std::ranges::count, std::ranges::count_if
(on partitioned ranges)
std::ranges
<algorithm>
class T, class Proj = std::identity >
requires std::indirect_binary_predicate
<ranges::equal_to, std::projected <I, Proj>, const T*>
constexpr std::iter_difference_t <I>
class Proj = std::identity,
class T = std::projected_value_t<I, Proj> >
requires std::indirect_binary_predicate
<ranges::equal_to, std::projected <I, Proj>, const T*>
constexpr std::iter_difference_t <I>
requires std::indirect_binary_predicate
<ranges::equal_to,
std::projected <ranges::iterator_t <R>, Proj>, const T*>
constexpr ranges::range_difference_t <R>
class T = std::projected_value_t<ranges::iterator_t <R>, Proj> >
requires std::indirect_binary_predicate
<ranges::equal_to,
std::projected <ranges::iterator_t <R>, Proj>, const T*>
constexpr ranges::range_difference_t <R>
class Proj = std::identity,
std::indirect_unary_predicate <std::projected <I, Proj>> Pred >
constexpr std::iter_difference_t <I>
std::indirect_unary_predicate <
std::projected <ranges::iterator_t <R>, Proj>> Pred >
constexpr ranges::range_difference_t <R>
Returns the number of elements in the range [
first,
last)
satisfying specific criteria.
The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:
Number of elements satisfying the condition.
Exactly last - first comparisons and projection.
For the number of elements in the range without any additional criteria, see std::ranges::distance.
Feature-test macro | Value | Std | Feature |
---|---|---|---|
__cpp_lib_algorithm_default_value_type |
202403 |
(C++26) | List-initialization for algorithms (1,2) |
count (1) |
---|
struct count_fn { template<std::input_iterator I, std::sentinel_for <I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj>> requires std::indirect_binary_predicate <ranges::equal_to, std::projected <I, Proj>, const T*> constexpr std::iter_difference_t <I> operator()(I first, S last, const T& value, Proj proj = {}) const { std::iter_difference_t <I> counter = 0; for (; first != last; ++first) if (std::invoke (proj, *first) == value) ++counter; return counter; } template<ranges::input_range R, class Proj = std::identity class T = std::projected_value_t<ranges::iterator_t <R>, Proj>> requires std::indirect_binary_predicate <ranges::equal_to, std::projected <ranges::iterator_t <R>, Proj>, const T*> constexpr ranges::range_difference_t <R> operator()(R&& r, const T& value, Proj proj = {}) const { return (*this)(ranges::begin (r), ranges::end (r), value, std::ref (proj)); } }; inline constexpr count_fn count; |
count_if (3) |
struct count_if_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 std::iter_difference_t <I> operator()(I first, S last, Pred pred, Proj proj = {}) const { std::iter_difference_t <I> counter = 0; for (; first != last; ++first) if (std::invoke (pred, std::invoke (proj, *first))) ++counter; return counter; } template<ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate < std::projected <ranges::iterator_t <R>, Proj>> Pred> constexpr ranges::range_difference_t <R> operator()(R&& r, Pred pred, Proj proj = {}) const { return (*this)(ranges::begin (r), ranges::end (r), std::ref (pred), std::ref (proj)); } }; inline constexpr count_if_fn count_if; |
#include <algorithm> #include <cassert> #include <complex> #include <iostream> #include <vector> int main() { std::vector <int> v{1, 2, 3, 4, 4, 3, 7, 8, 9, 10}; namespace ranges = std::ranges; // determine how many integers in a std::vector match a target value. int target1 = 3; int target2 = 5; int num_items1 = ranges::count(v.begin(), v.end(), target1); int num_items2 = ranges::count(v, target2); std::cout << "number: " << target1 << " count: " << num_items1 << '\n'; std::cout << "number: " << target2 << " count: " << num_items2 << '\n'; // use a lambda expression to count elements divisible by 3. int num_items3 = ranges::count_if(v.begin(), v.end(), [](int i){ return i % 3 == 0; }); std::cout << "number divisible by three: " << num_items3 << '\n'; // use a lambda expression to count elements divisible by 11. int num_items11 = ranges::count_if(v, [](int i){ return i % 11 == 0; }); std::cout << "number divisible by eleven: " << num_items11 << '\n'; std::vector <std::complex <double>> nums{{4, 2}, {1, 3}, {4, 2}}; #ifdef __cpp_lib_algorithm_default_value_type auto c = ranges::count(nums, {4, 2}); #else auto c = ranges::count(nums, std::complex <double>{4, 2}); #endif assert (c == 2); }
Output:
number: 3 count: 2 number: 5 count: 0 number divisible by three: 3 number divisible by eleven: 0
view
that consists of the elements of a range
that satisfies a predicate