CODEWITHSUNDEEP
c++vectorstl
polapts
polapts

Reputation: 5823

How to copy elements to a Vector if you are given a pair of iterators?

I am trying to find a way to copy elements of a vector to another vector. 

int main()
{
  std::vector<int> aVec{0,1,2,3,4};
  std::vector<int>::iterator itBegin = aVec.begin();
  std::vector<int>::iterator itEnd = aVec.begin()+3;

  std::vector<int> aVecNew;
  // How to insert elements ranging from itBegin till itEnd(including itEnd) to
  // the vector aVecNew

  return 0;

}

Most of the insert methods appear not to include itEnd. Is there a clean way to do this?

EDIT: If I am not sure ++itEnd is the end iterator or not. In such case it would fail. Is there any safer way without the mess ?

Upvotes: 2

Views: 1306

Answers (3)

decltype_auto
decltype_auto

Reputation: 1736

In the general case - the target vector does already exist - the copy onto a back_insert_iterator until just before ++itEnd is the right way, but in your case, 

std::vector<int> aVecNew(itBegin, ++itEnd);

is the appropriate measure. std::vector has ctor #(4) for that; no reason to first create and then populate the vector here.

#include <vector>
#include <iostream>
#include <iterator>     // ostream_iterator
#include <algorithm>    // copy

int main()
{

    std::vector<int> aVec{0,1,2,3,4}; // this is c++ of 2011 or later...

    // ... thus this 
    std::vector<int>::iterator itBegin = aVec.begin();
    std::vector<int>::iterator itEnd = aVec.begin() + 3;
    // (if used) would at least be a case for "auto" 

    std::vector<int> aVecNew(itBegin, ++itEnd);

    std::copy(aVecNew.begin(), aVecNew.end(),
              std::ostream_iterator<int>(std::cout, " ")); 

    std::cout << std::endl; 

    return EXIT_SUCCESS;
}

output:

0 1 2 3

live at Coliru's

Upvotes: 0

nh_
nh_

Reputation: 2241

Beyond the ways already mentioned, std::vector has a constructor that will do exactly what you want (take the elements from range given begin and end iterators).

std::vector<int> aVecNew(itBegin, ++itEnd);

Upvotes: 1

alexeykuzmin0
alexeykuzmin0

Reputation: 6440

You can use std::copy from <algorithms> and std::back_inserter from <iterator>:

int main(int a, char**){
  std::vector<int> aVec{ 0, 1, 2, 3, 4 };
  std::vector<int>::iterator itBegin = aVec.begin();
  std::vector<int>::iterator itEnd = aVec.begin() + 3;

  std::vector<int> aVecNew;
  std::copy(itBegin, itEnd, std::back_inserter(aVecNew));
  return 0;
}

PS: Also, as it was mentioned in the comment, this code copies excluding itEnd. If you want to copy elements including itEnd, just increment its value by 1:

int main(int a, char**){
  std::vector<int> aVec{ 0, 1, 2, 3, 4 };
  std::vector<int>::iterator itBegin = aVec.begin();
  std::vector<int>::iterator itEnd = aVec.begin() + 3;

  std::vector<int> aVecNew;
  std::copy(itBegin, ++itEnd, std::back_inserter(aVecNew));
  return 0;
}

Some documentation on back_inserter and copy.

Upvotes: 1

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