C++ Tutorial/STL Algorithms Helper/prev permutation

Illustrating the generic prev_permutation algorithms

<source lang="cpp">#include <iostream>

1. include <cassert>
2. include <algorithm>
3. include <vector>

using namespace std; int main() {

 vector<int> vector1(3);
 for (int i = 0; i < 3; ++i) 
    vector1[i] = i;
 // In lexicographical order the permutations of 0 1 2 are
 // 0 1 2, 0 2 1, 1 0 2, 1 2 0, 2 0 1, 2 1 0. 
 // Show that from 0 1 2 next_permutation produces 0 2 1:
 next_permutation(vector1.begin(), vector1.end());
 cout << vector1[0] << " ";
 cout << vector1[1] << " ";
 cout << vector1[2] << " ";

 cout << "\n\n\n\n\n\n";
 // Show that from 0 2 1 prev_permutation() produces 0 1 2:
 prev_permutation(vector1.begin(), vector1.end());
 cout << vector1[0] << " ";
 cout << vector1[1] << " ";
 cout << vector1[2] << " ";
 
 return 0;

}</source>

0 2 1


0 1 2

Use prev_permutation to permute elements until they are sorted in descending order

<source lang="cpp">/* The following code example is taken from the book

* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/

1. include <iostream>
2. include <vector>
3. include <deque>
4. include <list>
5. include <set>
6. include <map>
7. include <string>
8. include <algorithm>
9. include <iterator>
10. include <functional>
11. include <numeric>

/* PRINT_ELEMENTS()

* - prints optional C-string optcstr followed by
* - all elements of the collection coll
* - separated by spaces
*/

template <class T> inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="") {

   typename T::const_iterator pos;
   std::cout << optcstr;
   for (pos=coll.begin(); pos!=coll.end(); ++pos) {
       std::cout << *pos << " ";
   }
   std::cout << std::endl;

} /* INSERT_ELEMENTS (collection, first, last)

* - fill values from first to last into the collection
* - NOTE: NO half-open range
*/

template <class T> inline void INSERT_ELEMENTS (T& coll, int first, int last) {

   for (int i=first; i<=last; ++i) {
       coll.insert(coll.end(),i);
   }

} using namespace std; int main() {

   vector<int> coll;
   INSERT_ELEMENTS(coll,1,3);
   PRINT_ELEMENTS(coll,"on entry:  ");
   /* permute elements until they are sorted in descending order
    * - runs through all permutations because the elements are sorted
    *    in descending order now
    */
   while (prev_permutation(coll.begin(),coll.end())) {
       PRINT_ELEMENTS(coll," ");
   }
   PRINT_ELEMENTS(coll,"afterward: ");

}</source>

on entry:  1 2 3
afterward: 3 2 1

Use prev_permutation to permute until descending sorted

<source lang="cpp">/* The following code example is taken from the book

* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/

1. include <iostream>
2. include <vector>
3. include <deque>
4. include <list>
5. include <set>
6. include <map>
7. include <string>
8. include <algorithm>
9. include <iterator>
10. include <functional>
11. include <numeric>

/* PRINT_ELEMENTS()

* - prints optional C-string optcstr followed by
* - all elements of the collection coll
* - separated by spaces
*/

template <class T> inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="") {

   typename T::const_iterator pos;
   std::cout << optcstr;
   for (pos=coll.begin(); pos!=coll.end(); ++pos) {
       std::cout << *pos << " ";
   }
   std::cout << std::endl;

} /* INSERT_ELEMENTS (collection, first, last)

* - fill values from first to last into the collection
* - NOTE: NO half-open range
*/

template <class T> inline void INSERT_ELEMENTS (T& coll, int first, int last) {

   for (int i=first; i<=last; ++i) {
       coll.insert(coll.end(),i);
   }

} using namespace std; int main() {

   vector<int> coll;
   INSERT_ELEMENTS(coll,1,3);
   PRINT_ELEMENTS(coll,"on entry:  ");
   /* permute until descending sorted
    * - this is the next permutation after ascending sorting
    * - so the loop ends immediately
    */
   while (prev_permutation(coll.begin(),coll.end())) {
       PRINT_ELEMENTS(coll," ");
   }
   PRINT_ELEMENTS(coll,"now:       ");

}</source>

on entry:  1 2 3
now:       3 2 1