CODEWITHSUNDEEP
c++sortingobjectvectoroverloading
user3437460
user3437460

Reputation: 17454

Sort vector of objects with multiple sorting criteria

class Point2D
{
    friend ostream& operator<< (ostream&, const Point2D&);
    protected:
            int x;         //can sort by x
            int y;         //can sort by y
            double dist;   //can sort by dist

    public:  
            Point2D () {x=0; y=0;}              
            Point2D (int a, int b) {x=a; y=b;}      
            void setX (int);
            void setY (int);
            void setDist();
            double getDist() const;     
            int getX ();
            int getY ();

}

int main()
{
    vector<Point2D> vect;
    sort (vect.begin(), vect.end());

    //Print all vector elements
    for (int x=0; x<vect.size(); x++)
        cout <<  vect[x] << endl;      
}

I am trying to sort vector of objects using sort.

But when I run my codes above, I get plenty of repeating errors saying:

instantiated from here - sort (vect.begin(), vect.end());

I want to be able to sort by either x, y or dist. I guess I probably need to overload the > or == operators in order for me to use the sort provided by C++ std library?

How would the code for overloading look like? I know how to overload ostream operators like << to display data, but in this complicated situation, how do we do the overloading in order to allow us to use sort?

Upvotes: 2

Views: 1183

Answers (1)

Dimitrios Bouzas
Dimitrios Bouzas

Reputation: 42909

If your compiler supports C++11, You could do something like:

vector<Point2D> vect;
// sort by x
sort (vect.begin(), vect.end(), [](Point2D const &a, Point2D const &b) { return a.getX() < b.getX(); });
// sort by y
sort (vect.begin(), vect.end(), [](Point2D const &a, Point2D const &b) { return a.getY() < b.getY(); });

Note, for the above example to work you have either to define your member functions getX and getY as consts or remove the const qualifiers from the input arguments of the lambdas.

If your compiler doesn't support C++11 then you can define comparables like below:

bool compare_x(Point2D const &a, Point2D const &b)
{
  return a.getX() < b.getX();
}

bool compare_y(Point2D const &a, Point2D const &b)
{
  return a.getY() < b.getY();
}

and call sort like below:

  // sort by x
  sort(vect.begin(), vect.end(), compare_x);
  // sort by y
  sort(vect.begin(), vect.end(), compare_y);

Upvotes: 5

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