std::prev_permutation
<algorithm>
bool prev_permutation( BidirIt first, BidirIt last );
bool prev_permutation( BidirIt first, BidirIt last, Compare comp );
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.
If the type of *first is not Swappable (until C++11)BidirIt is not ValueSwappable (since C++11), the behavior is undefined.
Contents
[edit] Parameters
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.
[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
Given \(\scriptsize N\)N as std::distance (first, last):
[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] 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.
[edit] Example
The following code prints all six permutations of the string "cab" in reverse order.
#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'; }
Output:
cab bca bac acb abc cba
[edit] See also
(function template) [edit]
(algorithm function object)[edit]