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std::count, std::count_if

From cppreference.com
< cpp‎ | algorithm
 
 
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countcount_if

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Defined in header <algorithm>
(1)
template< class InputIt, class T >

typename iterator_traits<InputIt>::difference_type

    count( InputIt first, InputIt last, const T& value );
(until C++20)
template< class InputIt, class T >

constexpr typename iterator_traits<InputIt>::difference_type

    count( InputIt first, InputIt last, const T& value );
(since C++20)
template< class ExecutionPolicy, class ForwardIt, class T >

typename iterator_traits<ForwardIt>::difference_type
    count( ExecutionPolicy&& policy,

           ForwardIt first, ForwardIt last, const T& value );
(2) (since C++17)
(3)
template< class InputIt, class UnaryPredicate >

typename iterator_traits<InputIt>::difference_type

    count_if( InputIt first, InputIt last, UnaryPredicate p );
(until C++20)
template< class InputIt, class UnaryPredicate >

constexpr typename iterator_traits<InputIt>::difference_type

    count_if( InputIt first, InputIt last, UnaryPredicate p );
(since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >

typename iterator_traits<ForwardIt>::difference_type
    count_if( ExecutionPolicy&& policy,

              ForwardIt first, ForwardIt last, UnaryPredicate p );
(4) (since C++17)

Returns the number of elements in the range [firstlast) 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)

Contents

[edit] 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.
p - unary predicate which returns ​true for the required elements.

The expression p(v) must be convertible to bool for every argument v 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.

[edit] Return value

The number of iterators it in the range [firstlast) satisfying the following condition:

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

[edit] Complexity

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

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

[edit] 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.

[edit] Notes

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

[edit] 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 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;
}
count_if
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;
}

[edit] Example

#include <algorithm>
#include <array>
#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);
}

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

[edit] 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, but
the value type of InputIt is not always T
removed the requirement

[edit] See also

returns the distance between two iterators
(function template) [edit]
returns the number of elements satisfying specific criteria
(niebloid)[edit]