C# Arrays and Collections Explained — From Zero to Confident

using System;

class ArrayBasics
{
    static void Main()
    {
        // --- 1. Declaring and creating an array ---
        // We know there are exactly 5 weekdays, so an array is perfect here.
        string[] weekdays = new string[5];

        // Assign a value to each numbered slot (index starts at 0, not 1)
        weekdays[0] = "Monday";
        weekdays[1] = "Tuesday";
        weekdays[2] = "Wednesday";
        weekdays[3] = "Thursday";
        weekdays[4] = "Friday";

        // --- 2. Shorthand initialiser — declare and fill in one line ---
        int[] highScores = { 9800, 7650, 6200, 5100, 3400 };

        // --- 3. Reading a single value by its index ---
        Console.WriteLine("First weekday: " + weekdays[0]);   // Monday
        Console.WriteLine("Top score: " + highScores[0]);     // 9800
        Console.WriteLine("Third score: " + highScores[2]);   // 6200 (index 2 = 3rd item)

        // --- 4. Looping over every item with foreach ---
        Console.WriteLine("\n--- All Weekdays ---");
        foreach (string day in weekdays)
        {
            // 'day' is a temporary variable that holds the current item each loop
            Console.WriteLine(day);
        }

        // --- 5. Looping with index using a for loop (useful when you need the position) ---
        Console.WriteLine("\n--- High Score Leaderboard ---");
        for (int position = 0; position < highScores.Length; position++)
        {
            // position + 1 because humans count from 1, not 0
            Console.WriteLine($"#{position + 1}: {highScores[position]} points");
        }

        // --- 6. Array.Length tells you how many slots exist ---
        Console.WriteLine($"\nTotal scores stored: {highScores.Length}");
    }
}

using System;
using System.Collections.Generic; // Required to use List<T>

class ListBasics
{
    static void Main()
    {
        // --- 1. Creating an empty List and adding items dynamically ---
        List<string> shoppingCart = new List<string>();

        shoppingCart.Add("Apples");     // Adds to the end of the list
        shoppingCart.Add("Bread");
        shoppingCart.Add("Milk");
        shoppingCart.Add("Cheese");

        Console.WriteLine($"Items in cart: {shoppingCart.Count}"); // Count, not Length

        // --- 2. Initialiser syntax — create with starting values ---
        List<int> playerScores = new List<int> { 450, 820, 310, 990, 670 };

        // --- 3. Access by index — works just like an array ---
        Console.WriteLine("First cart item: " + shoppingCart[0]); // Apples

        // --- 4. Check if an item exists before acting on it ---
        if (shoppingCart.Contains("Milk"))
        {
            Console.WriteLine("Milk is in the cart.");
        }

        // --- 5. Remove a specific item by value ---
        shoppingCart.Remove("Bread"); // Finds and removes the first matching item
        Console.WriteLine($"After removing Bread: {shoppingCart.Count} items");

        // --- 6. Insert at a specific position (index 1 = second slot) ---
        shoppingCart.Insert(1, "Butter");

        // --- 7. Iterate with foreach — cleanest way to read all items ---
        Console.WriteLine("\n--- Final Shopping Cart ---");
        foreach (string item in shoppingCart)
        {
            Console.WriteLine("  - " + item);
        }

        // --- 8. Sort the list in place ---
        playerScores.Sort();
        Console.WriteLine("\n--- Sorted Scores (low to high) ---");
        foreach (int score in playerScores)
        {
            Console.Write(score + " ");
        }
        Console.WriteLine();

        // --- 9. Convert a List back to an array if ever needed ---
        string[] cartArray = shoppingCart.ToArray();
        Console.WriteLine($"\nArray version has {cartArray.Length} elements.");
    }
}

using System;
using System.Collections.Generic;

class DictionaryBasics
{
    static void Main()
    {
        // --- 1. Creating a Dictionary: key is country name, value is its capital ---
        Dictionary<string, string> capitalCities = new Dictionary<string, string>();

