Lesson 25.1 – Select a random monster at a location

A reader asked how to have several different monsters that could be at a location – and randomly select one when the player moves to that location.

This is a little complex, so I created this lesson, to show how.

Planning the change

Currently, the Location class has a MonsterLivingHere property, which holds a single Monster object (or nothing, if there isn’t a monster at that location).

We use that property in the World class, to set the monster at a location. We also use it in the UI code, to detect if there is a monster at the location – so we can hide, or show, the combat buttons. And, we use it to instantiate a new monster to fight – using the NewInstanceOfMonsterLivingHere function.

To allow different possible monsters, we need to change the PopulateLocation function in the World class, to let us add multiple monsters to a list, along with the percentage chance for them to appear at the location.

Then, in the UI, instead of checking that single Monster property, we need to check if there are any monsters in the Location’s monster list.

Finally, we need to change the NewInstanceOfMonsterLivingHere function to get a random monster from the list, when it creates the Monster object for the Player to fight.

Making the changes

Step 1: Change Location.cs

using System.Collections.Generic;
using System.Linq;
namespace Engine
{
    public class Location
    {
        private readonly SortedList<int, int> _monstersAtLocation = new SortedList<int, int>();
        public int ID { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
        public Item ItemRequiredToEnter { get; set; }
        public Quest QuestAvailableHere { get; set; }
        public Vendor VendorWorkingHere { get; set; }
        public Location LocationToNorth { get; set; }
        public Location LocationToEast { get; set; }
        public Location LocationToSouth { get; set; }
        public Location LocationToWest { get; set; }
        public bool HasAMonster { get { return _monstersAtLocation.Count > 0; } }
        public bool HasAQuest { get { return QuestAvailableHere != null; } }
        public bool DoesNotHaveAnItemRequiredToEnter { get { return ItemRequiredToEnter == null; } }
        public Location(int id, string name, string description, 
            Item itemRequiredToEnter = null, Quest questAvailableHere = null)
        {
            ID = id;
            Name = name;
            Description = description;
            ItemRequiredToEnter = itemRequiredToEnter;
            QuestAvailableHere = questAvailableHere;
        }
        public void AddMonster(int monsterID, int percentageOfAppearance)
        {
            if(_monstersAtLocation.ContainsKey(monsterID))
            {
                _monstersAtLocation[monsterID] = percentageOfAppearance;
            }
            else
            {
                _monstersAtLocation.Add(monsterID, percentageOfAppearance);
            }
        }
        public Monster NewInstanceOfMonsterLivingHere()
        {
            if(!HasAMonster)
            {
                return null;
            }
            // Total the percentages of all monsters at this location.
            int totalPercentages = _monstersAtLocation.Values.Sum();
            // Select a random number between 1 and the total (in case the total of percentages is not 100).
            int randomNumber = RandomNumberGenerator.NumberBetween(1, totalPercentages);
            // Loop through the monster list, 
            // adding the monster's percentage chance of appearing to the runningTotal variable.
            // When the random number is lower than the runningTotal,
            // that is the monster to return.
            int runningTotal = 0;
            
            foreach(KeyValuePair<int, int> monsterKeyValuePair in _monstersAtLocation)
            {
                runningTotal += monsterKeyValuePair.Value;
                if(randomNumber <= runningTotal)
                {
                    return World.MonsterByID(monsterKeyValuePair.Key).NewInstanceOfMonster();
                }
            }
            // In case there was a problem, return the last monster in the list.
            return World.MonsterByID(_monstersAtLocation.Keys.Last()).NewInstanceOfMonster();
        }
    }
}

We add a new private variable (_monstersAtLocation) that is a SortedList.

A SortedList is like a List, but lets you store two values per entry. These can be any types of values: strings, integers, your custom classes, etc.

For this SortedList, we will want to hold two integer values.

The first value is the “key”. You cannot have two items with the same key. The keys will be populated with the monster ID. So, we cannot add an entry to the SortedList that has the same key value (Monster ID) as an entry already in the SortedList.

The second value is the “value”. This is where we will store the percentage chance that this monster appears, when the Player moves to the Location.

To populate the SortedList, we add the “AddMonster” function.

This checks if the key (Monster ID) already exists in the SortedList, using the ContainsKey function. If the SortedList already has an entry with that key, it will replace the value for that entry with the new value. If there is no entry with the passed key, the function will add this entry to the variable.

