std::count, std::count_if
From Cppreference
| Defined in header
<algorithm> | ||
| template< class InputIterator, class T >
typename iterator_traits<InputIterator>::difference_type | (1) | |
| template< class InputIterator, class UnaryPredicate >
typename iterator_traits<InputIterator>::difference_type | (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 |
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:
The signature does not need to have const &, but the function must not modify the objects passed to it. | |||||||||
Return value
number of elements satisfying the condition.
Complexity
linear in the distance between first and last
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; }
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 << std::endl; std::cout << "number: " << target2 << " count: " << num_items2 << std::endl; return 0; }
Output:
number: 3 count: 2 number: 5 count: 0
