Re: count_unique or unique_count - why does it not exist?

From:
Carl Barron <cbarron413@adelphia.net>
Newsgroups:
comp.lang.c++.moderated
Date:
22 Oct 2006 04:26:39 -0400
Message-ID:
<221020060327393228%cbarron413@adelphia.net>
In article <PUn_g.16755$e66.11901@newssvr13.news.prodigy.com>, Gene
Bushuyev <spam@spamguard.com> wrote:

. Below is the code off the top of
my head,
so it may contain errors, but the idea I think should be clear:

std::size_t sum(std::size_t count, bool equal)
{
  return equal ? count : count + 1;
}

template<class It>
std::size_t unique_count(It first, It last)
{
  return first != last ?
     std::inner_product(first++, last, first, 1, sum,
std::equal_to<typename
It::value_type>())
     : 0;
}


This is what I think you want, assumes bool is convertable to size_t.
and For is a forward iterator.

template <class For>
std::size_t unique_count(For first,For last)
{
    if(first==last) return 0;
    // need a copy since order of argument evaluation
    // is unspecified in the standard.

    For x(first);
    return std::inner_product
       (
          first,
          last,
          ++x,
          (unsigned int)0,
          std::plus<std::size_t>(),
          std::not_equal_to
          <
             std::iterator_traits<For>::value_type
          >()
       );
}

I do not see the gain over using an output iterator to compute the
result, via unique_copy(). [see another post in this thread for
details]
in my opinion the output iterator solution is better, can be done with
boost::function_output_iterator like

class unique_count
{
    struct internal
    {
       std::size_t &n;
       internal(std::size_t &a):n(a){}
       template <class T>
       void operator ()(T const &){++n;}
    };
    std::size_t n;
public:
    unique_count():n(0){}
    template <class In>
    std::size_t operator ()(In first,In last)
    {
       std::unique_copy
       (
          first,
          last,
          boost::function_output_iterator
          <
             internal
          >(internal(n))
       );
       return n;
    }
};

usage:
    unique_count()(first,last)

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