std::count, std::count_if

 

C++

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Technical Specifications

 

Algorithm library

Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseq	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policy
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal lexicographical_compare	findfind_iffind_if_not (C++11) find_end find_first_of	adjacent_find search search_n
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17)	uninitialized_copy_n (C++11) uninitialized_move_n (C++17)	uninitialized_fill	uninitialized_fill_n
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

Defined in header <algorithm>
template< class InputIt, class T > typename iterator_traits<InputIt>::difference_type     count( InputIt first, InputIt last, const T &value );	(1)
template< class ExecutionPolicy, class InputIt, class T > typename iterator_traits<InputIt>::difference_type     count( ExecutionPolicy&& policy, InputIt first, InputIt last, const T &value );	(2)	(since C++17)
template< class InputIt, class UnaryPredicate > typename iterator_traits<InputIt>::difference_type     count_if( InputIt first, InputIt last, UnaryPredicate p );	(3)
template< class ExecutionPolicy, class InputIt, class UnaryPredicate > typename iterator_traits<InputIt>::difference_type     count_if( ExecutionPolicy&& policy, InputIt first, InputIt last, UnaryPredicate p );	(4)	(since C++17)

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 executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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 signature of the predicate function should be equivalent to the following:  bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​
Type requirements
- InputIt must meet the requirements of InputIterator.

Return value

number of elements satisfying the condition.

Complexity

exactly last - first comparisons / applications of the predicate

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, std::terminate is called.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes

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

Possible implementation

First version
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; }
Second version
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; }

Example

Run this code

#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v{ 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };
 
    // determine how many integers in a std::vector match a target value.
    int target1 = 3;
    int target2 = 5;
    int num_items1 = std::count(v.begin(), v.end(), target1);
    int num_items2 = std::count(v.begin(), v.end(), 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 = std::count_if(v.begin(), v.end(), [](int i){return i % 3 == 0;});
    std::cout << "number divisible by three: " << num_items3 << '\n';
}

Output:

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

See also

std::experimental::parallel::count (parallelism TS)	parallelized version of std::count (function template)
std::experimental::parallel::count_if (parallelism TS)	parallelized version of std::count_if (function template)