C++ boost::range::count_if














































C++ boost::range::count_if



Description:
The header 'boost/range/algorithm/count_if.hpp'   contains the algorithm count_if() in C++ boost library. This function is very similar to boost::range::count() the only difference in this version is that this function uses unary predicate which returns a value convertible to bool. The value returned indicates whether the element of the sequence has to be counted by the function.

Syntax:
template<class SinglePassRange, class UnaryPredicate>
typename range_difference<const SinglePassRange>::type count_if(const SinglePassRange& rng, UnaryPredicate pred);

Parameters:
The following parameters are accepted by the function:
1.) rng: It specifies the sequence which has to be reversed.
2.) pred: Unary predicate which accepts each elements of the sequence and returns the Boolean value where true indicates the element has to be counted.


Program:
#include <boost/range/algorithm/count_if.hpp>
#include <iostream>
#include <vector>
using namespace std;


int main()
{
    vector<int> Arr = {222, 819, 619, 7887, 6116, 535, 42};
    // Predicate to check palindrome numbers. Returns true if element is palindrome
    auto is_palindrome = [](int element)
    {
        string str1 = to_string(element), str2 = to_string(element);
        reverse(str1.begin(), str1.end());
        return str1 == str2;
    };
    // count_if to check total number of palindrome numbers in array.
    int cnt = boost::range::count_if(Arr, is_palindrome);

    cout << "There are " << cnt << " palindrome numbers in array.";
}
Output:
There are 4 palindrome numbers in array.

Complexity:
Time Complexity: The time complexity is distance(rng) number of invocations of predicate pred.

Please write on comment section if you find any mistake or for any suggestions.

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