std::count, std::count_if
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| Elemento definito nell'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) | |
Restituisce il numero di elementi nell'intervallo
[first, last) soddisfano criteri specifici. La prima versione conta gli elementi che sono uguali a value, la seconda versione conta elementi per i quali predicato p ritorna true. Original:
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. The text has been machine-translated via Google Translate.
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Indice |
[modifica] Parametri
| first, last | - | la gamma di elementi da esaminare
Original: the range of elements to examine The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. |
| value | - | il valore da cercare
Original: the value to search for The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. |
| p | - | unary predicate which returns true per gli elementi richiesti . Original: for the required elements The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. 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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[modifica] Valore di ritorno
numero di elementi che soddisfano la condizione.
Original:
number of elements satisfying the condition.
The text has been machine-translated via Google Translate.
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[modifica] Complessità
esattamente
last - first confronto / applicazioni del predicatoOriginal:
exactly
last - first comparisons / applications of the predicateThe text has been machine-translated via Google Translate.
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[modifica] Possibile implementazione
| 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; } |
[modifica] Esempio
Il codice seguente
count utilizza per determinare quanti numeri interi in un std::vector corrisponde un valore obiettivo .
Original:
The following code uses
count to determine how many integers in a std::vector match a target value.
The text has been machine-translated via Google Translate.
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#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
Questo esempio utilizza un lambda espressione per contare elementi divisibile per 3 .
Original:
This example uses a lambda espressione to count elements divisible by 3.
The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.
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
