prev_permutation

  • Article

Reorders the elements in a range so that the original ordering is replaced by the lexicographically previous greater permutation if it exists, where the sense of previous may be specified with a binary predicate.

template<class BidirectionalIterator>
   bool prev_permutation(
      BidirectionalIterator _First, 
      BidirectionalIterator _Last
   );
template<class BidirectionalIterator, class BinaryPredicate>
   bool prev_permutation(
      BidirectionalIterator _First, 
      BidirectionalIterator _Last,
      BinaryPredicate _Comp
   );

Parameters

  • _First
    A bidirectional iterator pointing to the position of the first element in the range to be permuted.

  • _Last
    A bidirectional iterator pointing to the position one past the final element in the range to be permuted.

  • _Comp
    User-defined predicate function object that defines the comparison criterion to be satisfied by successive elements in the ordering. A binary predicate takes two arguments and returns true when satisfied and false when not satisfied.

Return Value

true if the lexicographically previous permutation exists and has replaced the original ordering of the range; otherwise false, in which case the ordering is transformed into the lexicographically largest permutation.

Remarks

The range referenced must be valid; all pointers must be dereferenceable and within the sequence the last position is reachable from the first by incrementation.

The default binary predicate is less than and the elements in the range must be less-than comparable to ensure that the previous permutation is well defined.

The complexity is linear, with at most ( _Last – _First)/2 swaps.

Example

// alg_prev_perm.cpp
// compile with: /EHsc
#include <vector>
#include <deque>
#include <algorithm>
#include <iostream>
#include <ostream>

using namespace std;
class CInt;
ostream& operator<<( ostream& osIn, const CInt& rhs );

class CInt {
public:
   CInt( int n = 0 ) : m_nVal( n ){}
   CInt( const CInt& rhs ) : m_nVal( rhs.m_nVal ){}
   CInt&   operator=( const CInt& rhs ) {m_nVal =
   rhs.m_nVal; return *this;}
   bool operator<( const CInt& rhs ) const
      {return ( m_nVal < rhs.m_nVal );}
   friend ostream& operator<<( ostream& osIn, const CInt& rhs );

private:
   int m_nVal;
};

inline ostream& operator<<( ostream& osIn, const CInt& rhs ) {
   osIn << "CInt( " << rhs.m_nVal << " )";
   return osIn;
}

// Return whether modulus of elem1 is less than modulus of elem2
bool mod_lesser (int elem1, int elem2 ) {
   if ( elem1 < 0 )
      elem1 = - elem1;
   if ( elem2 < 0 )
      elem2 = - elem2;
   return elem1 < elem2;
};

int main() {
   // Reordering the elements of type CInt in a deque
   // using the prev_permutation algorithm
   CInt c1 = 1, c2 = 5, c3 = 10;
   bool deq1Result;
   deque<CInt> deq1, deq2, deq3;
   deque<CInt>::iterator d1_Iter;

   deq1.push_back ( c1 );
   deq1.push_back ( c2 );
   deq1.push_back ( c3 );

   cout << "The original deque of CInts is deq1 = (";
   for ( d1_Iter = deq1.begin( ); d1_Iter != --deq1.end( ); d1_Iter++ )
      cout << " " << *d1_Iter << ",";
   d1_Iter = --deq1.end( );
   cout << " " << *d1_Iter << " )." << endl;

   deq1Result = prev_permutation ( deq1.begin ( ) , deq1.end ( ) );

   if ( deq1Result )
      cout << "The lexicographically previous permutation "
           << "exists and has \nreplaced the original "
           << "ordering of the sequence in deq1." << endl;
   else
      cout << "The lexicographically previous permutation doesn't "
           << "exist\n and the lexicographically "
           << "smallest permutation\n has replaced the "
           << "original ordering of the sequence in deq1." << endl;

   cout << "After one application of prev_permutation,\n deq1 = (";
   for ( d1_Iter = deq1.begin( ); d1_Iter != --deq1.end( ); d1_Iter++ )
      cout << " " << *d1_Iter << ",";
   d1_Iter = --deq1.end( );
   cout << " " << *d1_Iter << " )." << endl << endl;

   // Permutating vector elements with binary function mod_lesser
   vector <int> v1;
   vector <int>::iterator Iter1;

   int i;
   for ( i = -3 ; i <= 3 ; i++ )
      v1.push_back( i );

   cout << "Vector v1 is ( " ;
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
      cout << *Iter1 << " ";
   cout << ")." << endl;

   prev_permutation ( v1.begin ( ) , v1.end ( ) , mod_lesser );

   cout << "After the first prev_permutation, vector v1 is:\n v1 = ( " ;
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
      cout << *Iter1 << " ";
   cout << ")." << endl;

   int iii = 1;
   while ( iii <= 5 ) {
      prev_permutation ( v1.begin ( ) , v1.end ( ) , mod_lesser );
      cout << "After another prev_permutation of vector v1,\n v1 =   ( " ;
      for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ;Iter1 ++ )
         cout << *Iter1  << " ";
      cout << ")." << endl;
      iii++;
   }
}

The original deque of CInts is deq1 = ( CInt( 1 ), CInt( 5 ), CInt( 10 ) ).
The lexicographically previous permutation doesn't exist
 and the lexicographically smallest permutation
 has replaced the original ordering of the sequence in deq1.
After one application of prev_permutation,
 deq1 = ( CInt( 10 ), CInt( 5 ), CInt( 1 ) ).

Vector v1 is ( -3 -2 -1 0 1 2 3 ).
After the first prev_permutation, vector v1 is:
 v1 = ( -3 -2 0 3 2 1 -1 ).
After another prev_permutation of vector v1,
 v1 =   ( -3 -2 0 3 -1 2 1 ).
After another prev_permutation of vector v1,
 v1 =   ( -3 -2 0 3 -1 1 2 ).
After another prev_permutation of vector v1,
 v1 =   ( -3 -2 0 2 3 1 -1 ).
After another prev_permutation of vector v1,
 v1 =   ( -3 -2 0 2 -1 3 1 ).
After another prev_permutation of vector v1,
 v1 =   ( -3 -2 0 2 -1 1 3 ).

Requirements

Header: <algorithm>

Namespace: std

See Also

Reference

prev_permutation (STL Samples)

Standard Template Library