std::ranges::is_permutation() algorithm
- since C++20
- Simplified
- Detailed
// (1)
constexpr bool
is_partitioned( I first, S last, Pred pred, Proj proj = {} );
// (2)
constexpr bool
is_partitioned( R&& r, Pred pred, Proj proj = {} );
The type of arguments are generic and have the following constraints:
I
-std::input_iterator
S
-std::sentinel_for<I>
R
-std::ranges::input_range
Pred
:- (1) -
std::indirect_unary_predicate<std::projected<I, Proj>>
- (2) -
std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
- (1) -
Proj
- (none)
The Proj
template argument has a default type std::identity
for all overloads.
// (1)
template<
std::forward_iterator I1,
std::sentinel_for<I1> S1,
std::forward_iterator I2,
std::sentinel_for<I2> S2,
class Proj1 = std::identity,
class Proj2 = std::identity,
std::indirect_equivalence_relation< std::projected<I1, Proj1>,
std::projected<I2, Proj2> > Pred = ranges::equal_to
>
constexpr bool is_permutation( I1 first1, S1 last1, I2 first2, S2 last2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
// (2)
template<
ranges::forward_range R1,
ranges::forward_range R2,
class Proj1 = std::identity,
class Proj2 = std::identity,
std::indirect_equivalence_relation< std::projected<ranges::iterator_t<R1>, Proj1>,
std::projected<ranges::iterator_t<R2>, Proj2> > Pred = ranges::equal_to
>
constexpr bool is_permutation( R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
-
(1) Returns
true
if there exists a permutation of the elements in range [first1
;last1
) that makes the range equal to [first2
;last2
) (after application of corresponding projectionsProj1
,Proj2
, and using the binary predicatePred
as a comparator). Otherwise, returnsfalse
. -
(2) Same as (1), but uses
r1
as the first source range andr2
as the second source range, as if usingranges::begin(r1)
asfirst1
,ranges::end(r1)
aslast1
,ranges::begin(r2)
asfirst2
, andranges::end(r2)
aslast2
.
The function-like entities described on this page are niebloids.
Parameters
first1 last1 | The first range of elements to process. |
r | The first range of elements to process. |
first2 last2 | The second range of elements to process. |
r | The second range of elements to process. |
pred | The predicate to apply to the projected elemenets. |
proj | The projection to apply to the elements. |
Return value
true
if the range [first1
; last1
) is a permutation of the range [first2
; last2
).
Complexity
Given N
is ranges::distance(first1, last1)
.
At most O(N2) applications of the predicate and each projection, or exactly N
if the sequences are already equal,
However if ranges::distance(first1, last1) != ranges::distance(first2, last2)
, no applications of the predicate and projections are made.
Exceptions
(none)
Possible implementation
is_permutation(1) and is_permutation(2)
struct is_permutation_fn
{
template<std::forward_iterator I1, std::sentinel_for<I1> S1,
std::forward_iterator I2, std::sentinel_for<I2> S2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_equivalence_relation<std::projected<I1, Proj1>,
std::projected<I2, Proj2>>
Pred = ranges::equal_to>
constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
// skip common prefix
auto ret = std::ranges::mismatch(first1, last1, first2, last2,
std::ref(pred), std::ref(proj1), std::ref(proj2));
first1 = ret.in1, first2 = ret.in2;
// iterate over the rest, counting how many times each element
// from [first1, last1) appears in [first2, last2)
for (auto i {first1}; i != last1; ++i)
{
const auto i_proj {std::invoke(proj1, *i)};
auto i_cmp = [&]<typename T>(T&& t)
{
return std::invoke(pred, i_proj, std::forward<T>(t));
};
if (i != ranges::find_if(first1, i, i_cmp, proj1))
continue; // this *i has been checked
if (const auto m {ranges::count_if(first2, last2, i_cmp, proj2)};
m == 0 or m != ranges::count_if(i, last1, i_cmp, proj1))
return false;
}
return true;
}
template<ranges::forward_range R1, ranges::forward_range R2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_equivalence_relation<std::projected<ranges::iterator_t<R1>, Proj1>,
std::projected<ranges::iterator_t<R2>, Proj2>>
Pred = ranges::equal_to>
constexpr bool operator()(R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1), ranges::end(r1),
ranges::begin(r2), ranges::end(r2),
std::move(pred), std::move(proj1), std::move(proj2));
}
};
inline constexpr is_permutation_fn is_permutation {};
Notes
The permutation relation is an equivalence relation.
The ranges::is_permutation
can be used in testing, namely to check the correctness of rearranging algorithms (e.g. sorting, shuffling, partitioning).
If x
is an original range and y
is a permuted range then ranges::is_permutation(x, y) == true
means that y
consist of "the same" elements, maybe staying at other positions.
Examples
#include <algorithm>
#include <array>
#include <cmath>
#include <iostream>
#include <ranges>
auto& operator<<(auto& os, std::ranges::forward_range auto const& v)
{
os << "{ ";
for (auto const& e : v) os << e << ' ';
return os << "}";
}
int main()
{
static constexpr auto r1 = {1, 2, 3, 4, 5};
static constexpr auto r2 = {3, 5, 4, 1, 2};
static constexpr auto r3 = {3, 5, 4, 1, 1};
static_assert(
std::ranges::is_permutation(r1, r1) &&
std::ranges::is_permutation(r1, r2) &&
std::ranges::is_permutation(r2, r1) &&
std::ranges::is_permutation(r1.begin(), r1.end(), r2.begin(), r2.end()));
std::cout
<< std::boolalpha
<< "is_permutation( " << r1 << ", " << r2 << " ): "
<< std::ranges::is_permutation(r1, r2) << '\n'
<< "is_permutation( " << r1 << ", " << r3 << " ): "
<< std::ranges::is_permutation(r1, r3) << '\n'
<< "is_permutation with custom predicate and projections: "
<< std::ranges::is_permutation(
std::array {-14, -11, -13, -15, -12}, // 1st range
std::array {'F', 'E', 'C', 'B', 'D'}, // 2nd range
[](int x, int y) { return abs(x) == abs(y); }, // predicate
[](int x) { return x + 10; }, // projection for 1st range
[](char y) { return int(y - 'A'); }) // projection for 2nd range
<< '\n';
}
is_permutation( { 1 2 3 4 5 }, { 3 5 4 1 2 } ): true
is_permutation( { 1 2 3 4 5 }, { 3 5 4 1 1 } ): false
is_permutation with custom predicate and projections: true
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