swap() function not working on a vector object passed by reference c++

Question

When I try to use swap() in in the function partitionk(), I get the error

" error: request for member 'swap' in '(& num_list)->std::vector<_Tp, _Alloc>::operator[] >(((std::vector::size_type)endn))', which is of non-class type '__gnu_cxx::__alloc_traits >::value_type {aka int}'| " `

enter code here

#include <iostream>
#include <vector>
#include <unordered_map>
#include <cstdlib>
using namespace std;

int partitionk(vector<int>& num_list , int start, int endn  ) {
        int pindex = start;
        int rand_num  = rand( ) % endn;
        num_list[endn].swap(num_list[rand_num]);    // getting error

        for (int i = 1 ; i < endn ; i++){
            if ( num_list[i] < num_list[endn] ){
                num_list[i].swap( num_list[pindex] );   // getting error
                pindex += 1;
            }
        }
        num_list[endn].swap(num_list[pindex]);      // getting error
        return pindex;
}

void quick_sort( vector<int>& num_list , int start,  int endn   ){
       if (start >= endn) return ;

       else{
             int index = partitionk(  num_list ,  start, endn  )   ;

              quick_sort( num_list ,  start, index );
             quick_sort( num_list , index+1,  endn  );

       }
}

int main()
{
    vector <int> nums= {4,7,1,3,9,5};
    quick_sort(nums , 0 , nums.size()-1 );

    for (auto i : nums){
        cout << i << " ";
    }

}

num_list[endn] is an int which doesn't have any member functions, yet you try to call swap on it. What is it you are trying to do? — DeiDei

Christophe Christophe · Accepted Answer · 2017-06-03T07:39:39

Use std::swap():

std::swap(num_list[endn], num_list[num]];

The vector member swap() is meant for swapping entire vectors. And your use of swap() tries to call a swap member of a vector item, that is an int : there's no swap() for this type.

swap() function not working on a vector object passed by reference c++

1 Answers