Namespaces
Variants
Views
Actions

std::ranges::is_partitioned

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
ranges::is_partitioned
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
Defined in header <algorithm>
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 = {} );
(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 = {} );
(2) (since C++20)
1) Returns true if all projected elements in the range [first, last) that satisfy the predicate p 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:

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

Contents

[edit] Parameters

first, last - the range of elements to reorder
r - the range of elements to reorder
pred - predicate to apply to the projected elements
proj - projection to apply to the elements

[edit] Return value

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

[edit] Complexity

At most ranges::distance(first, last) applications of pred and proj.

[edit] 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();

[edit] Example

#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

[edit] See also

divides a range of elements into two groups
(niebloid) [edit]
locates the partition point of a partitioned range
(niebloid) [edit]
determines if the range is partitioned by the given predicate
(function template) [edit]