Question

I’d like to do the following. but don’t know how:

    //have two vectors: vector1 (full of numbers), vector2 (empty)
    //with vectors, I mean STL vectors
    //outer loop
    {
    //inner loop 
    {
    //vector2 gets written more and more over iterations of inner loop
    //elements of vector1 are needed for this
    } //end of inner loop
    //now data of vector1 is not needed anymore and vector2 takes the role of
    //vector 1 in the next iteration of the outer loop

    //old approach (costly):
    //clear vector1 ,copy vector2's data to vector1, clear vector2

    //wanted: 
    //forget about vector1's data
    //somehow 'rename' vector2 as 'vector1' 
    //(e.g. call vector2's data 'vector1')
    //do something so vector2 is empty again
    //(e.g. when referring to vector2 in the next
    //iteration of outer loop it should be an empty vector in the
    //beginning.)

    } // end of outer loop

I was trying

     vector<double> &vector1 = vector2;
     vector2.clear();

but I think the problem is that vector1 then is a reference to vector2, which is then deleted.

Any ideas ?

Answer 1

Are you aware of this function: http://www.cplusplus.com/reference/stl/vector/swap/

// swap vectors
#include <iostream>
#include <vector>
using namespace std;

int main ()
{
  unsigned int i;
  vector<int> first;   // empty
  vector<int> second (5,200);  // five ints with a value of 200

  first.swap(second);

  cout << "first contains:";
  for (i=0; i<first.size(); i++) cout << " " << first[i];

  cout << "\nsecond contains:";
  for (i=0; i<second.size(); i++) cout << " " << second[i];

  cout << endl;

  return 0;
}

The complexity of this function is guarantee to be constant.