C++ Algorithm count_if function has a 'pred' value and returns the count of the elements in the range [first,last) for which the value of pred is true.
Syntax
Example
template <class InputIterator, class UnaryPredicate>
typename iterator_traits<InputIterator>::difference_type count_if(InputIterator first, InputIterator last,UnaryPredicate pred);
Parameter
first : It is an input iterator to the first element in the range.
last : It is an input iterator to the last element in the range.
val : It is the element whose occurrence is being searched in the range.
Return value
The function returns the number of element in the range [first,last) for which the value of pred is true.
Example 1
Example
#include<iostream>
#include<algorithm>
#include<vector>
bool isOdd(int k)
{
return((k%2)==1);
}
int main()
{
std::vector<int> newvector;
for(int k=1; k<10; k++)
newvector.push_back(k);
int newcount=count_if(newvector.begin(),newvector.end(),isOdd);
std::cout<<"newvector contains "<<newcount<<" odd values.\n";
return 0;
}
Output:
Output
newvector contains 5 odd values.
Example 2
Example
#include<bits/stdc++.h>
using namespace std;
bool isEven(int k)
{
if(k%2==0)
return true;
}
int main()
{
vector<int> u;
for(int i=0; i<10; i++)
{
u.push_back(i);
}
int noEven=count_if(u.begin(),u.end(),isEven);
cout<<"Count of even number is:"<<noEven;
return 0;
}
Output:
Output
Count of even number is: 10
Complexity
The complexity of the function is linear up to a distance between first and last element.
Data races
Some or all of the elements of the range are accessed
Exceptions
The function throws an exception if any of the argument throws one.