Count the Subarrays

// C++ implementation of the above approach
#include <bits/stdc++.h>
#define maxLen 30
using namespace std;

// Function to find required value
int findCnt(int arr[], int n, int k)
{
// Variable to store final answer
int ans = 0;

// Loop to find prefix-sum 
for (int i = 1; i < n; i++) { 
	arr[i] += arr[i - 1]; 
	if (arr[i] > k or arr[i] < -1 * k) 
		ans++; 
} 

if (arr[0] > k || arr[0] < -1 * k) 
	ans++; 

// Sorting prefix-sum array 
sort(arr, arr + n); 

// Loop to find upper_bound 
// for each element 
for (int i = 0; i < n; i++) 
	ans += n - 
(upper_bound(arr, arr + n, arr[i] + k) - arr); 

// Returning final answer 
return ans; 

}

// Driver code
int main()
{
int arr[] = { -1, 4, -5, 6 };
int n = sizeof(arr) / sizeof(int);
int k = 0;

// Function to find required value 
cout << findCnt(arr, n, k); 

}

I don’t understand why we use upper_bound in this code and what we get from it

https://www.geeksforgeeks.org/number-subarrays-sum-exactly-equal-k/ and refer
https://www.youtube.com/watch?v=HbbYPQc-Oo4

Now, I will undestand ,
Thank you.