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std::count, std::count_if

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
countcount_if
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Permutations
Numeric operations
C library
 
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 ForwardIt, class T >

typename iterator_traits<ForwardIt>::difference_type

    count( ExecutionPolicy&& policy, ForwardIt first, ForwardIt 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 ForwardIt, class UnaryPredicate >

typename iterator_traits<ForwardIt>::difference_type

    count_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt 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. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Contents

[edit] 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.
-
ForwardIt must meet the requirements of ForwardIterator.

[edit] Return value

number of elements satisfying the condition.

[edit] Complexity

exactly last - first comparisons / applications of the predicate

[edit] 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 and ExecutionPolicy is one of the three standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Notes

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

[edit] 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;
}

[edit] Example

#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

[edit] See also

returns the distance between two iterators
(function) [edit]