        // Add key-value pairs using Add()
        capitalCities.Add("France", "Paris");
        capitalCities.Add("Japan", "Tokyo");
        capitalCities.Add("Brazil", "Brasília");
        capitalCities.Add("Canada", "Ottawa");

        // --- 2. Initialiser syntax — cleaner when you know values upfront ---
        Dictionary<string, int> playerHighScores = new Dictionary<string, int>
        {
            { "Alice",   9800 },
            { "Bob",     7200 },
            { "Charlie", 8450 }
        };

        // --- 3. Look up a value by its key ---
        string japanCapital = capitalCities["Japan"]; // Returns "Tokyo"
        Console.WriteLine("Capital of Japan: " + japanCapital);

        // --- 4. Safe lookup with TryGetValue — prevents crashes on missing keys ---
        string countryToFind = "Germany";
        if (capitalCities.TryGetValue(countryToFind, out string capital))
        {
            Console.WriteLine($"Capital of {countryToFind}: {capital}");
        }
        else
        {
            // This branch runs because Germany is not in our dictionary
            Console.WriteLine($"{countryToFind} is not in the dictionary.");
        }

        // --- 5. Update a value — just assign to the key directly ---
        playerHighScores["Bob"] = 8100; // Bob beat his old score
        Console.WriteLine($"Bob's updated score: {playerHighScores["Bob"]}");

        // --- 6. Check if a key exists before adding to avoid duplicate key exception ---
        string newPlayer = "Alice";
        if (!playerHighScores.ContainsKey(newPlayer))
        {
            playerHighScores.Add(newPlayer, 5000);
        }
        else
        {
            Console.WriteLine($"{newPlayer} already has a score on record.");
        }

        // --- 7. Loop over all key-value pairs using KeyValuePair ---
        Console.WriteLine("\n--- All Player High Scores ---");
        foreach (KeyValuePair<string, int> entry in playerHighScores)
        {
            // entry.Key is the player name, entry.Value is their score
            Console.WriteLine($"  {entry.Key}: {entry.Value} points");
        }

        // --- 8. Loop over just the keys or just the values ---
        Console.WriteLine("\n--- Countries in our dictionary ---");
        foreach (string country in capitalCities.Keys)
        {
            Console.WriteLine("  " + country);
        }

        Console.WriteLine($"\nTotal entries: {capitalCities.Count}");
    }
}

using System;
using System.Collections.Generic;

class ChoosingCollections
{
    static void Main()
    {
        // SCENARIO 1: Fixed data that never changes → use an ARRAY
        // The number of months in a year is always 12.
        string[] monthNames = { "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                                 "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };
        Console.WriteLine("Month 3: " + monthNames[2]); // Index 2 = March

        // SCENARIO 2: Growing or shrinking list of items → use a LIST
        // A to-do list can have any number of tasks.
        List<string> todoList = new List<string>();
        todoList.Add("Buy groceries");
        todoList.Add("Book dentist");
        todoList.Add("Reply to emails");
        todoList.Remove("Book dentist"); // Done!

        Console.WriteLine("\n--- To-Do List ---");
        foreach (string task in todoList)
        {
            Console.WriteLine("  [ ] " + task);
        }

        // SCENARIO 3: Look up by a meaningful key → use a DICTIONARY
        // Product catalogue: look up price by product code instantly.
        Dictionary<string, decimal> productCatalogue = new Dictionary<string, decimal>
        {
            { "SKU-001", 29.99m },
            { "SKU-002", 14.50m },
            { "SKU-003", 59.00m }
        };

        string scannedCode = "SKU-002";
        if (productCatalogue.TryGetValue(scannedCode, out decimal price))
        {
            Console.WriteLine($"\nProduct {scannedCode} costs £{price:F2}");
        }

        // BONUS: HashSet — when you only want UNIQUE values (no duplicates allowed)
        HashSet<string> uniqueTags = new HashSet<string>();
        uniqueTags.Add("csharp");
        uniqueTags.Add("dotnet");
        uniqueTags.Add("csharp"); // Duplicate — silently ignored, no error thrown
        Console.WriteLine($"\nUnique tags count: {uniqueTags.Count}"); // 2, not 3
    }
}