Next, we add a new HasAMonster property, that checks if there are any entries in the _monstersAtLocation variable.

Before, when we wanted to check if there was a monster at a location, we used “MonsterLivingHere != null”. Since we’re replacing the MonsterLivingHere property with the new logic, we need a new way to check for monsters at a location.

The third step is to change the NewInstanceOfMonsterLivingHere function. Now, we need to randomly select one of the possible monsters, from the _monstersAtLocation variable, and create an instance of it.

This code sums all the percentages, and selects a random number that is less than, or equal to, that sum. This way, the function can handle situations when the total percentages do not equal 100.

Then, it loops through the list of possible monsters – adding the monster’s percentage chance of appearing to a running total variable.

If the random number is less than the running total, the function returns the monster. If not, it loops to the next monster in the list.

Finally, we can remove the MonsterLivingHere property – and its parameter in the constructor.

Step 2: Change World.cs

using System.Collections.Generic;
using System.Linq;
namespace Engine
{
    public static class World
    {
        private static readonly List<Item> _items = new List<Item>();
        private static readonly List<Monster> _monsters = new List<Monster>();
        private static readonly List<Quest> _quests = new List<Quest>();
        private static readonly List<Location> _locations = new List<Location>();
        public const int UNSELLABLE_ITEM_PRICE = -1;
        public const int ITEM_ID_RUSTY_SWORD = 1;
        public const int ITEM_ID_RAT_TAIL = 2;
        public const int ITEM_ID_PIECE_OF_FUR = 3;
        public const int ITEM_ID_SNAKE_FANG = 4;
        public const int ITEM_ID_SNAKESKIN = 5;
        public const int ITEM_ID_CLUB = 6;
        public const int ITEM_ID_HEALING_POTION = 7;
        public const int ITEM_ID_SPIDER_FANG = 8;
        public const int ITEM_ID_SPIDER_SILK = 9;
        public const int ITEM_ID_ADVENTURER_PASS = 10;
        public const int MONSTER_ID_RAT = 1;
        public const int MONSTER_ID_SNAKE = 2;
        public const int MONSTER_ID_GIANT_SPIDER = 3;
        public const int QUEST_ID_CLEAR_ALCHEMIST_GARDEN = 1;
        public const int QUEST_ID_CLEAR_FARMERS_FIELD = 2;
        public const int LOCATION_ID_HOME = 1;
        public const int LOCATION_ID_TOWN_SQUARE = 2;
        public const int LOCATION_ID_GUARD_POST = 3;
        public const int LOCATION_ID_ALCHEMIST_HUT = 4;
        public const int LOCATION_ID_ALCHEMISTS_GARDEN = 5;
        public const int LOCATION_ID_FARMHOUSE = 6;
        public const int LOCATION_ID_FARM_FIELD = 7;
        public const int LOCATION_ID_BRIDGE = 8;
        public const int LOCATION_ID_SPIDER_FIELD = 9;
        static World()
        {
            PopulateItems();
            PopulateMonsters();
            PopulateQuests();
            PopulateLocations();
        }
        private static void PopulateItems()
        {
            _items.Add(new Weapon(ITEM_ID_RUSTY_SWORD, "Rusty sword", "Rusty swords", 0, 5, 5));
            _items.Add(new Item(ITEM_ID_RAT_TAIL, "Rat tail", "Rat tails", 1));
            _items.Add(new Item(ITEM_ID_PIECE_OF_FUR, "Piece of fur", "Pieces of fur", 1));
            _items.Add(new Item(ITEM_ID_SNAKE_FANG, "Snake fang", "Snake fangs", 1));
            _items.Add(new Item(ITEM_ID_SNAKESKIN, "Snakeskin", "Snakeskins", 2));
            _items.Add(new Weapon(ITEM_ID_CLUB, "Club", "Clubs", 3, 10, 8));
            _items.Add(new HealingPotion(ITEM_ID_HEALING_POTION, "Healing potion", "Healing potions", 5, 3));
            _items.Add(new Item(ITEM_ID_SPIDER_FANG, "Spider fang", "Spider fangs", 1));
            _items.Add(new Item(ITEM_ID_SPIDER_SILK, "Spider silk", "Spider silks", 1));
            _items.Add(new Item(ITEM_ID_ADVENTURER_PASS, "Adventurer pass", "Adventurer passes", UNSELLABLE_ITEM_PRICE));
        }
        private static void PopulateMonsters()
        {
            Monster rat = new Monster(MONSTER_ID_RAT, "Rat", 5, 3, 10, 3, 3);
            rat.LootTable.Add(new LootItem(ItemByID(ITEM_ID_RAT_TAIL), 75, false));
            rat.LootTable.Add(new LootItem(ItemByID(ITEM_ID_PIECE_OF_FUR), 75, true));
            Monster snake = new Monster(MONSTER_ID_SNAKE, "Snake", 5, 3, 10, 3, 3);
            snake.LootTable.Add(new LootItem(ItemByID(ITEM_ID_SNAKE_FANG), 75, false));
            snake.LootTable.Add(new LootItem(ItemByID(ITEM_ID_SNAKESKIN), 75, true));
            Monster giantSpider = new Monster(MONSTER_ID_GIANT_SPIDER, "Giant spider", 20, 5, 40, 10, 10);
            giantSpider.LootTable.Add(new LootItem(ItemByID(ITEM_ID_SPIDER_FANG), 75, true));
            giantSpider.LootTable.Add(new LootItem(ItemByID(ITEM_ID_SPIDER_SILK), 25, false));
            _monsters.Add(rat);
            _monsters.Add(snake);
            _monsters.Add(giantSpider);
        }
