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std::count, std::count_if

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Bibliothèque d'algorithmes
Fonctions
Original:
Functions
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Non-modification de la séquence des opérations
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
Modification de la séquence des opérations
Original:
Modifying sequence operations
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Des opérations de partitionnement
Original:
Partitioning operations
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Opérations de tri (sur les gammes triés)
Original:
Sorting operations (on sorted ranges)
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is_sorted (C++11)
is_sorted_until (C++11)
sort
Opérations binaires de recherche (sur les gammes triés)
Original:
Binary search operations (on sorted ranges)
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Définir les opérations (sur les gammes triés)
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Set operations (on sorted ranges)
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Opérations Heap
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Heap operations
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Minimum / maximum de fonctionnement
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Minimum/maximum operations
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Opérations numériques
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Numeric operations
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Bibliothèque C
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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)
Retourne le nombre d'éléments dans la gamme [first, last) satisfaisant des critères spécifiques. La première version compte les éléments qui sont égaux à value, la deuxième version compte les éléments pour lesquels prédicat retourne p 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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Sommaire

[modifier] Paramètres

first, last -
l'éventail des éléments à examiner
Original:
the range of elements to examine
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value -
la valeur à rechercher
Original:
the value to search for
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p - unary predicate which returns ​true
pour les éléments requis
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.

[modifier] Retourne la valeur

certain nombre d'éléments satisfaisant la condition .
Original:
number of elements satisfying the condition.
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[modifier] Complexité

exactement last - first comparaisons / applications du prédicat
Original:
exactly last - first comparisons / applications of the predicate
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[modifier] Mise en œuvre possible

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;
}

[modifier] Exemple

Au code count utilise pour déterminer le nombre de nombres entiers dans un std::vector correspond à une valeur cible .
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';
}

Résultat :

number: 3 count: 2
number: 5 count: 0

Cet exemple utilise un expression lambda de compter des éléments divisible par 3 .
Original:
This example uses a expression lambda 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';
}

Résultat :

number divisible by three: 3