std::count_if() algorithm

od C++20

// (1)
template< class InputIt, class UnaryPredicate >
constexpr typename iterator_traits<InputIt>::difference_type
    count_if( InputIt first, InputIt last, UnaryPredicate p );

// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
typename iterator_traits<ForwardIt>::difference_type
    count_if( ExecutionPolicy&& policy,
              ForwardIt first, ForwardIt last, UnaryPredicate p );

od C++17

// (1)
template< class InputIt, class UnaryPredicate >
typename iterator_traits<InputIt>::difference_type
    count_if( InputIt first, InputIt last, UnaryPredicate p );

// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
typename iterator_traits<ForwardIt>::difference_type
    count_if( ExecutionPolicy&& policy,
              ForwardIt first, ForwardIt last, UnaryPredicate p );

do C++17

// (1)
template< class InputIt, class UnaryPredicate >
typename iterator_traits<InputIt>::difference_type
    count_if( InputIt first, InputIt last, UnaryPredicate p );

Returns the number of elements in the range [first1; last1) that satisfy specific criteria.

• (1) Counts elements for which predicate p returns true.
• (2) Same as (1), but executed according to policy.
  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (od C++20) is true.

Parameters

first last	The range of elements to examine.
policy	The execution policy to use. See execution policy for details.
count_if	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. (od C++11).

Type requirements

InputIt	LegacyInputIterator
ForwardIt	LegacyForwardIterator

Return value

• (1 - 2) The number of elements for which the predicate p holds true.

Complexity

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

• (1 - 2) exactly N applications of 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.

Possible implementation

count_if (1)

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;
}

Notes

If you want to obtain the number of elements in range [first; last) without any additional criteria, use std::distance

Examples

Main.cpp

#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