several functors for redefining order for sort function of vector

Question

Hello!

I have the following data structure:

typedef vector< vector<int> > MxInt2d;
typedef vector< vector<double> > MxDouble2d;

 class QSweep{   
 public:
 ....
 static MxDouble2d myPoints_;
 MxInt2d myEdges_;
 MxInt2d sweepEvents;

 class order{
 public:
    bool operator() (const vector<int>& edge1, const vector<int>& edge2){
            return (myPoints_[edge1[0]][0]<myPoints_[edge2[0]][0])|| 
                       (myPoints_[edge1[0]][0]==myPoints_[edge2[0]][0]&& 
                        myPoints_[edge1[0]][1]<myPoints_[edge2[0]][1]) 
                   ||
                    (myPoints_[edge1[0]][0]==myPoints_[edge2[0]][0]&& 
                         myPoints_[edge1[0]][1]==myPoints_[edge2[0]][1]&& 
                     getSlope(myPoints_[edge1[0]][0],myPoints_[edge1[0][1],  
                                  myPoints_[edge1[1]][0],myPoints_[edge1[1]][1])
                     <
                         getSlope(myPoints_[edge2[0][0],myPoints_[edge2[0][1],    
                                  myPoints_[edge2[1]][0],myPoints_[edge2[1]][1]));
                    }
};
static double getSlope(double a, double b, double c, double d);
static double computeDet(double a, double b, double c, double d, double x, double y);

};

I use the functor for order() when defining the constructor in the following way:

 QSweep::QSweep(const MxDouble2d& myPoints, const MxInt2d& myEdges){ 
....
    //code here for initializing myPoints_, myEdges_
sort(myEdges_.begin(),myEdges_.end(),order());
 }

In this way my data myEdges_ are arranged when initialized using the functor order(). Now I want to arrange the data sweepEvents (which has the same type as myEdges_ using the predefined data type from C++ vector) using a totally different criteria, that is I do not want to use the function getSlope(...) but the function computeDet(...) for sweepEvents. So I was thinking I still need other functor for redefining the < of sort() for the vector data type right? So I would have to write a new functor order1() in which I use the data computed with computeDet(...) and then I call the sort on my sweepEvents data types:

 sort(sweepEvents.begin(),sweepEvents.end(),order1());

I am not sure if this is the good solution? can I redefine in several ways < if I use different names for the functor? Is someone has some suggestions I would be really grateful. Thank you in advance, madalina

No the two functors does not use any common code. I have written the second functor insede the QSweep class as this:

 class orderDet{
public:
 bool operator() (const vector<int>& edgeSW1, const vector<int>& edgeSW2   
                              ,const vector<int>& edgeC){
  return  
          (
          computeDet(myPoints_[edgeSW1[0]],myPoints_[edgeSW1[1]],myPoints_[edgeC[0]]) 
           < 
          computeDet(myPoints_[edgeSW2[0]],myPoints_[edgeSW2[1]],myPoints_[edgeC[0]])
           );
  }}

and I called it like:

 sort(sweepEvents.begin(), sweepEvents.end(), orderDet());

But I got the following compilation error:

 error: no match for call to '(QSweepComplete::orderDet) (std::vector<int,  
 std::allocator<int> >&, std::vector<int, std::allocator<int> >&)'
 ./QSweepComplete.h:68: note: candidates are: bool 
 QSweepComplete::orderDet::operator()(const std::vector<int, std::allocator<int>>&,   
 const std::vector<int, std::allocator<int> >&, const std::vector<int, 
 std::allocator<int> >&)

I am guessing the parameter do not match, as the third parameter of orderDet(...., edgeC) is not part of sweepEvents list as the others are but is part of the variable myEdges_... Maybe one can give me some suggestions on how to implement this functor?

thank you in advance. best wishes, madalina

Answer 1

If you have 2 different criteria for sorting then you should have 2 different functors.

Give them meaningful names such as:

OrderBySlopes and OrderByXYZ

If the 2 functors use some common code, you can refactor this out. i.e. into a base class

Answer 2

What Glen said.

If you really really insist on having one function do double duty, you can rely on having different parameter types to overload two versions of operator().

Answer 3

Your orderDet::operator() takes three arguments. The std::sort wants only two to compare two values.

A solution may be to add the edges vector as a member of your functor:

struct orderDet {
    const vector<int>& edges;
    orderDet( const vector<int>& edges ):edges(edges){};

    bool operator()( const vector<int>& v1, const vector<int>& v2 ) const {
       ....
    }
 };

By the way, if you want people to understand your code when they read it, I rather think you should use a typedef for your inner vector than for the outer one. Next to that, it's better to define a struct containing a 'first' and 'second' point, than a very generic vector which you use only the first two points of. It will make your code more readable, too.

Regards.