Populate a vector with all multimap values with a given key

Here's the way to do it STL style :

// The following define is needed for select2nd with DinkumWare STL under VC++
#define _HAS_TRADITIONAL_STL 1

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

using namespace std;

void main()
{
    typedef multimap<string, int> MapType;
    MapType m;
    vector<int> v;

    // Test data
    for(int i = 0; i < 10; ++i)
    {
        m.insert(make_pair("123", i * 2));
        m.insert(make_pair("12", i));
    }

    MapType::iterator i = m.lower_bound("123");
    MapType::iterator j = m.upper_bound("123");

    transform(i, j, back_inserter(v), select2nd<MapType::value_type>());

    copy(v.begin(), v.end(),  ostream_iterator<int>(cout, ","));

}

Let's go lambda

given: multimap<A,B> M

requested: vector<B> (of all values in M with a specific key 'a'.)

method:

std::pair<M::iterator, M::iterator> aRange = M.equal_range('a')
std::vector<B> aVector;
std::transform(aRange.first, aRange.second,std::back_inserter(aVector), [](std::pair<A,B> element){return element.second;});         

System environment:

1. compiler: gcc (Ubuntu 5.3.1-14ubuntu2.1) 5.3.1 20160413 (with -std=c++11)
2. os: ubuntu 16.04

Code example:

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

int main()
{
    typedef std::multimap<std::string, int> MapType;
    MapType m;
    std::vector<int> v;

    /// Test data
    for(int i = 0; i < 10; ++i)
    {
        m.insert(std::make_pair("123", i * 2));
        m.insert(std::make_pair("12", i));
    }

    std::pair<MapType::iterator,MapType::iterator> aRange = m.equal_range("123");

    std::transform(aRange.first, aRange.second, std::back_inserter(v), [](std::pair<std::string,int> element){return element.second;});

    for(auto & elem: v)
    {
        std::cout << elem << std::endl;
    }
    return 0;
}

You need a loop anyway. All "loop-free" methods just abstract the loop away.

#include <map>
#include <vector>
#include <algorithm>
#include <ext/functional>
using namespace std;

int main () {
    multimap<int, double> mm;
    mm.insert(make_pair(1, 2.2));
    mm.insert(make_pair(4, 2.6));
    mm.insert(make_pair(1, 9.1));
    mm.insert(make_pair(1, 3.1));

    vector<double> v;
    transform(mm.lower_bound(1), mm.upper_bound(1),
              back_inserter(v), __gnu_cxx::select2nd<pair<int, double> >());
    // note: select2nd is an SGI extension.

    for (vector<double>::const_iterator cit = v.begin(); cit != v.end(); ++ cit)
        printf("%g, ", *cit);   // verify that you've got 2.2, 9.1, 3.1
    return 0;
}