std::count, std::count_if

From Cppreference

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.

[edit] Parameters

[edit] Return value

number of elements satisfying the condition.

[edit] Complexity

exactly last - first comparisons / applications of the predicate

[edit] Equivalent function

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;
}

[edit] 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