c++ STL copy_backward question

Question

int main () {
  vector<int> myvector;
  vector<int>::iterator it;

  // set some values:
  for (int i=1; i<=5; i++)
    myvector.push_back(i*10);          // myvector: 10 20 30 40 50

  myvector.resize(myvector.size()+3);  // allocate space for 3 more elements

  copy_backward ( myvector.begin(), myvector.begin()+5, myvector.end() );

  cout << "myvector contains:";
  for (it=myvector.begin(); it!=myvector.end(); ++it)
    cout << " " << *it;

  cout << endl;

  return 0;
}

Why the output is "myvector contains: 10 20 30 10 20 30 40 50"

why not "30 40 50 10 20 30 40 50"

The implementation of copy_backward is here:

template<class BidirectionalIterator1, class BidirectionalIterator2>
  BidirectionalIterator2 copy_backward ( BidirectionalIterator1 first,
                                         BidirectionalIterator1 last,
                                         BidirectionalIterator2 result )
{
  while (last!=first) *(--result) = *(--last);
  return result;
}

So confused. thank you for all your help.

Related: http://stackoverflow.com/questions/1853358/use-a-regular-iterator-to-iterate-backwards-or-struggle-with-reverse-iterator/1857314#1857314 — Billy ONeal, Aug 02 '11 at 22:02

score 2 · Accepted Answer · answered Aug 02 '11 at 22:07

That output looks right to me according to the way the code is written. You are copying from and into the same vector. You are copying from [begin, begin +5] (10 20 30 40 50) and you are copying to [end, end-5]. So 10 20 30 [10 20 30 40 50] is the right output for that code. The first 3 elements are untouched.

score 1 · Answer 2 · answered Aug 02 '11 at 22:00

1

If you want to copy something backwards, use reverse-iterators: rbegin() and rend(). Then just use the regular std::copy.

answered Aug 02 '11 at 22:00

Nicol Bolas

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I'm not sure how this helps. To be fair, I'm not sure exactly what the OP wants. Where would the data for the 'new' first three elements come from? – Michael Burr Aug 02 '11 at 22:09

c++ STL copy_backward question

2 Answers2