Sort a map on the basis of first value of pair

Question

Suppose I have to describe my map as:

map<int, pair<long, int>> mp;

Now I insert elements as:

int y; long x;
pair<long, int> p;

for(int i = 0; i < 5; i++)
{ 
    cin >> x >> y;
    p.first = x;
    p.second = y;
    mp.insert({i, p});   // What is wrong here syntax wise?
}

Further, I want to sort it on the basis of first value of the pair.

Answer 1

You can use a little trick here.

Map in c++ automatically sorts everything by key, so you can do following =>

map <long, (set,vector) < int > > mp; //Create this kind of map
//it will sort elements by first value and depending on your needs, select vector or set
//if you need to sort elements by second value use set
//if you do not care about it use vector

long x;
int y;
for (int i = 0; i < n; i++)
{
   cin >> x >> y;
   if (mp.find(x) != mp.end()) // if element exist
      {
         mp[x].push_back(y); // add it to vector
      }
      else
      {
         mp[x] = vector < int > (); // if not create new vector
         mp[x].push_back(y); // and then push new element
      }
}