Upper bound in pair vector c++

Question

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>
#include <utility>

using namespace std;

typedef pair<int,int> Pair;

inline bool less_than_second( const Pair& b1, const Pair& b2 ){
   return b1.second < b2.second;
}

int main()
{
   const int SP1[] = { 2,53,21,55,36,5,1};
   const int EP1[] = { 18, 20, 26, 30, 41,1,5 };
         int i;


   const int num_pairs = sizeof( SP1 ) / sizeof( SP1[0] );
    vector<int> sm(num_pairs);

      // vector <int> SP;

   vector<Pair> pair( num_pairs );
   transform( EP1, EP1+num_pairs, SP1,pair.begin(), make_pair<int,int> );// MAKE PAIR

   sort( pair.begin(), pair.end() );

   sort( pair.begin(), pair.end(), less_than_second );

   vector<Pair>::const_iterator pair_end = pair.end();
    vector<int> SP,EP;
    vector<int>::iterator low,up;

   for( vector<Pair>::const_iterator ptr = pair.begin();ptr != pair_end; ++ptr )
   {

            int SP = ptr->second;
        int EP = ptr->first;

      cout<<"("<<SP<<","<<EP<<")\n";
      } 
   //cout<<"("<<SP<<","<<EP<<")\n";
   low=lower_bound (SP.begin(), SP.end(), 20); 
   up= upper_bound (SP.begin(), SP.end(), 20);

  cout << "lower_bound at position " << int(low- SP.begin()) << endl;
  cout << "upper_bound at position " << int(up - SP.begin()) << endl;


   up= upper_bound (pair.begin(), pair.end(), 20);                


  cout << "upper_bound at position " << int(up - pair.begin()) << endl;

   getchar();
}

I Sorted the pair vector and I am trying to get the values of upper_bound of one the vector in the pair, but it gives me upper_bound at position = 0.

Please bear with me, I am a newbie to c++ and want to learn. Please help to fix this code. Thank you

Answer 1

As far as I can tell, you never put any data into the SP vector. It's possible that instead of int SP = ptr->second; you meant SP.push_back(ptr->second);.

As a side note, since sort isn't stable there's no point in calling sort( pair.begin(), pair.end() ); before you sort it with your predicate.

Finally, you may wish to pick up one of the books in The Definitive C++ Book Guide and List to help you learn the language.