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std::count, std::count_if

From cppreference.com
 
 
 
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.

Contents

[edit] 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. ​

[edit] Return value

number of elements satisfying the condition.

[edit] Complexity

exactly last - first comparisons / applications of the predicate

[edit] Possible implementation

[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

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