C++ Algorithm Function count_if ()

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

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

#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:

newvector contains 5 odd values.

Example 2

#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: