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,
std::indirect_unary_predicate<std::projected<I, Proj>> (1) (since C++20)
Pred >
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>, Proj>> Pred > (2) (since 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 pred 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 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.

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 pred, 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 <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

See also¶

ranges::partition divides a range of elements into two groups
(C++20) (niebloid)
ranges::partition_point locates the partition point of a partitioned range
(C++20) (niebloid)
is_partitioned determines if the range is partitioned by the given
(C++11) predicate
(function template)