        private static void PopulateQuests()
        {
            Quest clearAlchemistGarden =
                new Quest(
                    QUEST_ID_CLEAR_ALCHEMIST_GARDEN,
                    "Clear the alchemist's garden",
                    "Kill rats in the alchemist's garden and bring back 3 rat tails. You will receive a healing potion and 10 gold pieces.", 20, 10);
            clearAlchemistGarden.QuestCompletionItems.Add(new QuestCompletionItem(ItemByID(ITEM_ID_RAT_TAIL), 3));
            clearAlchemistGarden.RewardItem = ItemByID(ITEM_ID_HEALING_POTION);
            Quest clearFarmersField =
                new Quest(
                    QUEST_ID_CLEAR_FARMERS_FIELD,
                    "Clear the farmer's field",
                    "Kill snakes in the farmer's field and bring back 3 snake fangs. You will receive an adventurer's pass and 20 gold pieces.", 20, 20);
            clearFarmersField.QuestCompletionItems.Add(new QuestCompletionItem(ItemByID(ITEM_ID_SNAKE_FANG), 3));
            clearFarmersField.RewardItem = ItemByID(ITEM_ID_ADVENTURER_PASS);
            _quests.Add(clearAlchemistGarden);
            _quests.Add(clearFarmersField);
        }
        private static void PopulateLocations()
        {
            // Create each location
            Location home = new Location(LOCATION_ID_HOME, "Home", "Your house. You really need to clean up the place.");
            Location townSquare = new Location(LOCATION_ID_TOWN_SQUARE, "Town square", "You see a fountain.");
            Vendor bobTheRatCatcher = new Vendor("Bob the Rat-Catcher");
            bobTheRatCatcher.AddItemToInventory(ItemByID(ITEM_ID_PIECE_OF_FUR), 5);
            bobTheRatCatcher.AddItemToInventory(ItemByID(ITEM_ID_RAT_TAIL), 3);
            townSquare.VendorWorkingHere = bobTheRatCatcher;
            Location alchemistHut = new Location(LOCATION_ID_ALCHEMIST_HUT, "Alchemist's hut", "There are many strange plants on the shelves.");
            alchemistHut.QuestAvailableHere = QuestByID(QUEST_ID_CLEAR_ALCHEMIST_GARDEN);
            Location alchemistsGarden = new Location(LOCATION_ID_ALCHEMISTS_GARDEN, "Alchemist's garden", "Many plants are growing here.");
            alchemistsGarden.AddMonster(MONSTER_ID_RAT, 100);
            Location farmhouse = new Location(LOCATION_ID_FARMHOUSE, "Farmhouse", "There is a small farmhouse, with a farmer in front.");
            farmhouse.QuestAvailableHere = QuestByID(QUEST_ID_CLEAR_FARMERS_FIELD);
            Location farmersField = new Location(LOCATION_ID_FARM_FIELD, "Farmer's field", "You see rows of vegetables growing here.");
            farmersField.AddMonster(MONSTER_ID_SNAKE, 100);
            Location guardPost = new Location(LOCATION_ID_GUARD_POST, "Guard post", "There is a large, tough-looking guard here.", ItemByID(ITEM_ID_ADVENTURER_PASS));
            Location bridge = new Location(LOCATION_ID_BRIDGE, "Bridge", "A stone bridge crosses a wide river.");
            Location spiderField = new Location(LOCATION_ID_SPIDER_FIELD, "Forest", "You see spider webs covering covering the trees in this forest.");
            spiderField.AddMonster(MONSTER_ID_GIANT_SPIDER, 100);
            // Link the locations together
            home.LocationToNorth = townSquare;
            townSquare.LocationToNorth = alchemistHut;
            townSquare.LocationToSouth = home;
            townSquare.LocationToEast = guardPost;
            townSquare.LocationToWest = farmhouse;
            farmhouse.LocationToEast = townSquare;
            farmhouse.LocationToWest = farmersField;
            farmersField.LocationToEast = farmhouse;
            alchemistHut.LocationToSouth = townSquare;
            alchemistHut.LocationToNorth = alchemistsGarden;
            alchemistsGarden.LocationToSouth = alchemistHut;
            guardPost.LocationToEast = bridge;
            guardPost.LocationToWest = townSquare;
            bridge.LocationToWest = guardPost;
            bridge.LocationToEast = spiderField;
            spiderField.LocationToWest = bridge;
            // Add the locations to the static list
            _locations.Add(home);
            _locations.Add(townSquare);
            _locations.Add(guardPost);
            _locations.Add(alchemistHut);
            _locations.Add(alchemistsGarden);
            _locations.Add(farmhouse);
            _locations.Add(farmersField);
            _locations.Add(bridge);
            _locations.Add(spiderField);
        }
        public static Item ItemByID(int id)
        {
            return _items.SingleOrDefault(x => x.ID == id);
        }
        public static Monster MonsterByID(int id)
        {
            return _monsters.SingleOrDefault(x => x.ID == id);
        }
        public static Quest QuestByID(int id)
        {
            return _quests.SingleOrDefault(x => x.ID == id);
        }
        public static Location LocationByID(int id)
        {
            return _locations.SingleOrDefault(x => x.ID == id);
        }
    }
}

