Question

How do I use a permuted number outside while loop like to make comparisons.. Because I have checked, vector outside while is same as defined earlier.

void display(vector<int> vec){
    for (int i=0; i<vec.size();i++){
        cout<<vec[i];
    }
}
int main()
{
  vector <int> vec;
  vec.push_back(3);
  vec.push_back(2);
  vec.push_back(1);
  vec.push_back(4);
  sort(vec.begin(), vec.end());
  while (next_permutation( vec.begin(), vec.end() )){
  display(vec);
  cout<<endl;
}

Answer 1

The following complete program works fine both with Visual C++ and with g++:

#include <algorithm>
#include <vector>
#include <iostream>
#include <utility>
using namespace std;

template< class Item >
wostream& operator<<( wostream& stream, vector<Item> const& v )
{
    stream << "[";
    for( auto p = v.begin(); p != v.end();  ++p )
    {
        if( p != v.begin() ) { wcout << ", "; }
        wcout << *p;
    }
    stream << "]";
    return stream;
}

int main()
{
    int const   data[]  = {1, 2, 3, 4};
    vector <int> vec( begin( data ), end( data ) ); 
    sort(vec.begin(),vec.end());
    do
    {
        wcout << vec << endl; 
    } while (next_permutation( vec.begin(), vec.end() ) );
}

The main code is identical to your code (it was copied and pasted, and just edited a teeny tiny little bit to add the data).

So, given that the code works, what exactly is the problem?

---

EDIT: the OP has now updated the question with different code, a while loop instead of a do loop, and the order of the calls to next_permutation and display changed so that it fails to display the original permutation.

The new code given,

void display(vector<int> vec){
    for (int i=0; i<vec.size();i++){
        cout<<vec[i];
    }
}
int main()
{
  vector <int> vec;
  vec.push_back(3);
  vec.push_back(2);
  vec.push_back(1);
  vec.push_back(4);
  sort(vec.begin(), vec.end());
  while (next_permutation( vec.begin(), vec.end() )){
  display(vec);
  cout<<endl;
}

does not compile even after adding the requisite headers and a using namespace std;.

While this is only due to a missing right brace, it would have been avoided by copying and pasting the code from a working version.

That is also much less work to do. 🙂