Iterate through different subset of size k

Question

I have an array of n integers (not necessarily distinct!) and I would like to iterate over all subsets of size k. However I\'d like to exclude all duplicate subsets.

Answer 1

The basic idea of this solution is a function like next_permutation but which generates the next ascending sequence of "digits". Here called ascend_ordered.

template< class It >
auto ascend_ordered( const int n_digits, const It begin, const It end )
    -> bool
{
    using R_it = reverse_iterator< It >;
    const R_it r_begin  = R_it( end );
    const R_it r_end    = R_it( begin );

    int max_digit = n_digits - 1;
    for( R_it it = r_begin ; it != r_end; ++it )
    {
        if( *it < max_digit )
        {
            ++*it;
            const int n_further_items = it - r_begin;
            for( It it2 = end - n_further_items; it2 != end; ++it2 )
            {
                *it2 = *(it2 - 1) + 1;
            }
            return true;
        }
        --max_digit;
    }
    return false;
}

Main program for the case at hand:

auto main() -> int
{
    vector a = {1,2,2,3,3,3,3};
    assert( is_sorted( begin( a ), end( a ) ) );
    const int k = 2;
    const int n = a.size();
    vector indices( k );
    iota( indices.begin(), indices.end(), 0 );      // Fill with 0, 1, 2 ...
    set> encountered;
    for( ;; )
    {
        vector current;
        for( int const i : indices ) { current.push_back( a[i] ); }
        if( encountered.count( current ) == 0 )
        {
            cout << "Indices " << indices << " -> values " << current << endl;
            encountered.insert( current );
        }
        if( not ascend_ordered( n, begin( indices ), end( indices ) ) )
        {
            break;
        }
    }
}

Supporting includes and i/o:

#include 
using std::is_sorted;

#include 

#include 
using std::reverse_iterator;

#include 
using std::ostream; using std::cout; using std::endl;

#include 
using std::iota;

#include 
using std::set;

#include 
using std::begin; using std::end;

#include 
using std::vector;

template< class Container, class Enable_if = typename Container::value_type >
auto operator<<( ostream& stream, const Container& c )
    -> ostream&
{
    stream << "{";
    int n_items_outputted = 0;
    for( const int x : c )
    {
        if( n_items_outputted >= 1 ) { stream << ", "; }
        stream << x;
        ++n_items_outputted;
    }
    stream << "}";
    return stream;
}