CODEWITHSUNDEEP
c++
user1047092
user1047092

Reputation: 1481

error using c++ set container

Hi all I am a newbie to c++. After compiling this program, I get an error msg saying .

assign3_3.cpp:120:9: error: could not convert 'sPair' from 'std::set<pairT, clas
scomp>' to 'std::set<pairT>'

here is my code.

#include <set>
#include <string>
#include <iostream>
using namespace std;


struct pairT
{
  string first, second;
};

struct classcomp
{
bool operator() (const pairT &lhs, const pairT &rhs) const
{
    if (lhs.first == rhs.first && lhs.second == rhs.second)
    {
        return 0;
    }
    else if (lhs.first < rhs.first)
    {
        return -1;
    }
    else if (lhs.first == rhs.first && lhs.second < rhs.second)
    {
        return -1;
    }
    else
    {
        return 1;
    }
  }
};

set<pairT> CartesianProduct(set<string> & one, set<string> & two);

int main()
{

   string A = "ABC";
   string B = "XY";
   set<string> sA, sB;
   sA.insert(&A[0]);
   sA.insert(&A[1]);
   sA.insert(&A[2]);
   sA.insert(&B[0]);
   sA.insert(&B[1]);
   set<pairT> pT = CartesianProduct(sA, sB);
   //for (set<pairT>::iterator it = pT.begin(); it != pT.end(); it++)
   //   cout << pT.find(it).first << pT.find(it).second << endl;

   return 0;
}


set<pairT> CartesianProduct(set<string> &one, set<string> &two)
{
   set<string>::iterator itA, itB;
   pairT pT;
   set<pairT, classcomp> sPair;

for (itA = one.begin(); itA != one.end(); itA++)
{
    //cout << *itA << endl;
    for(itB = two.begin(); itB != two.end(); itB++)
    {
        pT.first = *itA;
        pT.second = *itB;
        sPair.insert(pT);
    }
}
return sPair;
}

First, I am not understanding about the making a comparison function for pairT. If this is the case here, please explain. I am having a trouble using set container please help thanks and merry christmas !

Upvotes: 1

Views: 263

Answers (2)

Blastfurnace
Blastfurnace

Reputation: 18652

In addition to what Kerrek SB said, your comparison function isn't correct.

The comparator required by std::set<std::pair> needs to follow the following logic:

if (lhs.first < rhs.first)
    return true;
else if (lhs.first == rhs.first && lhs.second < rhs.second)
    return true;
else
    return false;

This can be expressed more compactly as:

return lhs.first < rhs.first ||
     !(rhs.first < lhs.first) && lhs.second < rhs.second;

Fortunately, this is how std::pair::operator< is defined in the standard library. When you create a std::set<std::pair> this operator will be used by default so you don't have to provide your own.

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477454

The comparator is part of the type. You must say set<pairT, classcomp> everywhere. Best to use a typedef.

Upvotes: 3

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