Namespace
Varianti

std::count, std::count_if

Da cppreference.com.
< cpp‎ | algorithm

 
 
Algoritmo libreria
Funzioni
Original:
Functions
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Non modifica le operazioni di sequenza
Original:
Non-modifying sequence operations
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all_of
any_of
none_of
(C++11)
(C++11)
(C++11)
for_each
count
count_if
mismatch
equal
Modifica delle operazioni di sequenza
Original:
Modifying sequence operations
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Partizionamento operazioni
Original:
Partitioning operations
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Ordinamento delle operazioni (su intervalli ordinati)
Original:
Sorting operations (on sorted ranges)
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Binarie (le operazioni di ricerca sui campi ordinati)
Original:
Binary search operations (on sorted ranges)
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Impostare le operazioni (su intervalli ordinati)
Original:
Set operations (on sorted ranges)
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Heap operazioni
Original:
Heap operations
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Minimo / massimo le operazioni
Original:
Minimum/maximum operations
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Operazioni numeriche
Original:
Numeric operations
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Libreria C
Original:
C library
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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 InputIt, class UnaryPredicate >

typename iterator_traits<InputIt>::difference_type

    count_if( InputIt first, InputIt last, UnaryPredicate p );
(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.
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Indice

[modifica] Parametri

first, last -
la gamma di elementi da esaminare
Original:
the range of elements to examine
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value -
il valore da cercare
Original:
the value to search for
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p - unary predicate which returns ​true
per gli elementi richiesti
Original:
for the required elements
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.

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.

[modifica] Valore di ritorno

numero di elementi che soddisfano la condizione.
Original:
number of elements satisfying the condition.
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[modifica] Complessità

esattamente last - first confronto / applicazioni del predicato
Original:
exactly last - first comparisons / applications of the predicate
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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.
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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.
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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 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