prev_permutation (STL Sample)

The sample code below illustrates how to use the prev_permutation STL function in Visual C++.

Required Header:
<algorithm>

template<class BidirectionalIterator> inline
     bool prev_permutation(BidirectionalIterator first,
     BidirectionalIterator last)

Note: The class/parameter names in the prototype do not match the version in the header file. Some have been modified to improve readability.

Description:
The prev_permutation algorithm changes the order of the elements the range [first, last), to the previous lexicographic permutation and returns true. If there is no prev_permutation, it arranges the sequence to be the first permutation and returns false. IMPORTANT: The prev_permutation algorithm assumes the sequence is sorted in descending order using operator<. The non-predicate version uses the operator< to order the permutations.

Sample Code:

//////////////////////////////////////////////////////////////////////
//
// Compile options needed: /GX
//
// prev_permutation.cpp : Illustrates how to use the prev_permutation
//                        function.
//
// Functions:
//
//    prev_permutation : Change the order of the sequence to the
//                       previous lexicographic permutation.
//////////////////////////////////////////////////////////////////////

// disable warning C4786: symbol greater than 255 character,
// okay to ignore
#pragma warning(disable: 4786)

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <functional>

using namespace std ;

void main()
{
    const int VECTOR_SIZE = 3 ;

    // Define a template class vector of strings
    typedef vector<string> StrVector ;

    //Define an iterator for template class vector of strings
    typedef StrVector::iterator StrVectorIt ;

    //Define an ostream iterator for strings
    typedef ostream_iterator<string>
    StrOstreamIt;

    StrVector Pattern(VECTOR_SIZE) ;

    StrVectorIt start, end, it ;

    StrOstreamIt outIt(cout, " ") ;

    start = Pattern.begin() ;   // location of first
                                      // element of Pattern

    end = Pattern.end() ;       // one past the location last
                                       // element of Pattern

    //Initialize vector Pattern
    Pattern[0] = "C" ;
    Pattern[1] = "B" ;
    Pattern[2] = "A" ;

    // print content of Pattern
    cout << "Before calling prev_permutation...\n" << "Pattern: " ;
    for(it = start; it != end; it++)
        cout << *it << " " ;
    cout << "\n\n" ;

    // Generate all possible permutations

    cout << "After calling prev_permutation...." << endl ;
    while ( prev_permutation(start, end) )
    {
        copy(start, end, outIt) ;
        cout << endl ;
    }
}

 

Program Output is:

Before calling prev_permutation...
Pattern: C B A

After calling prev_permutation....
C A B
B C A
B A C
A C B
A B C