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std::count_if() algorithm

// (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 );

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

InputItLegacyInputIterator
ForwardItLegacyForwardIterator

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
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std::count_if() algorithm

// (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 );

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

InputItLegacyInputIterator
ForwardItLegacyForwardIterator

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
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.