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, |
(1) | (since C++20) |
| template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate< |
(2) | (since C++20) |
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
| (C++20) |
divides a range of elements into two groups (niebloid) |
| (C++20) |
locates the partition point of a partitioned range (niebloid) |
| (C++11) |
determines if the range is partitioned by the given predicate (function template) |