std::ranges::is_partitioned
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::input_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
constexpr bool is_partitioned( I first, S last, Pred pred, Proj proj = {} );
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(1) | (since C++20) |
template< ranges::input_range R, class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred >
constexpr bool is_partitioned( R&& r, Pred pred, Proj proj = {} );
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(2) | (since C++20) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I, std::sized_sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
bool is_partitioned( Ep&& policy, I first, S last, Pred pred, Proj proj = {} );
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(3) | (since C++26) |
template< /*execution-policy*/ Ep,
/*sized-random-access-range*/ R, class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred >
bool is_partitioned( Ep&& policy, R&& r, Pred pred, Proj proj = {} );
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(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
1,2) Checks whether the elements
e in the source range [first, last) or r are partitioned with respect to the expression bool(std::invoke(pred, std::invoke(proj, e))): all elements (projected by proj) satisfy pred appear before all elements that do not.3,4) Same as (1,2), but executed according to
policy.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 source range |
| r | - | the source range |
| pred | - | the predicate to be applied to the (projected) elements |
| proj | - | the projection to be applied to the elements |
| policy | - | the execution policy to use |
Return value
true if the elements of the source range are partiationed as descibed above or the source range is empty, false otherwise.
Complexity
Given N as ranges::distance(first, last) or ranges::distance(r):
1,2) At most N applications of
pred and proj.3,4) 𝓞(N) applications of
pred and proj.Possible implementation
struct is_partitioned_fn
{
template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
constexpr bool operator()(I first, S last, Pred pred, Proj proj = {}) const
{
for (; first != last; ++first)
if (!std::invoke(pred, std::invoke(proj, *first)))
break;
for (; first != last; ++first)
if (std::invoke(pred, std::invoke(proj, *first)))
return false;
return true;
}
template<ranges::input_range R, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred>
constexpr bool operator()(R&& r, Pred pred, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
}
template<ranges::forward_range R, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred>
constexpr bool operator()(R&& r, Pred pred, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::next(ranges::begin(r), ranges::end(r)),
std::ref(pred), std::ref(proj));
}
};
inline constexpr is_partitioned_fn is_partitioned;
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Example
Run this code
#include <algorithm>
#include <array>
#include <iostream>
#include <numeric>
#include <utility>
int main()
{
std::array<int, 9> v;
auto print = [&v](bool o)
{
for (int x : v)
std::cout << x << ' ';
std::cout << (o ? "=> " : "=> not ") << "partitioned\n";
};
auto is_even = [](int i) { return i % 2 == 0; };
std::iota(v.begin(), v.end(), 1); // or std::ranges::iota(v, 1);
print(std::ranges::is_partitioned(v, is_even));
std::ranges::partition(v, is_even);
print(std::ranges::is_partitioned(std::as_const(v), is_even));
std::ranges::reverse(v);
print(std::ranges::is_partitioned(v.cbegin(), v.cend(), is_even));
print(std::ranges::is_partitioned(v.crbegin(), v.crend(), is_even));
}
Output:
1 2 3 4 5 6 7 8 9 => not partitioned
2 4 6 8 5 3 7 1 9 => partitioned
9 1 7 3 5 8 6 4 2 => not partitioned
9 1 7 3 5 8 6 4 2 => partitioned