推回 map <int,vector<char> &gt;* </int,vector<char>

Question

c++

map<int, vector>* maxcounts;

When I have a pointer to map maxcount how do I write this next statement correctly?

maxcounts[-m.second]->push_back(m.first);

without referencing a pointer I write

maxcounts[-m.second].push_back(m.first);

map<int, vector<char>> maxcounts;
for (pair<char, int> m : counts) {
    if (maxcounts.count(-m.second))
        maxcounts[-m.second].push_back(m.first);
    else
       maxcounts.insert({ -m.second, {m.first} });
}

Answer 1

To figure out how to use a pointer to the map, first rewrite your loop this way:

std::map<char, int> counts;
//...
std::map<int, std::vector<char>> maxcounts;
for (std::pair<char, int> m : counts) 
    maxcounts.insert({-m.second, std::vector<char>()}).first->second.push_back(m.first);

Note that the return value for std::map::insert is a std::pair , where the first of the pair is an iterator to the existing item if the item already is in the map, or the iterator to the newly inserted item. Thus you can perform the test and insert in one line without need for an if statement.

The push_back will occur, regardless of whether the item inserted in the map is new or if the item existed. Note that for a new entry, the std::vector being inserted starts as empty.

Given this, the pointer to the map version is very simple:

std::map<char, int> counts;
//...
map<int, vector<char>>* maxcounts;
//
for (pair<char, int> m : counts) 
    maxcounts->insert({-m.second, std::vector<char>()}).first->second.push_back(m.first);

Now, why you need a pointer to a map in the first place is another issue, but to answer your question, the above should work.

Answer 2

I would likely write something like:

std::map<int, std::vector<int>>* maxcounts = ...;
for (std::pair<char, int> m : counts)
  (*maxcounts)[-m.second].push_back(m.first);