Namespaces
Variants
Actions

std::ranges::count, std::ranges::count_if

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations Copy operations
(C++11)
(C++11)
Swap operations Transformation operations Generation operations Removing operations Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations Binary search operations
(on partitioned ranges) Set operations (on sorted ranges) Merge operations (on sorted ranges) Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations Permutation operations


C library
Numeric operations
 
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
       
Binary search operations (on sorted ranges)
       
       
    
Set operations (on sorted ranges)
     
        
Heap operations
     
         
Minimum/maximum operations
       
       
Permutation operations
Fold operations
(C++23)
(C++23)  
(C++23)
(C++23)  
Defined in header <algorithm>
Call signature
(1)
template< std::input_iterator I, std::sentinel_for <I> S,

          class T, class Proj = std::identity >
requires std::indirect_binary_predicate
             <ranges::equal_to, std::projected <I, Proj>, const T*>
constexpr std::iter_difference_t <I>

    count( I first, S last, const T& value, Proj proj = {} );
(since C++20)
(until C++26)
template< std::input_iterator I, std::sentinel_for <I> S,

          class Proj = std::identity,
          class T = std::projected_value_t<I, Proj> >
requires std::indirect_binary_predicate
             <ranges::equal_to, std::projected <I, Proj>, const T*>
constexpr std::iter_difference_t <I>

    count( I first, S last, const T& value, Proj proj = {} );
(since C++26)
(2)
template< ranges::input_range R, class T, class Proj = std::identity >

requires std::indirect_binary_predicate
             <ranges::equal_to,
              std::projected <ranges::iterator_t <R>, Proj>, const T*>
constexpr ranges::range_difference_t <R>

    count( R&& r, const T& value, Proj proj = {} );
(since C++20)
(until C++26)
template< ranges::input_range R, class Proj = std::identity,

          class T = std::projected_value_t<ranges::iterator_t <R>, Proj> >
requires std::indirect_binary_predicate
             <ranges::equal_to,
              std::projected <ranges::iterator_t <R>, Proj>, const T*>
constexpr ranges::range_difference_t <R>

    count( R&& r, const T& value, Proj proj = {} );
(since C++26)
template< std::input_iterator I, std::sentinel_for <I> S,

          class Proj = std::identity,
          std::indirect_unary_predicate <std::projected <I, Proj>> Pred >
constexpr std::iter_difference_t <I>

    count_if( I first, S last, Pred pred, Proj proj = {} );
(3) (since C++20)
template< ranges::input_range R, class Proj = std::identity,

          std::indirect_unary_predicate <
              std::projected <ranges::iterator_t <R>, Proj>> Pred >
constexpr ranges::range_difference_t <R>

    count_if( R&& r, Pred pred, Proj proj = {} );
(4) (since C++20)

Returns the number of elements in the range [first, last) satisfying specific criteria.

1) Counts the elements that are equal to value.
3) Counts elements for which predicate p returns true.
2,4) Same as (1,3), but uses r as the source range, as if using ranges::begin (r) as first and ranges::end (r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Contents

[edit] Parameters

first, last - the iterator-sentinel pair defining the range of elements to examine
r - the range of the elements to examine
value - the value to search for
pred - predicate to apply to the projected elements
proj - projection to apply to the elements

[edit] Return value

Number of elements satisfying the condition.

[edit] Complexity

Exactly last - first comparisons and projection.

[edit] Notes

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

Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for algorithms (1,2)

[edit] Possible implementation

count (1) 
struct count_fn
{
 template<std::input_iterator I, std::sentinel_for <I> S,
 class Proj = std::identity, class T = std::projected_value_t<I, Proj>>
 requires std::indirect_binary_predicate <ranges::equal_to,
 std::projected <I, Proj>, const T*>
 constexpr std::iter_difference_t <I>
 operator()(I first, S last, const T& value, Proj proj = {}) const
 {
 std::iter_difference_t <I> counter = 0;
 for (; first != last; ++first)
 if (std::invoke (proj, *first) == value)
 ++counter;
 return counter;
 }
 
 template<ranges::input_range R, class Proj = std::identity
 class T = std::projected_value_t<ranges::iterator_t <R>, Proj>>
 requires std::indirect_binary_predicate <ranges::equal_to,
 std::projected <ranges::iterator_t <R>, Proj>,
 const T*>
 constexpr ranges::range_difference_t <R>
 operator()(R&& r, const T& value, Proj proj = {}) const
 {
 return (*this)(ranges::begin (r), ranges::end (r), value, std::ref (proj));
 }
};
 
inline constexpr count_fn count;
count_if (3) 
struct count_if_fn
{
 template<std::input_iterator I, std::sentinel_for <I> S,
 class Proj = std::identity,
 std::indirect_unary_predicate <std::projected <I, Proj>> Pred>
 constexpr std::iter_difference_t <I>
 operator()(I first, S last, Pred pred, Proj proj = {}) const
 {
 std::iter_difference_t <I> counter = 0;
 for (; first != last; ++first)
 if (std::invoke (pred, std::invoke (proj, *first)))
 ++counter;
 return counter;
 }
 
 template<ranges::input_range R, class Proj = std::identity,
 std::indirect_unary_predicate <
 std::projected <ranges::iterator_t <R>, Proj>> Pred>
 constexpr ranges::range_difference_t <R>
 operator()(R&& r, Pred pred, Proj proj = {}) const
 {
 return (*this)(ranges::begin (r), ranges::end (r),
 std::ref (pred), std::ref (proj));
 }
};
 
inline constexpr count_if_fn count_if;

[edit] Example

Run this code
#include <algorithm>
#include <cassert>
#include <complex>
#include <iostream>
#include <vector>
 
int main()
{
 std::vector <int> v{1, 2, 3, 4, 4, 3, 7, 8, 9, 10};
 
 namespace ranges = std::ranges;
 
 // determine how many integers in a std::vector match a target value.
 int target1 = 3;
 int target2 = 5;
 int num_items1 = ranges::count(v.begin(), v.end(), target1);
 int num_items2 = ranges::count(v, 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 = ranges::count_if(v.begin(), v.end(), [](int i){ return i % 3 == 0; });
 std::cout << "number divisible by three: " << num_items3 << '\n';
 
 // use a lambda expression to count elements divisible by 11.
 int num_items11 = ranges::count_if(v, [](int i){ return i % 11 == 0; });
 std::cout << "number divisible by eleven: " << num_items11 << '\n';
 
 std::vector <std::complex <double>> nums{{4, 2}, {1, 3}, {4, 2}};
 #ifdef __cpp_lib_algorithm_default_value_type
 auto c = ranges::count(nums, {4, 2});
 #else
 auto c = ranges::count(nums, std::complex <double>{4, 2});
 #endif
 assert (c == 2);
}

Output: 

number: 3 count: 2
number: 5 count: 0
number divisible by three: 3
number divisible by eleven: 0

[edit] See also

returns the distance between an iterator and a sentinel, or between the beginning and end of a range
(algorithm function object)[edit]
creates a subrange from an iterator and a count
(customization point object)[edit]
a view that consists of the elements of a range that satisfies a predicate
(class template) (range adaptor object)[edit]
returns the number of elements satisfying specific criteria
(function template) [edit]

AltStyle によって変換されたページ (->オリジナル) /