How to get the first n elements of a std::map

Question

Since there is no .resize() member function in C++ std::map I was wondering, how one can get a std::map with at most n elements.

The obvious solution is to create a loop from 0 to n and use the nth iterator as the first parameter for std::erase().

I was wondering if there is any solution that does not need the loop (at least not in my user code) and is more "the STL way to go".

Answer 1

You can use std::advance( iter, numberofsteps ) for that.

Answer 2

Universal solution for almost any container, such as std::list, std::map, boost::multi_index. You must check the size of your map only.

template<class It>
It myadvance(It it, size_t n) {
   std::advance(it, n);
   return it;
}

template<class Cont>
void resize_container(Cont & cont, size_t n) {
    cont.erase(myadvance(cont.begin(), std::min(n, cont.size())), 
                 cont.end());
}

Answer 3

The correct way for this is to use std::advance. But here is a funny (slow) way allowing to 'use resize on map'. More generally, this kind of trick can be used for other things working on vector but not on map.

map<K,V> m; //your map
vector< pair<K,V> > v(m.begin(), m.end());
v.resize(n);
m = map<K,V>(v.begin(),v.end());