std::prev_permutation

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Defined in header <algorithm>

template< class BidirectionalIterator >
bool prev_permutation( BidirectionalIterator first, BidirectionalIterator last);
(1)
template< class BidirectionalIterator >
bool prev_permutation( BidirectionalIterator first, BidirectionalIterator 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.

Contents

[edit] Parameters

first, last - the range of elements to permute
comp - comparison function 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 must not modify the objects passed to it.
The types Type1 and Type2 must be such that an object of type RandomAccessIterator can be dereferenced and then implicitly converted to both of them. ​

[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] Complexity

At most (last-first)/2 swaps.

[edit] Equivalent function

[edit] Example

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

#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)