std::ranges::prev_permutation, std::ranges::prev_permutation_result

 

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Metaprogramming library(C++11)

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Ranges library(C++20)

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Filesystem library(C++17)

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Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges(C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Constrained algorithms

All names in this menu belong to namespace std::ranges

Non-modifying sequence operations

all_ofany_ofnone_of for_each for_each_n    countcount_if adjacent_find mismatch equal lexicographical_compare    containscontains_subrange (C++23)(C++23) findfind_iffind_if_not find_lastfind_last_iffind_last_if_not (C++23)(C++23)(C++23)   find_end find_first_of search search_n starts_with (C++23) ends_with (C++23)

Modifying sequence operations

copycopy_if copy_n copy_backward    move move_backward shift_leftshift_right (C++23)(C++23)   fill fill_n generate generate_n    transform swap_ranges    shuffle sample removeremove_if replacereplace_if reverse rotate remove_copyremove_copy_if    unique replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy

Partitioning operations

is_partitioned    partition_point    partition    partition_copy    stable_partition

Sorting operations

is_sorted is_sorted_until     sort stable_sort         partial_sort       nth_element partial_sort_copy

Binary search operations (on sorted ranges)

lower_bound         upper_bound         binary_search      equal_range

Set operations (on sorted ranges)

merge inplace_merge       set_difference set_intersection    set_union          includes set_symmetric_difference

Heap operations

is_heap is_heap_until       make_heap           sort_heap push_heap pop_heap

Minimum/maximum operations

max max_element         min min_element         minmax minmax_element     clamp

Permutation operations

is_permutation      next_permutation    prev_permutation

Fold operations

fold_left (C++23) fold_left_first (C++23)   fold_right (C++23) fold_right_last (C++23)   fold_left_with_iter (C++23) fold_left_first_with_iter (C++23)

Numeric operations iota (C++23)             Random number generation generate_random (C++26)

Operations on uninitialized storage

construct_at    destroy destroy_n destroy_at uninitialized_copy    uninitialized_move uninitialized_fill uninitialized_copy_n    uninitialized_move_n uninitialized_fill_n uninitialized_default_construct uninitialized_default_construct_n uninitialized_value_construct uninitialized_value_construct_n

Return types

in_fun_result in_in_result in_found_result    in_out_result in_in_out_result    in_out_out_result    min_max_result out_value_result (C++23) in_value_result (C++23)

 

Defined in header <algorithm>
Call signature
template<std::bidirectional_iterator I, std::sentinel_for<I> S,           class Comp =ranges::less, class Proj =std::identity> requires std::sortable<I, Comp, Proj> constexpr prev_permutation_result<I>     prev_permutation( I first, S last, Comp comp ={}, Proj proj ={});	(1)	(since C++20)
template<ranges::bidirectional_range R, class Comp =ranges::less,           class Proj =std::identity> requires std::sortable<ranges::iterator_t<R>, Comp, Proj> constexpr prev_permutation_result<ranges::borrowed_iterator_t<R>>     prev_permutation( R&& r, Comp comp ={}, Proj proj ={});	(2)	(since C++20)
Helper type
template<class I > using prev_permutation_result =ranges::in_found_result<I>;	(3)	(since C++20)

1) Transforms the range [first, last) into the previous permutation, where the set of all permutations is ordered lexicographically with respect to binary comparison function object comp and projection function object proj.

Returns:

• {last, true} if "previous" permutation exists. Otherwise,
• {last, false}, and transforms the range into the (lexicographically) last permutation, as if by

ranges::sort(first, last, comp, proj);ranges::reverse(first, last);

2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first, last	-	the iterator-sentinel pair defining the range of elements to "permute"
r	-	the range of elements to "permute"
comp	-	comparison FunctionObject which returns true if the first argument is less than the second
proj	-	projection to apply to the elements

Return value

1)ranges::prev_permutation_result<I>{last, true} if the new permutation is lexicographically less than the old one. ranges::prev_permutation_result<I>{last, false} if the first permutation was reached and the range was reset to the last permutation.

2) Same as (1) except that the return type is ranges::prev_permutation_result<ranges::borrowed_iterator_t<R>>.

Exceptions

Any exceptions thrown from iterator operations or the element swap.

