C++ – Copy map values to vector in STL

ccontainersstl

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?

Best Answer

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;
    m[0]=true;
    m[1]=false;
    //...
    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.

Related Solutions

C++ – using STL to find all elements in a vector

Boost Lambda makes this easy.

#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
#include <boost/lambda/if.hpp>

std::for_each( v.begin(), v.end(), 
               if_( MyPred() )[ std::mem_fun(&MyType::myfunc) ] 
             );

You could even do away with defining MyPred(), if it is simple. This is where lambda really shines. E.g., if MyPred meant "is divisible by 2":

std::for_each( v.begin(), v.end(), 
               if_( _1 % 2 == 0 )[ std::mem_fun( &MyType::myfunc ) ]
             );

Update: Doing this with the C++0x lambda syntax is also very nice (continuing with the predicate as modulo 2):

std::for_each( v.begin(), v.end(),
               [](MyType& mt ) mutable
               {
                 if( mt % 2 == 0)
                 { 
                   mt.myfunc(); 
                 }
               } );

At first glance this looks like a step backwards from boost::lambda syntax, however, it is better because more complex functor logic is trivial to implement with c++0x syntax... where anything very complicated in boost::lambda gets tricky quickly. Microsoft Visual Studio 2010 beta 2 currently implements this functionality.

C++ – Erasing elements from a vector

Use the remove/erase idiom:

std::vector<int>& vec = myNumbers; // use shorter name
vec.erase(std::remove(vec.begin(), vec.end(), number_in), vec.end());

What happens is that remove compacts the elements that differ from the value to be removed (number_in) in the beginning of the vector and returns the iterator to the first element after that range. Then erase removes these elements (whose value is unspecified).

Best Answer

Related Solutions

C++ – using STL to find all elements in a vector

C++ – Erasing elements from a vector

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