Is it possible to to have a custom comparator function for priority queue which keeps on changing. If not is their any way to implement this functionality. Ex:
#define fi first
#define se second
int X,Y;
struct comp
{
bool operator()(pair<int,int>& A, pair<int,int> & B)
{
int x1=A.fi,y1=A.se,x2=B.fi,y2=B.se;
if((abs(X-x1)+abs(Y-y1))>(abs(X-x2)+abs(Y-y2)))
{
return false;
}
else
return true;
}
};
////in MAin
X=1,Y=1;
priority_queue<pair<int,int>, vector<pair<int,int> >,comp>PQ;
for(int i=1;i<=N;i++)
{
for(int j=1;j<=M;j++)
{
if(i==1 && j==1) continue;
PQ.push(m_p(i,j));
}
}
cout<<1<<" "<<1<<endl;
while(!PQ.empty())
{
int x=PQ.top().fi;
int y=PQ.top().se;
cout<<x<<" "<<y<<endl;
PQ.pop();
X=x;Y=y;
}
as we keep on changing X,Y, our PQ will change itself accordingly?
No.
priority_queue does not work like this, but you can implement the functionality based on set and/or multiset.
The update operation is an erase followed by an insert.
In fact you need to keep the function's result stable all time , or STL will give you an unpredicted error.
priority_queueprovides constant lookup for the highest priority element and logarithmic time for insertion. To guarantee those time complexities, it maintains a data structure (usually binary heap) with all the elements in some order. You can see, how by definition it's not possible to do what you are asking. As whenever you change the comparator function, the already maintained order of elements is not useful anymore. So after changing the comparator, it's the same as finding the highest value from some new set of elements.