Get value from iterator.first

Question

I've started a little toy project as a way to teach myself C++ and have hit a wall. In the code below the highlighted line is causing a build error saying:

welcome.cc:65:26: error: cannot convert 'std::_Rb_tree_iterator >' to 'char' in assignment current = result.first;

and I'm not sure why. The goal is to create a Markov chain for English placenames, the intent of the line in question is meant to update the current letter in the generated placename with a randomly selected value of the previous one, my random selection method is taken from this question. As I say, new to C++ and no idea what I've done wrong.

    int main(int argc, char**argv) {
    string line;
    ifstream myfile;
    std::multimap<char, char> m;
    myfile.open ("C:\\Users\\james\\Desktop\\placenames.txt");
    if (myfile.is_open()){
        while ( getline (myfile,line) )
        {
            for(std::string::size_type i = 0; i < line.size(); ++i) {
                std::cout<<line[i]<<std::endl;
                m.insert(std::pair<char, char>(line[i], line[i+1]));
            }
        }
    for (std::multimap<char, char>::iterator it = m.begin();it != m.end(); ++it)
        cout << "  [" << (*it).first << ", " << (*it).second << "]" << endl;
    myfile.close();
//    for( auto it = m.begin(), end = m.end();it != end; it = m.upper_bound(it->first))
//        cout << (*it).first << ' ' << m.count( (*it).first) << endl;
    auto current = 'A';
    std::string name = "A";
    typedef std::multimap<const char, char>::iterator MMAPIterator;
    for (int j=0; j<8; ++j){
        std::pair<MMAPIterator, MMAPIterator> result = m.equal_range(current);
        std::size_t sz = std::distance(result.first, result.second);
        std::size_t idx = std::rand() % sz;
        std::advance(result.first, idx);

        current = result.first; <------THIS LINE

       //name+=current;
    }
    cout << name;
  }
  else cout << "Unable to open file"; 

  return 0;  
}

---

I'm aware its a pretty naive implementation of a markov chain, btw. As I say, intent was to play around with c++ rather than anything else.

Answer 1

You have a dual layer of pairs here, firstly the equal_range function returns a range, so your

std::size_t sz = std::distance(result.first, result.second);

Tells you how many items are in the range, to get an element of that range (your pair of chars) you need to

auto random_element = result.first;
auto the_const_char = random_element.first;
auto the_non_const_char = random_element.second;