Math - Intersection of HashSet

Part of the Logic & Flow section of Coddy's C# journey — lesson 63 of 66.

The intersection of two sets contains only the elements that appear in both sets.

Create two HashSets:

HashSet<int> set1 = new HashSet<int>() { 1, 2, 3, 4, 5 };
HashSet<int> set2 = new HashSet<int>() { 3, 4, 5, 6, 7 };

To find the intersection, use the IntersectWith method:

set1.IntersectWith(set2);

After executing this code, set1 will contain:

{ 3, 4, 5 }

The IntersectWith method modifies the set it's called on, keeping only elements that exist in both sets.

To create a new set with the intersection without modifying the original sets, you can:

HashSet<int> intersection = new HashSet<int>(set1);
intersection.IntersectWith(set2);

Challenge

Medium

Create a method named FindCommonElements that takes two arguments:

A HashSet of integers (set1)
A HashSet of integers (set2)

The method should return a new HashSet containing only the elements that appear in both sets (the intersection).

Try it yourself

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

class Program {
    public static HashSet<int> FindCommonElements(HashSet<int> set1, HashSet<int> set2) {
        // Write your code here
        return null;
    }
    
    static void Main(string[] args) {
        // Read first line for first set
        string line1 = Console.ReadLine();
        HashSet<int> set1 = new HashSet<int>();
        
        // Check if first set is in JSON array format
        if (line1 != null && line1.StartsWith("[") && line1.EndsWith("]")) {
            try {
                // Extract content between square brackets
                string arrayContent = line1.Substring(1, line1.Length - 2);
                
                // Try to parse the JSON array content
                string[] values = Regex.Split(arrayContent, @",\s*");
                foreach (string value in values) {
                    // Remove any quotes if present
                    string cleanValue = value.Trim();
                    if (cleanValue.StartsWith("\"") && cleanValue.EndsWith("\"")) {
                        cleanValue = cleanValue.Substring(1, cleanValue.Length - 2);
                    }
                    
                    // Parse as integer and add to set
                    if (int.TryParse(cleanValue, out int num)) {
                        set1.Add(num);
                    }
                }
            }
            catch (Exception ex) {
                Console.WriteLine($"Error parsing first set: {ex.Message}");
                return;
            }
        }
        else {
            // Process traditional space-separated input for first set
            string[] input1 = line1.Split();
            foreach (string s in input1) {
                if (int.TryParse(s, out int num)) {
                    set1.Add(num);
                }
            }
        }
        
        // Read second line for second set
        string line2 = Console.ReadLine();
        HashSet<int> set2 = new HashSet<int>();
        
        // Check if second set is in JSON array format
        if (line2 != null && line2.StartsWith("[") && line2.EndsWith("]")) {
            try {
                // Extract content between square brackets
                string arrayContent = line2.Substring(1, line2.Length - 2);
                
                // Try to parse the JSON array content
                string[] values = Regex.Split(arrayContent, @",\s*");
                foreach (string value in values) {
                    // Remove any quotes if present
                    string cleanValue = value.Trim();
                    if (cleanValue.StartsWith("\"") && cleanValue.EndsWith("\"")) {
                        cleanValue = cleanValue.Substring(1, cleanValue.Length - 2);
                    }
                    
                    // Parse as integer and add to set
                    if (int.TryParse(cleanValue, out int num)) {
                        set2.Add(num);
                    }
                }
            }
            catch (Exception ex) {
                Console.WriteLine($"Error parsing second set: {ex.Message}");
                return;
            }
        }
        else {
            // Process traditional space-separated input for second set
            string[] input2 = line2.Split();
            foreach (string s in input2) {
                if (int.TryParse(s, out int num)) {
                    set2.Add(num);
                }
            }
        }
        
        // Find common elements
        HashSet<int> common = FindCommonElements(set1, set2);
        
        // Print result (sorted)
        Console.WriteLine(string.Join(" ", common.OrderBy(x => x)));
    }
}