std::prev_permutation

Defined in header `<algorithm>`
template< class BidirIt > bool prev_permutation( BidirIt first, BidirIt last);	(1)
template< class BidirIt, class Compare > bool prev_permutation( BidirIt first, BidirIt last, Compare comp);	(2)

Transforms the range [first, last) into the previous permutation from the set of all permutations that are lexicographically ordered with respect to operator< or comp. Returns true if such permutation exists, otherwise transforms the range into the last permutation (as if by std::sort(first, last); std::reverse(first, last);) and returns false.

first, last	-	the range of elements to permute
comp	-	comparison function object (i.e. an object that satisfies the requirements of `Compare`) which returns true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function object must not modify the objects passed to it. The types Type1 and Type2 must be such that an object of type BidirIt can be dereferenced and then implicitly converted to both of them.
Type requirements
- `BidirIt` must meet the requirements of `ValueSwappable` and `BidirectionalIterator`.

[edit] Return value

true if the new permutation precedes the old in lexicographical order. false if the first permutation was reached and the range was reset to the last permutation.

[edit] Exceptions

Any exceptions thrown from iterator operations or the element swap.

[edit] Complexity

At most (last-first)/2 swaps.

[edit] Possible implementation

template<class BidirIt>
bool prev_permutation(BidirIt first, BidirIt last)
{
    if (first == last) return false;
    BidirIt i = last;
    if (first == --i) return false;
 
    while (1) {
        BidirIt i1, i2;
 
        i1 = i;
        if (*i1 < *--i) {
            i2 = last;
            while (!(*--i2 < *i))
                ;
            std::iter_swap(i, i2);
            std::reverse(i1, last);
            return true;
        }
        if (i == first) {
            std::reverse(first, last);
            return false;
        }
    }
}

[edit] Example

The following code prints all six permutations of the string "abc" in reverse order

Run this code

#include <algorithm>
#include <string>
#include <iostream>
#include <functional>
int main()
{
    std::string s="abc";
    std::sort(s.begin(), s.end(), std::greater<char>());
    do {
        std::cout << s << ' ';
    } while(std::prev_permutation(s.begin(), s.end()));
    std::cout << '\n';
}

Output:

cba cab bca bac acb abc

[edit] See also

is_permutation (C++11)	determines if a sequence is a permutation of another sequence (function template)
prev_permutation	generates the next smaller lexicographic permutation of a range of elements (function template)

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Technical Specifications