NAME

std::prev_permutation - std::prev_permutation

Synopsis

Defined in header <algorithm>
template< class BidirIt >
bool prev_permutation( BidirIt first, BidirIt (until C++20)
last );
template< class BidirIt >
constexpr bool prev_permutation( BidirIt first, (since C++20)
BidirIt last ); (1)
template< class BidirIt, class Compare >
bool prev_permutation( BidirIt first, BidirIt (until C++20)
last, Compare comp ); (2)
template< class BidirIt, class Compare >
constexpr bool prev_permutation( BidirIt first, (since C++20)
BidirIt last, Compare comp );

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.

Parameters

first, last - the range of elements to permute
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);
comp -
While the signature does not need to have const &, the function must
not modify the objects passed to it and must be able to accept all
values of type (possibly const) Type1 and Type2 regardless of value
category (thus, Type1 & is not allowed
, nor is Type1 unless for Type1 a move is equivalent to a copy
(since C++11)).
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
LegacyBidirectionalIterator.

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.

Exceptions

Any exceptions thrown from iterator operations or the element swap.

Complexity

At most (last-first)/2 swaps. Averaged over the entire sequence of permutations,
typical implementations use about 3 comparisons and 1.5 swaps per call.

Notes

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type
satisfies LegacyContiguousIterator and swapping its value type calls neither
non-trivial special member function nor ADL-found swap.

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;
}
}
}

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