Copy map values to vector in STL


Working my way through Effective STL at the moment. Item 5 suggests that it's usually preferable to use range member functions to their single element counterparts. I currently wish to copy all the values in a map (i.e. - I don't need the keys) to a vector.

What is the cleanest way to do this?

4/21/2009 7:30:42 AM

Accepted Answer

You can't easily use a range here becuase the iterator you get from a map refers to a std::pair, where the iterators you would use to insert into a vector refers to an object of the type stored in the vector, which is (if you are discarding the key) not a pair.

I really don't think it gets much cleaner than the obvious:

#include <map>
#include <vector>
#include <string>
using namespace std;

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

    // populate map somehow

    for( MapType::iterator it = m.begin(); it != m.end(); ++it ) {
        v.push_back( it->second );

which I would probably re-write as a template function if I was going to use it more than once. Something like:

template <typename M, typename V> 
void MapToVec( const  M & m, V & v ) {
    for( typename M::const_iterator it = m.begin(); it != m.end(); ++it ) {
        v.push_back( it->second );
4/21/2009 7:48:42 AM

You could probably use std::transform for that purpose. I would maybe prefer Neils version though, depending on what is more readable.

Example by xtofl (see comments):

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

template< typename tPair >
struct second_t {
    typename tPair::second_type operator()( const tPair& p ) const { return     p.second; }

template< typename tMap > 
second_t< typename tMap::value_type > second( const tMap& m ) { return second_t<     typename tMap::value_type >(); }

int main() {
    std::map<int,bool> m;
    std::vector<bool> v;
    std::transform( m.begin(), m.end(), std::back_inserter( v ), second(m) );
    std::transform( m.begin(), m.end(), std::ostream_iterator<bool>( std::cout,     ";" ), second(m) );

Very generic, remember to give him credit if you find it useful.

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