NAME¶

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

Synopsis¶

Defined in header <algorithm>
template< class InputIt, class T >
(constexpr
typename since C++20)
std::iterator_traits<InputIt>::difference_type (until
C++26)
count( InputIt first, InputIt last, const T&
value );
template< class InputIt, class T = typename
std::iterator_traits

<InputIt>::value_type > (since
constexpr typename C++26)
std::iterator_traits<InputIt>::difference_type

count( InputIt first, InputIt last, const T&
value );
template< class ExecutionPolicy, class ForwardIt,
class T >
(since
typename C++17)
std::iterator_traits<ForwardIt>::difference_type (until
count( ExecutionPolicy&& policy, C++26)

ForwardIt first, ForwardIt last, const T&
value );
template< class ExecutionPolicy, (1)

class ForwardIt, class T = typename
std::iterator_traits

<ForwardIt>::value_type > (since
typename C++26)
std::iterator_traits<ForwardIt>::difference_type
count( ExecutionPolicy&& policy,
(2)
ForwardIt first, ForwardIt last, const T&
value );
template< class InputIt, class UnaryPred >

typename (constexpr
std::iterator_traits<InputIt>::difference_type (3) since
C++20)
count_if( InputIt first, InputIt last, UnaryPred
p );
template< class ExecutionPolicy, class ForwardIt,
class UnaryPred >

typename (since
std::iterator_traits<ForwardIt>::difference_type (4) C++17)
count_if( ExecutionPolicy&& policy,

ForwardIt first, ForwardIt last,
UnaryPred p );

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

1) Counts the elements that are equal to value (using operator==).
3) Counts elements for which predicate p returns true.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
C++20)

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.
-
UnaryPred must meet the requirements of Predicate.

Return value¶

The number of iterators it in the range [first, last) satisfying the following
condition:

1,2) *it == value is true.
3,4) p(*it) != false is true.

Complexity¶

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N comparisons with value using operator==.
3,4) Exactly \(\scriptsize N\)N applications of the predicate p.

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.

Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
algorithms (1,2)

Possible implementation¶

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

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

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

Example¶

// Run this code

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

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);

std::array<std::complex<double>, 3> nums{{{4, 2}, {1, 3}, {4, 2}}};
#ifdef __cpp_lib_algorithm_default_value_type
// T gets deduced making list-initialization possible
auto c = std::count(nums.cbegin(), nums.cend(), {4, 2});
#else
auto c = std::count(nums.cbegin(), nums.cend(), std::complex<double>{4, 2});
#endif
assert(c == 2);
}

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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
T was required to be EqualityComparable,
LWG 283 C++98 but removed the requirement
the value type of InputIt is not always T

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)