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count

Count appearances of value in range

Definition

The count() algorithm is defined in the standard header <algorithm> and in the nonstandard backward-compatibility header <algo.h>.

Interface

#include <algorithm>
template < class InputIterator, class Type>
typename iterator_traits<InputIterator>::difference_type count(
InputIterator first,
InputIterator last,
const Type& val
);

Parameters:

Parameter Description
first An input iterator addressing the position of the first element in the range to be traversed
last An input iterator addressing the position one past the final element in the range to be traversed
val The value of the elements to be counted

Description

Count the number of values in an input range [first,last) that equal val.

Return Value

Count algorithm returns the number of iterators i in [first, last) such that *i == value.

Complexity

The complexity is linear. Exactly last - first comparisons.

Example:

Example - count algorithm
Problem
This short program illustrates the functionality of count() algorithm.
Workings
#include <vector>
#include <algorithm>
#include <iostream>

using namespace std;

int main()
{
vector<int> v1;
vector<int>::iterator Iter;

v1.push_back(10);
v1.push_back(20);
v1.push_back(10);
v1.push_back(40);
v1.push_back(10);

cout <<"v1 = ( " ;
for (Iter = v1.begin(); Iter != v1.end(); Iter++)
cout <<*Iter<<" ";
cout <<")"<<endl;

vector<int>::iterator::difference_type result;
result = count(v1.begin(), v1.end(), 10);
cout <<"The number of 10s in v2 is: "<<result<<"."<<endl;

return 0;
}
Solution
Output:

v1 = ( 10 20 10 40 10 )
The number of 10s in v2 is: 3.