Complexity

At most \(\scriptsize N/2\)N / 2 swaps, where \(\scriptsize N\)N is ranges::distance(first, last) in case (1) or ranges::distance(r) in case (2). 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 models contiguous_iterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

Possible implementation

struct prev_permutation_fn {template<std::bidirectional_iterator I, std::sentinel_for<I> S, class Comp =ranges::less, class Proj =std::identity> requires std::sortable<I, Comp, Proj>constexpr ranges::prev_permutation_result<I> operator()(I first, S last, Comp comp ={}, Proj proj ={})const{// check that the sequence has at least two elementsif(first == last)return{std::move(first), false};auto i{first};++i;if(i == last)return{std::move(i), false};auto i_last{ranges::next(first, last)}; i = i_last;--i;// main "permutating" loopfor(;;){auto i1{i};--i;if(std::invoke(comp, std::invoke(proj, *i1), std::invoke(proj, *i))){auto j{i_last};while(!std::invoke(comp, std::invoke(proj, *--j), std::invoke(proj, *i)));ranges::iter_swap(i, j);ranges::reverse(i1, last);return{std::move(i_last), true};}// permutation "space" is exhaustedif(i == first){ranges::reverse(first, last);return{std::move(i_last), false};}}}   template<ranges::bidirectional_range R, class Comp =ranges::less, class Proj =std::identity> requires std::sortable<ranges::iterator_t<R>, Comp, Proj>constexpr ranges::prev_permutation_result<ranges::borrowed_iterator_t<R>> operator()(R&& r, Comp comp ={}, Proj proj ={})const{return(*this)(ranges::begin(r), ranges::end(r), std::move(comp), std::move(proj));}};   inlineconstexpr prev_permutation_fn prev_permutation {};

Example

Run this code

#include <algorithm>#include <array>#include <compare>#include <functional>#include <iostream>#include <string>   struct S {char c{};int i{};auto operator<=>(const S&)const=default;friendstd::ostream& operator<<(std::ostream& os, const S& s){return os <<"{'"<< s.c<<"', "<< s.i<<"}";}};   auto print =[](autoconst& v, char term =' '){std::cout<<"{ ";for(constauto& e : v)std::cout<< e <<' ';std::cout<<'}'<< term;};   int main(){std::cout<<"Generate all permutations (iterators case):\n";std::string s{"cba"};do print(s);while(std::ranges::prev_permutation(s.begin(), s.end()).found);   std::cout<<"\nGenerate all permutations (range case):\n";std::array a{'c', 'b', 'a'};do print(a);while(std::ranges::prev_permutation(a).found);   std::cout<<"\nGenerate all permutations using comparator:\n";usingnamespace std::literals;std::array z{"▁"s, "▄"s, "█"s};do print(z);while(std::ranges::prev_permutation(z, std::greater()).found);   std::cout<<"\nGenerate all permutations using projection:\n";std::array<S, 3> r{S{'C',1}, S{'B',2}, S{'A',3}};do print(r, '\n');while(std::ranges::prev_permutation(r, {}, &S::c).found);}

Output:

Generate all permutations (iterators case): { c b a } { c a b } { b c a } { b a c } { a c b } { a b c } Generate all permutations (range case): { c b a } { c a b } { b c a } { b a c } { a c b } { a b c } Generate all permutations using comparator: { ▁ ▄ █ } { ▁ █ ▄ } { ▄ ▁ █ } { ▄ █ ▁ } { █ ▁ ▄ } { █ ▄ ▁ } Generate all permutations using projection: { {'C', 1} {'B', 2} {'A', 3} } { {'C', 1} {'A', 3} {'B', 2} } { {'B', 2} {'C', 1} {'A', 3} } { {'B', 2} {'A', 3} {'C', 1} } { {'A', 3} {'C', 1} {'B', 2} } { {'A', 3} {'B', 2} {'C', 1} }

See also

ranges::next_permutation (C++20)	generates the next greater lexicographic permutation of a range of elements (algorithm function object)
ranges::is_permutation (C++20)	determines if a sequence is a permutation of another sequence (algorithm function object)
next_permutation	generates the next greater lexicographic permutation of a range of elements (function template)
prev_permutation	generates the next smaller lexicographic permutation of a range of elements (function template)
is_permutation (C++11)	determines if a sequence is a permutation of another sequence (function template)