NAME¶

std::ranges::count,std::ranges::count_if - std::ranges::count,std::ranges::count_if

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S,

class T, class Proj = std::identity >
requires std::indirect_binary_predicate (since
<ranges::equal_to, std::projected<I, Proj>, C++20)
const T*> (until
constexpr std::iter_difference_t<I> C++26)

count( I first, S last, const T& value, Proj proj = {}
);
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 (since
<ranges::equal_to, std::projected<I, Proj>, C++26)
const T*>
constexpr std::iter_difference_t<I>

count( I first, S last, const T& value, Proj proj = {}
);
template< ranges::input_range R, class T, class Proj =
std::identity >

requires std::indirect_binary_predicate (since
<ranges::equal_to, C++20)
std::projected<ranges::iterator_t<R>, Proj>, (until
const T*> C++26)
constexpr ranges::range_difference_t<R> (1)

count( R&& r, const T& value, Proj proj = {} );
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 (since
<ranges::equal_to, C++26)
std::projected<ranges::iterator_t<R>, Proj>,
const T*>
constexpr ranges::range_difference_t<R> (2)

count( R&& r, const T& value, Proj proj = {} );
template< std::input_iterator I, std::sentinel_for<I> S,

class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, (3) (since
Proj>> Pred > C++20)
constexpr std::iter_difference_t<I>

count_if( I first, S last, Pred pred, Proj proj = {} );
template< ranges::input_range R, class Proj = std::identity,

std::indirect_unary_predicate<
std::projected<ranges::iterator_t<R>, Proj>> (4) (since
Pred > C++20)
constexpr ranges::range_difference_t<R>

count_if( R&& r, Pred pred, Proj proj = {} );

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

1) Counts the elements that are equal to value.
3) 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 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 - 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

Return value¶

Number of elements satisfying the condition.

Complexity¶

Exactly last - first comparisons and projection.

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)

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;

Example¶

// Run this code

#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

See also¶

ranges::distance returns the distance between an iterator and a sentinel, or
(C++20) between the beginning and end of a range
(niebloid)
views::counted creates a subrange from an iterator and a count
(C++20) (customization point object)
ranges::filter_view a view that consists of the elements of a range that satisfies a
views::filter predicate
(C++20) (class template) (range adaptor object)
count returns the number of elements satisfying specific criteria
count_if (function template)