std::prev_permutation

Header: <algorithm>

Transforms the range [first,last) into the previous permutation. Returns true if such permutation exists, otherwise transforms the range into the last permutation (as if by std::sort followed by std::reverse) and returns false.

# Declarations

template< class BidirIt >
bool prev_permutation( BidirIt first, BidirIt last );

(constexpr since C++20)

template< class BidirIt, class Compare >
bool prev_permutation( BidirIt first, BidirIt last, Compare comp );

(constexpr since C++20)

# Parameters

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); 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.

# 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.

# Notes

Averaged over the entire sequence of permutations, typical implementations use about 3 comparisons and 1.5 swaps per call.

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.

# Example

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