count_if
Returns the number of elements that match a criterion
Definition
The count_if() algorithm is defined in the standard header <algorithm> and in the nonstandard backward-compatibility header <algo.h>.
Interface
#include <algorithm>
template < class InputIterator, class Predicate >
typename iterator_traits<InputIterator>::difference_type count_if(
InputIterator first,
InputIterator last,
Predicate pred
);Parameters:
| Parameter | Description |
| first | An input iterator addressing the position of the first element in the range to be searched |
| last | An input iterator addressing the position one past the final element in the range to be searched |
| pred | User-defined predicate function object that defines the condition to be satisfied if an element is to be counted. A predicate takes single argument and returns true or false |
Description
The count_if algorithm counts the element in a range using a predicate instead of operator==.
Return Value
The return value is the number of elements that satisfy the condition specified by the predicate.
Complexity
The complexity is linear, there are last - first applications of pred.
References
Example 1
Problem
Here is a simple program which illustrates how to use count_if() algorithm.
Workings
#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;
bool greater10(int value)
{
return value >10;
}
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 result1;
result1 = count_if(v1.begin(), v1.end(), greater10);
cout <<"The number of elements in v1 greater than 10 is: "
<<result1<<"."<<endl;
return 0;
}Solution
Output:
v1 = ( 10 20 10 40 10 )
The number of elements in v1 greater than 10 is: 2.
References