Re: [Boost-users] Any "2D" iterator?

20 Sep 2010

      I have the same problem, to view a map (k1->k2->v) as a sequence(k1, k2) ->
v. With iterator_facade and iterator_transform, the problem solved
verbosely.

in the code, Begin and End is unary functors applied to first iterator to
get second begin, end iterator.

template <class F, class S, class Begin, class End,
          class Tag=boost::bidirectional_traversal_tag>
class chain_iter :
    public boost::iterator_facade< chain_iter<F, S, Begin, End, Tag>,
                                   std::pair<F, S>,
                                   Tag,
                                   std::pair<F, S>
                                   > {
 public:
  typedef F first_type;
  typedef S second_type;
  typedef std::pair<first_type, second_type> value_type;
  typedef value_type reference; // no, no reference to temporary

  chain_iter() {}

  chain_iter(const F &f, const F &f_end, const S &s)
      : m_first(f), m_first_end(f_end), m_second(s) {
    normalize();
  }

  chain_iter(const chain_iter &r)
      : m_first(r.m_first), m_first_end(r.m_first_end), m_second(r.m_second)
{
    normalize();
  }

 private:
  friend class boost::iterator_core_access;

  void normalize() {
    while (m_first != m_first_end && m_second == End()(m_first)) {
      ++m_first;
      if (m_first != m_first_end) m_second = Begin()(m_first);
    }
  }

  void increment() {
    assert(m_first != m_first_end);
    ++m_second;
    normalize();
  }

  void decrement() {
    while (m_second == Begin()(m_first)) {
      --m_first;
      m_second = End()(m_first);
    }
    --m_second;
  }

  reference dereference() const {
    return std::make_pair(m_first, m_second);
  }

  bool equal(const chain_iter &r) const {
    if (m_first == m_first_end) {
      return r.m_first == r.m_first_end;
    }
    return m_first == r.m_first && m_second == r.m_second;
  }

  F m_first;
  F m_first_end;
  S m_second;
};

                                                      Changsheng Jiang

On Sun, Sep 19, 2010 at 22:57, Richard
<legalize+jeeves@mail.xmission.com<legalize%2Bjeeves@mail.xmission.com>
...
wrote:
...
[Please do not mail me a copy of your followup]
boost-users@lists.boost.org spake the secret code
<AANLkTi=RiR=piq5CzT8kNchjht=KR1co11OsVeF1DaqC@mail.gmail.com> thusly:
...
On Sat, Sep 18, 2010 at 10:37 AM, TONGARI <tongari95@gmail.com> wrote:
...
2010/9/17 <lfrfly@icqmail.com>
...
It seems like there ought to be a natural way to do this using an
iterator
...
...
class.
I don't think that'll be a good solution to this problem, not efficient
enough.
Whether it's efficient "enough" really depends on the OP's
requirements.  However, if you're worried about efficiency you should
look up Matt Austern's 1998 paper "Segmented Iterators and
Hierarchical Algorithms"
It was an interesting read and not hard to implement.
<http://lafstern.org/matt/segmented.pdf>
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