std::count, std::count_if
| Defined in header <algorithm>
|
||
| (1) | ||
template< class InputIt, class T > typename std::iterator_traits<InputIt>::difference_type |
(constexpr since C++20) (until C++26) |
|
| template< class InputIt, class T = typename std::iterator_traits <InputIt>::value_type > |
(since C++26) | |
| (2) | ||
| template< class ExecutionPolicy, class ForwardIt, class T > typename std::iterator_traits<ForwardIt>::difference_type |
(since C++17) (until C++26) |
|
| template< class ExecutionPolicy, class ForwardIt, class T = typename std::iterator_traits |
(since C++26) | |
template< class InputIt, class UnaryPred > typename std::iterator_traits<InputIt>::difference_type |
(3) | (constexpr since C++20) |
| template< class ExecutionPolicy, class ForwardIt, class UnaryPred > typename std::iterator_traits<ForwardIt>::difference_type |
(4) | (since C++17) |
Returns the number of elements in the range [first, last) satisfying specific criteria.
|
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 |
| p | - | unary predicate which returns true for the required elements. The expression p(v) must be convertible to bool for every argument |
| 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:
Complexity
Given N as std::distance(first, last):
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
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy, 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
#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 |
|---|---|---|---|
| LWG 283 | C++98 | T was required to be EqualityComparable, butthe value type of InputIt is not always T
|
removed the requirement |
See also
| returns the distance between two iterators (function template) | |
| (C++20)(C++20) |
returns the number of elements satisfying specific criteria (algorithm function object) |