From cppreference.com
Defined in header
template<classBidirIt>boolprev_permutation(BidirItfirst,BidirItlast); (1) (constexpr since C++20)template<classBidirIt,classCompare>boolprev_permutation(BidirItfirst,BidirItlast,Comparecomp); (2) (constexpr since C++20)Transforms the range [first, last) into the previous
. Returns true if such permutation exists, otherwise transforms the range into the last permutation (a descending order) and returns false.
1) The set of all permutations is ordered lexicographically with respect to operator<(until C++20)std::less{}(since C++20).
2) The set of all permutations is ordered lexicographically with respect to comp.
If the type of *first is not
(until C++11)BidirIt is not
(since C++11), the behavior is undefined.
Parameters
first, last - the pair of iterators defining the
of elements to permute comp - comparison function object (i.e. an object that satisfies the requirements of
) which returns true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following:
boolcmp(constType1&a,constType2&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
(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
and
. 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
Given N as std::distance(first,last):
1,2) At most N2 swaps.
Possible implementation
template<classBidirIt>boolprev_permutation(BidirItfirst,BidirItlast){if(first==last)returnfalse;BidirIti=last;if(first==--i)returnfalse;while(1){BidirIti1,i2;i1=i;if(*i1<*--i){i2=last;while(!(*--i2<*i));std::iter_swap(i,i2);std::reverse(i1,last);returntrue;}if(i==first){std::reverse(first,last);returnfalse;}}}Notes
Averaged over the entire sequence of permutations, typical implementations use about 3 comparisons and 1.5 swaps per call.
Implementations (e.g.
) may enable vectorization when the iterator type satisfies
and swapping its value type calls neither non-trivial special member function nor
-found swap.
Example
The following code prints all six permutations of the string "cab" in reverse order.
Run this code
#include<algorithm>#include<iostream>#include<string>intmain(){std::strings="cab";do{std::cout<<s<<' ';}while(std::prev_permutation(s.begin(),s.end()));std::cout<<s<<'\n';}Output:
cab bca bac acb abc cba See also