使用std :: sort排序集

Question

I would like to know if we can sort a pre created set. When I first create the set s_p2, I sort using a different element point.getLength(). but after user input i would like to sort the items according to the x value point.getX(). How i do this ?

It seems like set container does not have a sort function. And i am advised to use vector. But sets are able to store unique elements only.

Q1: How can i sort a set depending on the criteria

Q2: If set is unable to do this than which STL container is the best choice and how can i sort the elements in the container.

Answer 1

You cannot resort a set , how it sorts is part of the type of the particular set . A given set has a fixed set order that cannot be changed.

You could create a new set with the same data relatively easily. Just create a new set that sorts based on the new criteria.

If you want to use the two set s in the same code, you'll have to abstract the access to the underlying set .

Now, if you are doing rare reads and modifications, using a vector that you sort manually is often a better idea. You can remove duplicates by using the std::unique - erase idiom.

Answer 2

std::set stores its members in a sorted fashion. If you walk through the set from .begin() to .end() , you will have a sorted list of items.

If you don't like the default sort criteria, you may supply a 2nd template parameter to std::set<>

Answer 3

You can have two sets and keep them in sync or copy one to the other.

#include <iostream>
#include <set>

using namespace std;

struct AB
{
   AB(int a,int b) : _a(a),_b(b) {}

   int _a;
   int _b;
};

struct byA
{
    bool operator () (const AB& lhs, const AB& rhs)
    {
          return lhs._a <= rhs._a;
    }
}; 

struct byB
{
    bool operator () (const AB& lhs, const AB& rhs)
    {
       return lhs._b <= rhs._b;
    }
}; 

typedef set<AB,byA> ByA;
typedef set<AB,byB> ByB;
typedef ByA::const_iterator ByAIt;
typedef ByB::const_iterator ByBIt;

void getByB(const ByA &sA,ByB &sB)
{
   for(ByAIt iter=sA.begin(); iter!=sA.end();++iter) {
      const AB &ab=*iter;
      sB.insert(ab);
   }
}

int main(int argc, const char **argv)
{
   ByA sA;
   sA.insert(AB(3,6));
   sA.insert(AB(1,8));
   sA.insert(AB(2,7));

   ByB sB;
   getByB(sA,sB);

   cout << "ByA:" << endl;
   for(ByAIt iter=sA.begin(); iter!=sA.end();++iter) {
      const AB &ab=*iter;
      cout << ab._a << "," << ab._b << " ";
   }
   cout << endl << endl;

   cout << "ByB:" << endl;
   for(ByBIt iter=sB.begin(); iter!=sB.end();++iter) {
      const AB &ab=*iter;
      cout << ab._a << "," << ab._b << " ";
   }
   cout << endl;
   return 0;
}

program returns: ByA 1,8 2,7 3,6

ByB: 3,6 2,7 1,8