std::ranges::count, std::ranges::count_if - cppreference.com (original) (raw)

Defined in header
Call signature
(1)
template< std::input_iterator I, std::sentinel_for<I> S, 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> count( I first, S last, const T& value, Proj proj = {} ); (since C++20) (until C++26)
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> count( I first, S last, const T& value, Proj proj = {} ); (since C++26)
(2)
template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::range_difference_t<R> count( R&& r, const T& value, Proj proj = {} ); (since C++20) (until C++26)
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> count( R&& r, const T& value, Proj proj = {} ); (since C++26)
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> count_if( I first, S last, Pred pred, Proj proj = {} ); (3) (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 ranges::range_difference_t<R> count_if( R&& r, Pred pred, Proj proj = {} ); (4) (since C++20)

Returns the number of elements in the range [first, last) satisfying specific criteria.

  1. Counts the elements that are equal to value.

  2. Counts elements for which predicate p returns true.

2,4) Same as (1,3), 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:

Contents

[edit] Parameters

first, last - the iterator-sentinel pair defining the range of elements to examine
r - the range of the elements to examine
value - the value to search for
pred - predicate to apply to the projected elements
proj - projection to apply to the elements

[edit] Return value

Number of elements satisfying the condition.

[edit] Complexity

Exactly last - first comparisons and projection.

[edit] Notes

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)

[edit] Possible implementation

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;

[edit] Example

#include #include #include #include #include   int main() { std::vector 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> 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{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

[edit] See also