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std::count, std::count_if

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C++
 
Algorithm Bibliothek
Funktionen
Original:
Functions
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Nicht-modifizierende Sequenz Operationen
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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
Modifizierende Sequenz Operationen
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Modifying sequence operations
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Partitionierungsoperationen
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Partitioning operations
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Sortierung Operationen (auf sortierten Bereiche)
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Sorting operations (on sorted ranges)
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Binary Suchaktionen (auf sortierten Bereiche)
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Binary search operations (on sorted ranges)
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Set-Operationen (auf sortierten Bereiche)
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Set operations (on sorted ranges)
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Heap-Operationen
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Heap operations
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Minimum / Maximum Operationen
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Minimum/maximum operations
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Numerische Operationen
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Numeric operations
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C-Bibliothek
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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)
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 .
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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Inhaltsverzeichnis

[Bearbeiten] Parameter

first, last -
das Spektrum der Elemente zu untersuchen
Original:
the range of elements to examine
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value -
der Wert für die Suche
Original:
the value to search for
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p - unary predicate which returns ​true
für die erforderlichen Elemente
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.

[Bearbeiten] Rückgabewert

Anzahl der Elemente, die der Bedingung .
Original:
number of elements satisfying the condition.
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[Bearbeiten] Komplexität

genau last - first Vergleiche / Anwendungen des Prädikats
Original:
exactly last - first comparisons / applications of the predicate
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[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

Der folgende Code verwendet count zu bestimmen, wie viele Zahlen in einer std::vector überein einen Zielwert .
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

Dieses Beispiel verwendet eine Lambda-Ausdruck zu zählen Elemente durch 3 teilbar .
Original:
This example uses a Lambda-Ausdruck 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
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