i=0;i<3;++i) { cout<<"Visiting hash table:"<<i<<endl; ++mp[i]; } } When using g++, there's no surprise with the output: Visiting hash table:0 Hash function is called:0 Visiting hash table:1 Hash function is called:1 Visiting hash

问题 I'd like to find the index of the minimum value in a C++ std::vector<double> . Here's a somewhat verbose implementation of this: //find index of smallest value in the vector int argMin(std::vector<double> vec) { std::vector<double>::iterator mins = std::min_element(vec.begin(), vec.end()); //returns all mins double min = mins[0]; //select the zeroth min if multiple mins exist for(int i=0; i < vec.size(); i++) { //Note: could use fabs( (min - vec[i]) < 0.01) if worried about floating-point

问题 I'd like to find the index of the minimum value in a C++ std::vector<double> . Here's a somewhat verbose implementation of this: //find index of smallest value in the vector int argMin(std::vector<double> vec) { std::vector<double>::iterator mins = std::min_element(vec.begin(), vec.end()); //returns all mins double min = mins[0]; //select the zeroth min if multiple mins exist for(int i=0; i < vec.size(); i++) { //Note: could use fabs( (min - vec[i]) < 0.01) if worried about floating-point

问题 The set_difference algorithm requires the following The elements in the ranges shall already be ordered according to this same criterion which is not the case for hash tables. I'm thinking of implementing a set difference A-B in terms of std::remove_copy where the removal criterion would be the existence of an element of A in the set B. Is there a standard-valid-fastest-safest way to do it? 回答1: If you have two hash tables, the most efficient way should be to iterate over one of them, looking

问题 What's the use of value_type in STL containers? From the MSDN: // vector_value_type.cpp // compile with: /EHsc #include <vector> #include <iostream> int main( ) { using namespace std; vector<int>::value_type AnInt; AnInt = 44; cout << AnInt << endl; } I don't understand what does value_type specifically achieve here? The variable could be an int as well? Is it used because the coders are lazy to check what's the type of objects present in the vector? Once I am clear about this I think I will

问题 I'm looking to insert values into a std::vector in a way like std::transform. std::transform needs a pre-sized third argument, but in my case, the size depends on transformers() and is not predictable. ... // std::vector<int> new_args(); <-- not working std::vector<int> new_args(args.size()); std::transform(args.begin(),args.end(),new_args.begin(),transformers()); Is there a std:transform-ish way to insert values into a std::vector? 回答1: You do not need to pre size the vector that is going to

问题 I'm looking to insert values into a std::vector in a way like std::transform. std::transform needs a pre-sized third argument, but in my case, the size depends on transformers() and is not predictable. ... // std::vector<int> new_args(); <-- not working std::vector<int> new_args(args.size()); std::transform(args.begin(),args.end(),new_args.begin(),transformers()); Is there a std:transform-ish way to insert values into a std::vector? 回答1: You do not need to pre size the vector that is going to

问题 I found myself writing the following a lot: int location =2; vector<int> vec; vector<int>::iterator it=vec.begin(); /..../ std::advance(it, location); instead of it= it + 5; what is the Preferred/Recommended way ? 回答1: Adding will only work with random access iterators. std::advance will work with all sorts of iterators. As long as you're only dealing with iterators into vectors, it makes no real difference, but std::advance keeps your code more generic (e.g. you could substitute a list for