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? — Ivaylo Valchev
– Ivaylo Valchev, Commented Jun 3, 2017 at 7:33

Christophe · Accepted Answer · 2017-06-03 07:47:56Z

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.

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