How to sort a pair of vector in some given range

Question

I know how to sort pair of vectors using std:sort(v.begin(), v.end()); But this will sort all the pairs of the vector. But I want to sort the vector in a specified range.

#include<bits/stdc++.h>
#define ll long long

using namespace std;

bool sortbysecdesc(const pair<int,int> &a,const pair<int,int> &b)
{
       return a.second>b.second;
}


int main(){
    int n;
    cin>>n;
    
    ll a[n];
    ll b[n];
    
    vector<pair<ll, ll>> v;
    vector<pair<ll, ll>>::iterator itr;

    for (int i = 0; i < n; i++) {
        cin>>a[i];
    }
    for (int i = 0; i < n; i++) {
        cin>>b[i];
    }
    
    for (int i = 0; i < n; i++) {
        v.push_back(make_pair(a[i],b[i]));
    }
    
    //sorting the pair in descending order with second element
    sort(v.begin(), v.end(), sortbysecdesc);
    
 
    for (itr=v.begin(); itr!=v.end(); itr++) {
        
        /* Logic to be implement */
        
    }
    return 0;
}

Code explanation- Here I'm taking input 2 arrays and making a pair of vector by combining first_array[i] element with second_element[i] element. Such as - first_array = {1, 5, 4, 9} and second_array = {2, 10, 5, 7} then the vector of pair's will be of size = 2*size_of(first/second array) and elements will be [{1, 2}, {5, 10}, {4, 5}, {9, 7}] . Then I'm sorting these pair in descending order according to second elements of pairs ie, all elements of second array.

Now I wanna sort the vector form where second elements are equal.

eg - if the vector is [{1,6} , {4,5}, {3,5}, {8,5}, {1,1}] . Here 5 occurs three time in second element of pair so sort their first index in ascending order. Expected result - [{1,6} , {3,5}, {4,5}, {8,5}, {1,1}]

Note - We had already sorted the second element in descending order.

Answer 1

You have to update your sorting function. Try this.

bool sortbysecdesc(const pair<int,int> &a,const pair<int,int> &b)
{
       return a.second == b.second ? a.first < b.first : a.second > b.second;
}

Demo

Answer 2

What you need is something like the following

#include <tuple>
#include <vector>
#include <iterator>
#include <algorithm>

//...

std::sort( std::begin( v ), std::end( v ),
           []( const auto &p1, const auto &p2 )
           {
               return std::tie( p2.second, p1.first ) < std::tie( p1.second, p2.first );
           } ); 

Here is a demonstration program.

#include <iostream>
#include <tuple>
#include <vector>
#include <iterator>
#include <algorithm>

int main() 
{
    std::vector<std::pair<long long, long long>> v =
    {
        {1,6} , {4,5}, {3,5}, {8,5}, {1,1}
    };
    
    std::sort( std::begin( v ), std::end( v ),
               []( const auto &p1, const auto &p2 )
               {
                    return std::tie( p2.second, p1.first ) <
                           std::tie( p1.second, p2.first );
               } );
               
    for ( const auto &p : v )
    {
        std::cout << "{ " << p.first << ", " << p.second << " } ";
    }
    
    std::cout << '\n';
    
               
    return 0;
}

The program output is

{ 1, 6 } { 3, 5 } { 4, 5 } { 8, 5 } { 1, 1 }