Re: copy from keys from multimap into the vector

From:
Salt_Peter <pj_hern@yahoo.com>
Newsgroups:
comp.lang.c++
Date:
Wed, 29 Oct 2008 16:28:55 -0700 (PDT)
Message-ID:
<4fc17fc6-e74c-4f86-a5b2-c208ccd69664@l64g2000hse.googlegroups.com>
On Oct 29, 2:33 pm, puzzlecracker <ironsel2...@gmail.com> wrote:

I am using while loop for that but I am sure you can do it quicker and
more syntactically clear with copy function.

Here is what I do and would like to if someone has a cleaner solution:

    vector<string> vec;
    multimap<stirng, int> myMap

   // populate myMap

    multimap<string, int >::iterator iter = myMap.begin();

    while(iter != myMap.end())
    {
           vec.push_back(iter->first)
    }


You could use std::transform with std::back_inserter to load vector
and a functor to extract the std::string from multimap's value_type.

#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <algorithm>
#include <iterator>

template< typename P >
struct extract_first
{
  const typename P::first_type&
  operator()(const P& p) const
  {
    return p.first;
  }
};

int main()
{
  std::vector< std::string > vec;
  std::multimap< std::string, int > mm;
  // populate mm

  typedef std::multimap< std::string, int >::value_type VType;

  std::transform( mm.begin(),
                  mm.end(),
                  std::back_inserter(vec),
                  extract_first< VType >() );
}

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