std::count, std::count_if
| definiert in Header <algorithm>
|
||
| template< class InputIt, class T > typename iterator_traits<InputIt>::difference_type |
(1) | |
| template< class InputIt, class UnaryPredicate > typename iterator_traits<InputIt>::difference_type |
(2) | |
Gibt die Anzahl der Elemente im Bereich [first, last) erfüllen bestimmte Kriterien. Die erste Version zählt die Elemente, die gleich value sind, zählt die zweite Version Elemente, für die Prädikats p kehrt true .
Inhaltsverzeichnis |
[Bearbeiten] Parameter
| first, last | - | das Spektrum der Elemente zu untersuchen |
| value | - | der Wert für die Suche |
| p | - | unary predicate which returns true für die erforderlichen Elemente. 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.
| ||
[Bearbeiten] Rückgabewert
Anzahl der Elemente, die der Bedingung entsprechen.
[Bearbeiten] Komplexität
genau last - first Vergleiche / Anwendungen des Prädikats.
[Bearbeiten] Mögliche Implementierung
| 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; } |
[Bearbeiten] Beispiel
count um zu bestimmen, wie viele Zahlen in einem std::vector mit einem Zielwert übereinstimmen.
count to determine how many integers in a std::vector match a target value.
You can help to correct and verify the translation. Click here for instructions.
#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
You can help to correct and verify the translation. Click here for instructions.
#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
