Re: Initialize a std::set with keys from a std::map

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 15 Jan 2008 23:07:25 -0500

Message-ID:

<daniel_t-7041CC.23072515012008@earthlink.vsrv-sjc.supernews.net>

brzrkr0@gmail.com wrote:

A portion of my program needs to initialize a std::set<int> to have
all the keys in a std::map<int, double>. The code I've pasted below
works, but I'm wondering if there's a more elegant way to do it (an
STL algorithm, maybe?). I'm a bit of an STL noob, so any advice
people can give would be greatly appreciated.

   transform( map->begin(), map->end(), inserter( v, v.begin() ),
      select1st<intDoubleMap::value_type>() );

Note, "select1st" is part of the STL, but not part of the standard
library. It's easy to implement though and is generally useful:

   template <typename Pair>
struct select1st : unary_function<Pair, typename Pair::first_type>
{
   const typename Pair::first_type& operator()(const Pair& x) const {
      return x.first;
   }
};

Or you can use the boost lambda library:

   transform( map->begin(), map->end(), inserter( v, v.begin() ),
      bind(&intDoubleMap::value_type::first, _1 ) );

typedef std::map<int, double> intDoubleMap;
typedef std::set<int> intSet;

intSet v;
// map is an intDoubleMap* and has been initialized with some values
for (intDoubleMap::const_iterator it = map->begin();
        it != map->end();
        it++) {
    int i = it->first;
    v.insert(i);
}