std::count, std::count_if
Defined in header <algorithm>
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(1) | ||
template< class InputIt, class T > typename iterator_traits<InputIt>::difference_type |
(until C++20) | |
template< class InputIt, class T > constexpr typename iterator_traits<InputIt>::difference_type |
(since C++20) | |
template< class ExecutionPolicy, class ForwardIt, class T > typename iterator_traits<ForwardIt>::difference_type |
(2) | (since C++17) |
(3) | ||
template< class InputIt, class UnaryPredicate > typename iterator_traits<InputIt>::difference_type |
(until C++20) | |
template< class InputIt, class UnaryPredicate > constexpr typename iterator_traits<InputIt>::difference_type |
(since C++20) | |
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate > typename iterator_traits<ForwardIt>::difference_type |
(4) | (since C++17) |
Returns the number of elements in the range [first, last)
satisfying specific criteria.
value
.p
returns true.policy
. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is trueParameters
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. |
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.
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-ForwardIt must meet the requirements of LegacyForwardIterator.
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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 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.
Possible implementation
First version |
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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
#include <algorithm> #include <iostream> #include <vector> int main() { std::vector<int> v{ 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 }; // determine how many integers in a std::vector match a target value. int target1 = 3; int target2 = 5; int num_items1 = std::count(v.begin(), v.end(), target1); int num_items2 = std::count(v.begin(), v.end(), 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 = std::count_if(v.begin(), v.end(), [](int i){return i % 3 == 0;}); std::cout << "number divisible by three: " << num_items3 << '\n'; }
Output:
number: 3 count: 2 number: 5 count: 0 number divisible by three: 3
See also
returns the distance between two iterators (function template) |