NAME

std::ranges::is_partitioned - std::ranges::is_partitioned

Synopsis

Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S, class Proj =
std::identity,
(since
std::indirect_unary_predicate<std::projected<I, Proj>> Pred > (1) C++20)

constexpr bool is_partitioned( I first, S last, Pred pred, Proj proj =
{} );
template< ranges::input_range R, class Proj = std::identity,

std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, (2) (since
Proj>> Pred > C++20)

constexpr bool is_partitioned( R&& r, Pred pred, Proj proj = {} );

1) Returns true if all elements in the range [first, last) that satisfy the
predicate p after projection appear before all elements that don't. Also returns
true if [first, last) is empty.
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 niebloids, that is:

* Explicit template argument lists may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters

first, last - iterator-sentinel pair denoting the range of elements to examine
r - the range of elements to examine
pred - predicate to apply to the projected elements
proj - projection to apply to the elements

Return value

true if the range [first, last) is empty or is partitioned by p. false otherwise.

Complexity

At most ranges::distance(first, last) 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));
} };

inline constexpr auto is_partitioned = is_partitioned_fn();

Example

// Run this code

#include <algorithm>
#include <array>
#include <iostream>
#include <utility>

int main()
{
std::array<int, 9> v;

auto is_even = [](int i){ return i % 2 == 0; };
auto print = [&](bool o) {
for (int x : v) std::cout << x << ' ';
std::cout << (o ? "=> " : "=> not ") << "partitioned\n";
};

std::iota(v.begin(), v.end(), 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