std::count, std::count_if

 

C++

Language

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Algorithm library

Functions
Non-modifying sequence operations
all_of any_of none_of (C++11) (C++11) (C++11) for_each count count_if mismatch equal	find find_if find_if_not (C++11) find_end find_first_of adjacent_find search search_n
Modifying sequence operations
copy copy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) fill fill_n transform generate generate_n remove remove_if remove_copy remove_copy_if	replace replace_if replace_copy replace_copy_if swap swap_ranges iter_swap reverse reverse_copy rotate rotate_copy random_shuffle shuffle   (C++11) unique unique_copy
Partitioning operations
is_partitioned (C++11) partition partition_copy (C++11)	stable_partition partition_point (C++11)
Sorting operations (on sorted ranges)
is_sorted (C++11) is_sorted_until (C++11) sort	partial_sort partial_sort_copy stable_sort nth_element
Binary search operations (on sorted ranges)
lower_bound upper_bound	binary_search equal_range
Set operations (on sorted ranges)
merge inplace_merge includes	set_difference set_intersection set_symmetric_difference set_union
Heap operations
is_heap is_heap_until (C++11) make_heap	push_heap pop_heap sort_heap
Minimum/maximum operations
max max_element min min_element minmax (C++11) minmax_element (C++11)	lexicographical_compare is_permutation (C++11) next_permutation prev_permutation
Numeric operations
iota (C++11) accumulate inner_product	adjacent_difference partial_sum
C library
qsort	bsearch

 

Defined in header <algorithm>
template< class InputIterator, class T > typename iterator_traits<InputIterator>::difference_type     count( InputIterator first, InputIterator last, const T &value );	(1)
template< class InputIterator, class UnaryPredicate > typename iterator_traits<InputIterator>::difference_type     count_if( InputIterator first, InputIterator last, UnaryPredicate p );	(2)

Returns the number of elements in the range [first, last) satisfying specific criteria. The first version counts the elements that are equal to value, the second version counts elements for which predicate p returns true.

Parameters

first, last	-	the range of elements to examine
value	-	the value to search for
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 InputIterator can be dereferenced and then implicitly converted to Type. ​

Return value

number of elements satisfying the condition.

Complexity

exactly last - first comparisons / applications of the predicate

Possible implementation

First version
template<class InputIterator, class T> typename iterator_traits<InputIterator>::difference_type count(InputIterator first, InputIterator last, const T& value) { typename iterator_traits<InputIterator>::difference_type ret = 0; for (; first != last; ++first) { if (*first == value) { ret++; } } return ret; }
Second version
template<class InputIterator, class UnaryPredicate> typename iterator_traits<InputIterator>::difference_type count_if(InputIterator first, InputIterator last, UnaryPredicate p) { typename iterator_traits<InputIterator>::difference_type ret = 0; for (; first != last; ++first) { if (p(*first)) { ret++; } } return ret; }

Example

The following code uses count to determine how many integers in a vector match a target value.

#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    int data[] = { 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };
    std::vector<int> v(data, data+10);
 
    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';
}

Output:

number: 3 count: 2
number: 5 count: 0

This example uses an anonymous lambda to count elements divisible by 3.

#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    int data[] = { 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };
    std::vector<int> v(data, data+10);
 
    int num_items1 = std::count_if(v.begin(), v.end(), [](int i) {return i % 3 == 0;});
 
    std::cout << "number divisible by three: " << num_items1 << '\n';
}

Output:

number divisible by three: 3