std::count, std::count_if
Defined in header <algorithm>
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template< class InputIt, class T > typename iterator_traits<InputIt>::difference_type |
(1) | |
template< class InputIt, class UnaryPredicate > typename iterator_traits<InputIt>::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 |
[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. |
Type requirements | ||
-InputIt must meet the requirements of InputIterator .
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[edit] Return value
number of elements satisfying the condition.
[edit] Complexity
exactly last
- first
comparisons / applications of the predicate
[edit] Notes
For the number of elements in the range [first, last)
without any additional criteria, see std::distance.
[edit] Possible implementation
First version |
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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
The following code uses count
to determine how many integers in a std::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 a lambda expression 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
[edit] See also
(parallelism TS) |
parallelized version of std::count (function template) |
(parallelism TS) |
parallelized version of std::count_if (function template) |