Copy map values to vector in STL

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?

Answer by anon for Copy map values to vector in STL

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 

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

template    void MapToVec( const  M & m, V & v ) {      for( typename M::const_iterator it = m.begin(); it != m.end(); ++it ) {      	v.push_back( it->second );      }  }  

Answer by Skurmedel for Copy map values to vector in STL

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

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Example by xtofl (see comments):

#include 

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

Answer by aJ. for Copy map values to vector in STL

One way is to use functor:

 template       class CopyMapToVec      {      public:           CopyMapToVec(std::vector& aVec): mVec(aVec){}            bool operator () (const std::pair& mapVal) const          {          	mVec.push_back(mapVal.second);          	return true;          }      private:          std::vector& mVec;      };      int main()  {      std::map myMap;      myMap["test1"] = 1;      myMap["test2"] = 2;        std::vector  myVector;        //reserve the memory for vector      myVector.reserve(myMap.size());      //create the functor      CopyMapToVec aConverter(myVector);        //call the functor      std::for_each(myMap.begin(), myMap.end(), aConverter);  }  

Answer by Loki Astari for Copy map values to vector in STL

Here is what I would do.
Also I would use a template function to make the construction of select2nd easier.

#include 

Answer by Matthieu N. for Copy map values to vector in STL

Using lambdas one can perform the following:

{     std::map m;     std::vector v;     v.reserve(m.size());     std::for_each(m.begin(),m.end(),                   [&v](const std::map::value_type& p)                    { v.push_back(p.second); });  }  

Answer by OK. for Copy map values to vector in STL

If you are using the boost libraries, you can use boost::bind to access the second value of the pair as follows:

#include   #include 

This solution is based on a post from Michael Goldshteyn on the boost mailing list.

Answer by OJMAN for Copy map values to vector in STL

I thought it should be

std::transform( map.begin(), map.end(), std::back_inserter(vec), boost::bind(&MapT::value_type::first,_1) );

Answer by skyhawk for Copy map values to vector in STL

Why not:

template  std::vector MapValuesAsVector(std::map map)  {     std::vector vec(map.size());     std::for_each(std::begin(map), std::end(map),          [&vec] (const std::map::value_type& entry)           {              vec.push_back(entry.second);          });      return vec;  }  

usage:

auto vec = MapValuesAsVector(anymap);

Answer by Seth for Copy map values to vector in STL

Old question, new answer. With C++11 we have the fancy new for loop:

for (const auto &s : schemas)     names.push_back(s.first);  

where schemas is a std::map and names is an std::vector.

This populates the array (names) with keys from the map (schemas); change s.first to s.second to get an array of values.

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