NAME¶

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

Synopsis¶

Defined in header <algorithm>
template< class InputIt, class T >
(until
typename iterator_traits<InputIt>::difference_type C++20)

count( InputIt first, InputIt last, const T& value );
template< class InputIt, class T >
(since
constexpr typename iterator_traits<InputIt>::difference_type C++20)

count( InputIt first, InputIt last, const T& value );
template< class ExecutionPolicy, class ForwardIt, class T >

typename iterator_traits<ForwardIt>::difference_type (2) (since
count( ExecutionPolicy&& policy, C++17)

ForwardIt first, ForwardIt last, const T& value );
template< class InputIt, class UnaryPredicate > (1)
(until
typename iterator_traits<InputIt>::difference_type C++20)

count_if( InputIt first, InputIt last, UnaryPredicate p );
template< class InputIt, class UnaryPredicate >
(since
constexpr typename iterator_traits<InputIt>::difference_type C++20)
(3)
count_if( InputIt first, InputIt last, UnaryPredicate p );
template< class ExecutionPolicy, class ForwardIt, class
UnaryPredicate >
(since
typename iterator_traits<ForwardIt>::difference_type (4) C++17)
count_if( ExecutionPolicy&& policy,

ForwardIt first, ForwardIt last, UnaryPredicate p );

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 executed according to policy. These overloads do not
participate in overload resolution unless
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.

Parameters¶

first, last - the range of elements to examine
value - the value to search for
policy - the execution policy to use. See execution policy for details.
unary predicate which returns true for the required elements.

The expression p(v) must be convertible to bool for every argument v
p - of type (possibly const) VT, where VT is the value type of InputIt,
regardless of value category, and must not modify v. Thus, a parameter
type of VT&is not allowed
, nor is VT unless for VT a move is equivalent to a copy
(since C++11).

Type requirements¶

-
InputIt must meet the requirements of LegacyInputIterator.
-
ForwardIt must meet the requirements of LegacyForwardIterator.

Return value¶

number of elements satisfying the condition.

Complexity¶

exactly last - first comparisons / applications of the predicate

Exceptions¶

The overloads with a template parameter named ExecutionPolicy report errors as
follows:

* If execution of a function invoked as part of the algorithm throws an exception
and ExecutionPolicy is one of the standard policies, std::terminate is called.
For any other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes¶

For the number of elements in the range [first, last) without any additional
criteria, see std::distance.

Possible implementation¶

See also the implementations of count in libstdc++ and libc++.

See also the implementations of count_if in libstdc++ and libc++.

First version¶

template<class InputIt, class T>
typename iterator_traits<InputIt>::difference_type
count(InputIt first, InputIt last, const T& value)
{
typename iterator_traits<InputIt>::difference_type ret = 0;
for (; first != last; ++first) {
if (*first == value) {
ret++;
}
}
return ret;
}

Second version¶

template<class InputIt, class UnaryPredicate>
typename iterator_traits<InputIt>::difference_type
count_if(InputIt first, InputIt last, UnaryPredicate p)
{
typename iterator_traits<InputIt>::difference_type ret = 0;
for (; first != last; ++first) {
if (p(*first)) {
ret++;
}
}
return ret;
}

Example¶

// Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <array>

int main()
{
constexpr std::array v = { 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };
std::cout << "v: ";
std::copy(v.cbegin(), v.cend(), std::ostream_iterator<int>(std::cout, " "));
std::cout << '\n';

// determine how many integers match a target value.
for (const int target: {3, 4, 5}) {
const int num_items = std::count(v.cbegin(), v.cend(), target);
std::cout << "number: " << target << ", count: " << num_items << '\n';
}

// use a lambda expression to count elements divisible by 4.
int count_div4 = std::count_if(v.begin(), v.end(), [](int i){return i % 4 == 0;});
std::cout << "numbers divisible by four: " << count_div4 << '\n';

// A simplified version of `distance` with O(N) complexity:
auto distance = [](auto first, auto last) {
return std::count_if(first, last, [](auto){return true;});
};
static_assert(distance(v.begin(), v.end()) == 10);
}

Output:¶

v: 1 2 3 4 4 3 7 8 9 10
number: 3, count: 2
number: 4, count: 2
number: 5, count: 0
numbers divisible by four: 3

See also¶

distance returns the distance between two iterators
(function template)
ranges::count
ranges::count_if returns the number of elements satisfying specific criteria
(C++20) (niebloid)
(C++20)