std::greater in C++ with examples

Hello learners, today in this tutorial we will learn about std:: greater method of STL library in C++. std:: greater is a functional object. It is used for changing the functionality of the given function. Let’s understand this in more detail using easy and comprehensible examples.

std:: greater is used for performing a comparison between the two arguments passed. It is also defined as the functional object class for greater( > ) or equal-to( = ) inequality comparison. ( >=).

It is defined in the functional header file as:

template<class T > struct greater{
    bool operator() (const T& a, T& b) const{ return  a > b ; }
    typedef  T  first_argument_type;
    typedef  T  second_argument_type;
    typedef bool result_type;
};

In this, the member function is a bool operator() (const T& a, const T& b) Member function compares the two arguments based on which it returns the result.

std::greater is used in many standard algorithms. Let’s see one of them.

Example 1: Sorting using std:: greater.

Here is the Code:

#include<bits/stdc++.h>
#include<functional>
using namespace std;

int main()
{
    int arr[] = { 44, 1, 76, 20, 86, 19, 5 };

    int size = sizeof(arr)/sizeof(arr[0]);

    std::cout << "Input Array: "<< "{ 44 1 76 20 86 19 5}" << std::endl;

    // Sort array in descending order
    // using greater functional object

    sort(arr, arr + size, greater<int>());

     std::cout << "Output Array: {" ;

    // for range-based loop

    for (int i : arr){
        std::cout << i << " ";
    }

    std::cout << "}" << std::endl;
    return 0;
}

Output:
Input Array: { 44 1 76 20 86 19 5}
Output Array: {86 76 44 20 19 5 1 }

Example 2: Creating a Min_Heap using priority_queue.

Explanation: Generally priority_queue is used for creating Max_Heap but by using the greater <int>() function we can create Min_Heap.

Here is the Code:

#include<bits/stdc++.h>
using namespace std;

int main()
{
    // Create Min_Heap

    priority_queue<int, vector<int>, greater<int>> p_queue;

    p_queue.push(10);
    p_queue.push(5);
    p_queue.push(87);
    p_queue.push(22);
    p_queue.push(1);
    p_queue.push(43);
    p_queue.push(101);


    std::cout << "Priority Queue is : " ;

    while( !p_queue.empty() )
    {
        std::cout << p_queue.top() << " ";

        // remove top element
        // so that top points to next element
        p_queue.pop();
    }

    return 0;
}

Output:
Priority Queue is : 1  5  10  22  43  87  101

Hope this article was helpful to you. Keep Coding Keep Learning.