Question

If I have a structure like

std::map<string, int> myMap;
myMap["banana"] = 1;
myMap["apple"] = 1;
myMap["orange"] = 1;

How can I access myMap[0]?

I know that the map sorts internally and I’m fine with this, I want to get a value in the map by index. I’ve tried myMap[0] but I get the error:

Error   1   error C2679: binary '[' : no operator found which takes a right-hand operand of type 'int' (or there is no acceptable conversion)   

I realise I could do something like this:

string getKeyAtIndex (map<string, int>& myMap, int index){
    map<string, int>::const_iterator end = myMap.end(); 

    int counter = -1;
    for (map<string, int>::const_iterator it = myMap.begin(); it != end; ++it) {
        counter++;
        
        if (counter == index)
            return it->first;
    }
}

But surely this is hugely inefficient? Is there a better way?

Answer 1

Your map is not supposed to be accessed that way, it’s indexed by keys not by positions. A map iterator is bidirectional, just like a list, so the function you are using is no more inefficient than accessing a list by position. If you want random access by position then use a vector or a deque.

Your function could be written with help from std::advance(iter, index) starting from begin():

auto it = myMap.begin();
std::advance(it, index);
return it->first;