std::count, std::count_if
Aus cppreference.com
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| definiert 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 .
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:
The signature does not need to have |
| Type requirements | ||
-InputIt must meet the requirements of InputIterator.
| ||
Rückgabewert
Anzahl der Elemente, die der Bedingung entsprechen.
Komplexität
genau last - first Vergleiche / Anwendungen des Prädikats.
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;
}
|
Beispiel
Der folgende Code verwendet
count um zu bestimmen, wie viele Zahlen in einem std::vector mit einem Zielwert übereinstimmen.
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 einen Lambda-Ausdruck, um die durch 3 teilbaren Elemente zu zählen.
Original:
This example uses a Lambda-Ausdruck 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