std::count,std::count_if(3)

🌸 NAME 🌸

Okay bestie, this is std::count,std::count_if(3), straight from the C++ Standard Libary — and yes, the header spells it “Libary,” we’re keeping that, it’s iconic. The NAME line is literally std::count,std::count_if — std::count,std::count_if, and the whole page is giving “how many things match?” main character energy. 💅

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✨ Synopsis ✨

Defined in header <algorithm>.

Defined in header <algorithm>

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

template< class InputIt, class T = typename std::iterator_traits<InputIt>::value_type >
constexpr typename std::iterator_traits<InputIt>::difference_type
    count( InputIt first, InputIt last, const T& value );
(1) (since C++26)

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

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

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

template< class ExecutionPolicy, class ForwardIt, class UnaryPred >
typename std::iterator_traits<ForwardIt>::difference_type
    count_if( ExecutionPolicy&& policy,
              ForwardIt first, ForwardIt last, UnaryPred p );
(4) (since C++17)

So basically: returns the number of elements in the range [first, last) satisfying specific criteria. Obsessed.

1. Counts the elements that are equal to value (using operator==).
2. Counts elements for which predicate p returns true.
3. 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)

Wait, bestie, the numbering in the original is a little feral: the description says 1) equals value, 3) uses p, and 2,4) are the policy versions of 1,3). We’re keeping it exactly like that. No cap. 🫶

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🎀 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).

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🦋 Type requirements 🦋

• InputIt must meet the requirements of LegacyInputIterator.
• ForwardIt must meet the requirements of LegacyForwardIterator.
• UnaryPred must meet the requirements of Predicate.

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💅 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.

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🔥 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.

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🫶 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.

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🌈 Notes 🌈

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

Feature-test macro

• __cpp_lib_algorithm_default_value_type — value 202403, Std (C++26), Feature: List-initialization for algorithms (1,2).

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👑 Possible implementation 👑

See also the implementations of count in libstdc++ and libc++. See also the implementations of count_if in libstdc++ and libc++. Here’s the vibe:

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;
}

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🌸 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

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💀 Defect reports 💀

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

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

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⭐ See also ⭐

• distance — returns the distance between two iterators (function template).
• ranges::count — returns the number of elements satisfying specific criteria (C++20) (niebloid).
• ranges::count_if — returns the number of elements satisfying specific criteria (C++20) (niebloid).

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That’s the whole page, bestie — source/version: http://cppreference.com, date: 2024.06.10, manual page: std::count,std::count_if(3). Slay, go count something. 💖✨