Library: Algorithms
Function
An algorithm that generates successive permutations of a sequence based on an ordering function
#include <algorithm>
namespace std{
template <class BidirectionalIterator>
bool prev_permutation(BidirectionalIterator start,
BidirectionalIterator finish);
template <class BidirectionalIterator, class Compare>
bool prev_permutation(BidirectionalIterator start,
BidirectionalIterator finish,
Compare comp);
}
The permutation-generating algorithms, next_permutation() and prev_permutation(), assume that the set of all permutations of the elements in a sequence is lexicographically sorted with respect to operator<() or the binary predicate comp. For example, if a sequence includes the integers 1 2 3, that sequence has six permutations. In order from first to last, they are: 1 2 3, 1 3 2, 2 1 3, 2 3 1, 3 1 2, and 3 2 1.
The prev_permutation() algorithm takes a sequence defined by the range [start, finish) and transforms it into its previous permutation, if possible. If such a permutation does exist, the algorithm completes the transformation and returns true. If the permutation does not exist, prev_permutation() returns false, and transforms the permutation into its last permutation. prev_permutation() does the transformation according to the lexicographical ordering defined by either operator<() (used in the first version of the algorithm) or the binary predicate comp (which is user-supplied in the second version of the algorithm).
For example, if the sequence defined by [start, finish) contains the integers 1 2 3 (in that order), there is not a previous permutation. Therefore, the algorithm transforms the sequence into its last permutation (3 2 1) and returns false.
At most (finish - start)/2 swaps are performed.
//
// permute.cpp
//
#include <algorithm> // for next_permutation, prev_permutation
#include <functional> // for less
#include <iostream> // for cout, endl
#include <numeric> // for accumulate
#include <vector> // for vector
int main ()
{
typedef std::vector<int, std::allocator<int> > IntVec;
typedef std::vector<char, std::allocator<char> > CharVec;
// Initialize a vector using an array of integers.
const IntVec::value_type a1[] = { 0, 0, 0, 0, 1,
0, 0, 0, 0, 0 };
const CharVec::value_type a2[] = "abcdefghji";
// Create the initial set and copies for permuting.
IntVec m1 (a1, a1 + sizeof a1 / sizeof *a1);
IntVec prev_m1 (10);
IntVec next_m1 (10);
CharVec m2 (a2, a2 + sizeof a2 / sizeof *a2 - 1);
CharVec prev_m2 (10);
CharVec next_m2 (10);
std::copy (m1.begin (), m1.end (), prev_m1.begin ());
std::copy (m1.begin (), m1.end (), next_m1.begin ());
std::copy (m2.begin (), m2.end (), prev_m2.begin ());
std::copy (m2.begin (), m2.end (), next_m2.begin ());
// Create permutations.
typedef std::less<IntVec::value_type> IntLess;
typedef std::less<CharVec::value_type> CharLess;
std::prev_permutation (prev_m1.begin (), prev_m1.end (),
IntLess ());
std::next_permutation (next_m1.begin (), next_m1.end (),
IntLess ());
std::prev_permutation (prev_m2.begin (), prev_m2.end (),
CharLess ());
std::next_permutation (next_m2.begin (), next_m2.end (),
CharLess ());
// Output results.
typedef std::ostream_iterator<IntVec::value_type, char,
std::char_traits<char> >
IntOSIter;
typedef std::ostream_iterator<CharVec::value_type, char,
std::char_traits<char> >
CharOSIter;
std::cout << "Example 1: \n Original values: ";
std::copy (m1.begin (), m1.end (),
IntOSIter (std::cout, " "));
std::cout << "\n Previous permutation: ";
std::copy (prev_m1.begin (), prev_m1.end (),
IntOSIter (std::cout, " "));
std::cout << "\n Next Permutation: ";
std::copy (next_m1.begin (), next_m1.end (),
IntOSIter (std::cout, " "));
std::cout << "\n\nExample 2: \n ";
<< "Original values: ";
std::copy (m2.begin (), m2.end (),
CharOSIter (std::cout, " "));
std::cout << "\n Previous Permutation: ";
std::copy (prev_m2.begin (), prev_m2.end (),
CharOSIter (std::cout, " "));
std::cout << "\n Next Permutation: ";
std::copy (next_m2.begin (), next_m2.end (),
CharOSIter (std::cout, " "));
std::cout << std::endl << std::endl;
return 0;
}
Program Output:
Example 1:
Original values: 0 0 0 0 1 0 0 0 0 0
Previous permutation: 0 0 0 0 0 1 0 0 0 0
Next Permutation: 0 0 0 1 0 0 0 0 0 0
Example 2:
Original values: a b c d e f g h j i
Previous Permutation: a b c d e f g h i j
Next Permutation: a b c d e f g i h j
ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.3.9