01_Dictionary.cs

using System;
using System.Collections.Generic;

namespace DataStructures
{
    /// <summary>
    /// Demonstrates the Dictionary<TKey, TValue> collection in C#.
    /// A dictionary stores key-value pairs and provides O(1) average-time lookups by key.
    /// 
    /// Common use cases: phone books, configuration settings, caching, data lookups.
    /// </summary>
    public class DictionaryExamples
    {
        /// <summary>
        /// Simple Employee (Personel) class used to demonstrate value-type storage in dictionaries.
        /// </summary>
        public class Employee
        {
            public int Id { get; set; }
            public string FullName { get; set; }
            public string Department { get; set; }

            public Employee(int id, string fullName, string department)
            {
                Id = id;
                FullName = fullName;
                Department = department;
            }

            public override string ToString()
            {
                return $"[{Id}] {FullName} — {Department}";
            }
        }

        // ─── Entry Point ───────────────────────────────────────────────
        public static void Main()
        {
            Console.WriteLine("═══════════════════════════════════════════════");
            Console.WriteLine("       C# Dictionary<TKey, TValue> Examples    ");
            Console.WriteLine("═══════════════════════════════════════════════\n");

            // ── 1. City Area-Code Lookup (int key → string value) ───────
            // Maps international city names to their telephone area codes.
            var phoneCodes = new Dictionary<int, string>
            {
                { 212, "New York" },
                { 213, "Los Angeles" },
                { 312, "Chicago" },
                { 415, "San Francisco" },
                { 202, "Washington D.C." },
                { 617, "Boston" },
                { 713, "Houston" },
                { 206, "Seattle" }
            };

            Console.WriteLine("── City Phone Codes ──────────────────────────");
            foreach (var entry in phoneCodes)
            {
                Console.WriteLine($"  Area code {entry.Key,4} → {entry.Value}");
            }
            Console.WriteLine();

            // ── 2. Adding & Removing Entries ────────────────────────────
            Console.WriteLine("── Add / Remove Operations ───────────────────");

            // Adding a new key-value pair
            phoneCodes.Add(305, "Miami");
            Console.WriteLine("  Added: 305 → Miami");

            // Removing an entry by key
            phoneCodes.Remove(213);
            Console.WriteLine("  Removed: 213 (Los Angeles)");

            // Safely adding with TryAdd (.NET Core 2.0+)
            bool added = phoneCodes.TryAdd(305, "Miami Again"); // won't add — key exists
            Console.WriteLine($"  TryAdd 305 again: succeeded = {added}");
            Console.WriteLine();

            // ── 3. Lookup: ContainsKey & ContainsValue ──────────────────
            Console.WriteLine("── Lookup Operations ─────────────────────────");
            Console.WriteLine($"  ContainsKey(312)    : {phoneCodes.ContainsKey(312)}");
            Console.WriteLine($"  ContainsKey(999)    : {phoneCodes.ContainsKey(999)}");
            Console.WriteLine($"  ContainsValue(\"Boston\"): {phoneCodes.ContainsValue("Boston")}");
            Console.WriteLine($"  ContainsValue(\"Paris\")  : {phoneCodes.ContainsValue("Paris")}");

            // TryGetValue — avoids KeyNotFoundException
            if (phoneCodes.TryGetValue(202, out string city))
            {
                Console.WriteLine($"  TryGetValue(202)   : {city}");
            }
            Console.WriteLine();

            // ── 4. Iterating with foreach ───────────────────────────────
            Console.WriteLine("── Iterating All Entries ──────────────────────");
            foreach (KeyValuePair<int, string> kvp in phoneCodes)
            {
                Console.WriteLine($"  {kvp.Key,4}  ↔  {kvp.Value}");
            }
            Console.WriteLine();

            // ── 5. Dictionary with Custom Object Values ────────────────
            Console.WriteLine("── Employee Directory (int → Employee) ───────");
            var employees = new Dictionary<int, Employee>
            {
                { 1001, new Employee(1001, "Alice Johnson",   "Engineering") },
                { 1002, new Employee(1002, "Bob Smith",       "Marketing") },
                { 1003, new Employee(1003, "Charlie Davis",   "Engineering") },
                { 1004, new Employee(1004, "Diana Prince",    "Human Resources") }
            };

            foreach (var emp in employees.Values)
            {
                Console.WriteLine($"  {emp}");
            }
            Console.WriteLine();

            // ── 6. Useful Properties ────────────────────────────────────
            Console.WriteLine("── Dictionary Properties ─────────────────────");
            Console.WriteLine($"  Total phone codes : {phoneCodes.Count}");
            Console.WriteLine($"  Total employees   : {employees.Count}");
            Console.WriteLine($"  Keys (phone codes): {string.Join(", ", phoneCodes.Keys)}");
            Console.WriteLine($"  Values (cities)   : {string.Join(", ", phoneCodes.Values)}");

            Console.WriteLine("\n═══════════════════════════════════════════════");
            Console.WriteLine("  Complexity: Add O(1) | Remove O(1) | Lookup O(1)");
            Console.WriteLine("  (Average case; worst case O(n) due to hash collisions)");
            Console.WriteLine("═══════════════════════════════════════════════");
        }
    }
}