Re: [Boost-users] [boost-users] istream_iterator question

12 May 2009

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Hello,
I don't know why the following code does not work as expected.
I hope the guru's here could lend me a hand.
For a test data file:
3
1 2
3 4
5 6
2
1.2 4.3 234.2
3.5656 67.88 345.3
<eof>
and 2 simple test helper class:
struct a
{ int i, j; };
std::istream& operator>>(std::istream& in, a & p)
{  in >> p.i >> p.j;  return in;  }
std::ostream& operator<<(std::ostream& out, const a & p)
{  out << p.i << " " << p.j; return out; }
struct b
{ float i, j, k; };
std::istream& operator>>(std::istream& in, b & p)
{  in >> p.i >> p.j >> p.k; return in;  }
std::ostream& operator<<(std::ostream& out, const b & p)
{  out << p.i << " " << p.j << " " << p.k; return out;  }
[Snip]
copy_n is defined as:
template <class InputIterator, class Size, class OutputIterator>
  OutputIterator copy_n( InputIterator first, InputIterator last,
  Size n,
  OutputIterator result)
{
  // copies the first `n' items from `first' to `result'. Returns
  // the value of `result' after inserting the `n' items.
  // it ends before n iterations, if the last iterator is reached
  while( n-- && first != last)
  {
      *result = *first;
      ++first;
      ++result;
  }
  return result;
}
Change your copy_n() as follows:
template <class InputIterator, class Size, class OutputIterator>
  OutputIterator copy_n( InputIterator first, InputIterator last,
  Size n,
  OutputIterator result)
{
  // copies the first `n' items from `first' to `result'. Returns
  // the value of `result' after inserting the `n' items.
  // it ends before n iterations, if the last iterator is reached
  while( n-- && first != last)
  {
      *result = *first;
      if (n) ++first;
      ++result;
  }
  return result;
}
Regards,
Dmitry
Hello Dmitry,

I really appreciate your help on such a naive question! 
I'm prepared to getting no response from the list, for my 
message is really a little bit lengthy, and somewhat off 
the main stream of the list, I'm afraid. :-)

With your code change, the test code runs as expected,
so does my real program. But I just don't know why.

In fact, the copy_n function is a safe version of another
one:

template <class InputIterator, class Size, class OutputIterator>
	OutputIterator copy_n( InputIterator first,
	Size n,
	OutputIterator result) 
{
	// copies the first `n' items from `first' to `result'. Returns
	// the value of `result' after inserting the `n' items.
	while( n-- ) 
	{
		*result = *first;
		++first;
		++result;
	}
	return result;
}

I cannot find bug from it, and I think the safer one is also 
straightforward and correct. :-)

I think 'first' and 'result' should both increment with 1 if needed
to keep sync. With your extra 'if(n)' condition they will be out of sync.
But that is just working well. I'm really confused.

Thanks again for your help.

B/Rgds
Max