## C# || How To Find Friends Of Appropriate Ages Using C#

The following is a module with functions which demonstrates how to find friends of appropriate ages using C#.

1. Num Friend Requests – Problem Statement

There are **n** persons on a social media website. You are given an integer array **ages** where **ages[i]** is the age of the **i ^{th}** person.

A Person **x** will not send a friend request to a person **y** (**x != y**) if any of the following conditions is true:

**age[y] <= 0.5 * age[x] + 7****age[y] > age[x]****age[y] > 100 && age[x] < 100**

Otherwise, **x** will send a friend request to **y**.

Note that if **x** sends a request to **y**, **y** will not necessarily send a request to **x**. Also, a person will not send a friend request to themself.

Return *the total number of friend requests made*.

**Example 1:**

Input: ages = [16,16]

Output: 2

Explanation: 2 people friend request each other.

**Example 2:**

Input: ages = [16,17,18]

Output: 2

Explanation: Friend requests are made 17 -> 16, 18 -> 17.

**Example 3:**

Input: ages = [20,30,100,110,120]

Output: 3

Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100.

2. Num Friend Requests – Solution

The following is a solution which demonstrates how to find friends of appropriate ages.

In this solution, we sort the ages and use a map that keeps track of the ages, and the total friend request count that age can recieve.

Binary search is used to get the age range in the array that satisfies the friend request conditions for a given age.

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// ============================================================================// Author: Kenneth Perkins// Date: Nov 5, 2021// Taken From: http://programmingnotes.org/// File: Solution.cs// Description: Demonstrates how to find friends of appropriate ages// ============================================================================public class Solution { public int NumFriendRequests(int[] ages) { var result = 0; // Sort ages Array.Sort(ages); // Keep track of results calculated var seen = new Dictionary<int, int>(); // Iterate array backwards for (int index = ages.Length - 1; index >= 0; --index) { // Check if we already calculated result for age if (!seen.ContainsKey(ages[index])) { // Binary search var lo = 0; var hi = index; while (lo < hi) { var mid = lo + (hi - lo) / 2; // Check if friends can be made in this range if (!CanFriend(ages[index], ages[mid])) { lo = mid + 1; } else { hi = mid; } } // Get distance between index and hi. This is the friend count for the age seen[ages[index]] = index - hi; } // Update result count result += seen[ages[index]]; } return result; } private bool CanFriend(int x, int y) { return !((y > x) || (y > 100 && x < 100) || (y <= 0.5 * x + 7)); }}// http://programmingnotes.org/

**QUICK NOTES**:

The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Once compiled, you should get this as your output for the example cases:

2

2

3

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