std::ranges::is_partitioned - cppreference.com (original) (raw)

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 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.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Possible implementation
• 5 Example
• 6 See also

[edit] Parameters

[edit] Return value

true if the range [first, last) is empty or is partitioned by pred, 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 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 ((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, 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 #include #include #include #include   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

[edit] See also