We were setting the location’s MonsterLivingHere property in the PopulateMonsters function. Now, we need to change it to use the new AddMonster function.

To set the monsters at a location, pass in the monster ID and the percentage chance it will be selected.

If you wanted rats to appear in the farmersField 20% of the time, and snakes the other 80%, your code could look like this:

farmersField.AddMonster(MONSTER_ID_RAT, 20);

farmersField.AddMonster(MONSTER_ID_SNAKE, 80);

STEP 3: Update the code that was using the MonsterLivingHere property.

There were three other classes that used the MonsterLivingHere property. We need to change them, to use the new property.

Player.cs

Inside the SetTheCurrentMonsterForTheCurrentLocation function (line 385), change this line:

RaiseMessage("You see a " + location.MonsterLivingHere.Name);

To this:

RaiseMessage("You see a " + CurrentMonster.Name);

SuperAdventure.cs (in the Windows Form UI project)

Inside the PlayerOnProperyChanged function (line 150), change this:

if(_player.CurrentLocation.MonsterLivingHere == null)

To this:

if(!_player.CurrentLocation.HasAMonster)

Program.cs (in the SuperAdventureConsole project)

Inside the AttackMonster function (on line 229), change this:

if(_player.CurrentLocation.MonsterLivingHere == null)

To this:

if(!_player.CurrentLocation.HasAMonster)

If you’ve modified the program, with your own changes, do a search for any other code that uses “MonsterLivingHere”, and replace them with the new property, or function.

Summary

When you modify an existing program, you need to find all the places where the current code is used. Then, plan how to get them to all use the new code.

Sometimes, this can be difficult – especially in larger programs. However, the time you spend in preparation will almost always save you more time, once you begin making the changes.

Source code for this lesson

Source code on GitHub

Source code on Dropbox

Next lesson: Lesson 26.1 Displaying a World Map

Previous lesson: Lesson 24.1 – Make the SuperAdventure source code easier to understand and modify

All lessons: Learn C# by Building a Simple RPG Index