What is the best way to concatenate two vectors

Question

I m using multitreading and want to merge the results  For example   std  vector lt int gt  A  std  vector lt int gt  B  std  vector lt int gt  AB    I want AB to have to contents of A and the contents of B in that order  What s the most efficient way of doing something like this

User · Answer

One more simple variant which was not yet mentioned:

copy(A.begin(),A.end(),std::back_inserter(AB));
copy(B.begin(),B.end(),std::back_inserter(AB));

And using merge algorithm:



#include <algorithm>
#include <vector>
#include <iterator>
#include <iostream>
#include <sstream>
#include <string>

template<template<typename, typename...> class Container, class T>
std::string toString(const Container<T>& v)
{
    std::stringstream ss;
    std::copy(v.begin(), v.end(), std::ostream_iterator<T>(ss, ""));
    return ss.str();
};


int main()
{
    std::vector<int> A(10);
    std::vector<int> B(5);  //zero filled
    std::vector<int> AB(15);

    std::for_each(A.begin(), A.end(),
            [](int& f)->void
            {
                f = rand() % 100;
            });

    std::cout << "before merge: " << toString(A) << "\n";
    std::cout << "before merge: " << toString(B) << "\n";
    merge(B.begin(),B.end(), begin(A), end(A), AB.begin(), [](int&,int&)->bool {});
    std::cout << "after merge:  " << toString(AB) << "\n";

    return 1;
}

User · Answer

Based on Kiril  V  Lyadvinsky answer  I made a new version  This snippet use template and overloading  With it  you can write vector3   vector1   vector2 and vector4    vector3  Hope it can help   template  lt typename T gt  std  vector lt T gt  operator  const std  vector lt T gt   amp A  const std  vector lt T gt   amp B        std  vector lt T gt  AB      AB reserve A size     B size                       preallocate memory     AB insert AB end    A begin    A end               add A      AB insert AB end    B begin    B end               add B      return AB     template  lt typename T gt  std  vector lt T gt   amp operator   std  vector lt T gt   amp A  const std  vector lt T gt   amp B        A reserve A size     B size                       preallocate memory without erase original data     A insert A end    B begin    B end                add B      return A                                            here A could be named AB

User · Answer

AB reserve  A size     B size         preallocate memory AB insert  AB end    A begin    A end      AB insert  AB end    B begin    B end

User · Answer

If your vectors are sorted   check out set union from  lt algorithm gt    set union A begin    A end    B begin    B end    AB begin       There s a more thorough example in the link   thanks rlbond

User · Answer

Depends on whether you really need to physically concatenate the two vectors or you want to give the appearance of concatenation of the sake of iteration  The boost  join function  http   www boost org doc libs 1 43 0 libs range doc html range reference utilities join html  will give you this   std  vector lt int gt  v0  v0 push back 1   v0 push back 2   v0 push back 3    std  vector lt int gt  v1  v1 push back 4   v1 push back 5   v1 push back 6        BOOST FOREACH const int  amp  i  boost  join v0  v1        cout  lt  lt  i  lt  lt  endl      should give you   1 2 3 4 5 6   Note boost  join does not copy the two vectors into a new container but generates a pair of iterators  range  that cover the span of both containers  There will be some performance overhead but maybe less that copying all the data to a new container first

User · Answer

This is precisely what the member function std  vector  insert is for  std  vector lt int gt  AB   A  AB insert AB end    B begin    B end

User · Answer

In the direction of Bradgonesurfing s answer  many times one doesn t really need to concatenate two vectors  O n    but instead just work with them as if they were concatenated  O 1    If this is your case  it can be done without the need of Boost libraries   The trick is to create a vector proxy  a wrapper class which manipulates references to both vectors  externally seen as a single  contiguous one    USAGE  std  vector lt int gt  A  1  2  3  4  5   std  vector lt int gt  B  10  20  30     VecProxy lt int gt  AB A  B       ---- gt  O 1   No copies performed   for  size t i   0  i  lt  AB size      i      std  cout  lt  lt  AB i   lt  lt           1 2 3 4 5 10 20 30   IMPLEMENTATION  template  lt class T gt  class VecProxy   private      std  vector lt T gt  amp  v1  v2  public      VecProxy std  vector lt T gt  amp  ref1  std  vector lt T gt  amp  ref2    v1 ref1   v2 ref2         const T amp  operator   const size t amp  i  const      const size t size   const      template  lt class T gt  const T amp  VecProxy lt T gt   operator   const size t amp  i  const      return  i  lt  v1 size      v1 i    v2 i - v1 size         template  lt class T gt  const size t VecProxy lt T gt   size   const   return v1 size     v2 size         MAIN BENEFIT  It s O 1   constant time  to create it  and with minimal extra memory allocation     SOME STUFF TO CONSIDER   You should only go for it if you really know what you re doing when dealing with references  This solution is intended for the specific purpose of the question made  for which it works pretty well  To employ it in any other context may lead to unexpected behavior if you are not sure on how references work  In this example  AB does not provide a non-const access operator        Feel free to include it  but keep in mind  since AB contains references  to assign it values will also affect the original elements within A and or B  Whether or not this is a desirable feature  it s an application-specific question one should carefully consider  Any changes directly made to either A or B  like assigning values  sorting  etc   will also  modify  AB  This is not necessarily bad  actually  it can be very handy  AB does never need to be explicitly updated to keep itself synchronized to both A and B   but it s certainly a behavior one must be aware of  Important exception  to resize A and or B to sth bigger may lead these to be reallocated in memory  for the need of contiguous space   and this would in turn invalidate AB  Because every access to an element is preceded by a test  namely   i  lt  v1 size      VecProxy access time  although constant  is also a bit slower than that of vectors  This approach can be generalized to n vectors  I haven t tried  but it shouldn t be a big deal

User · Answer

All the solutions are correct  but I found it easier just write a function to implement this  like this   template  lt class T1  class T2 gt  void ContainerInsert T1 t1  T2 t2        t1- gt insert t1- gt end    t2- gt begin    t2- gt end         That way you can avoid the temporary placement like this   ContainerInsert vec  GetSomeVector

[c++] What is the best way to concatenate two vectors?

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