Daily Archives: 02/05/2021

Developing a text-based adventure game (Part 3)

Implementing an inventory system

In this tutorial we will look at how a simple inventory system can be added to the game. We will then use the contents of the inventory to differentiate between different outcomes for certain events. In particular, possessing the sword from location 2 will allow the player to slay the Ogre, which will in turn allow them to collect the key to the chest in location 1.

Modelling the inventory using a list

We will use a list in Python to represent the player’s inventory. A list represents an ordered collection of items. A variety of operations can be performed on lists, including adding elements to a list, removing elements from a list, determine the size of a list, and checking whether an item is contained in a list. The basic list operations are demonstrated in the following code listing.

New lists are defined within a pair of square brackets. Individual elements of the list are separated by commas.

#define a new list
list1=['a', 'b', 'c']
print(list1)

The number of elements in a list is calculated using the len operator.

#the number of elements in a list
print(len(list1))

To check whether an element is in a list we use the in operator. Because ‘a’ is in list1 the first use of the in operator returns true. The second use of the in operator return false. We will use the in operator later to check whether or not an item is in the player’s inventory.

#check whether elements are in a list
print('a' in list1)
print('d' in list1)

New elements are added to a list using the append operator. Note the usage of the append operation – the name of the variable storing the list is given first, followed by a period, followed by the append operator. The append operator takes a single argument, corresponding to the element to be added, enclosed in brackets. This operator can be used to add new items to the inventory.

#add an element to a list
list1.append('d')
print(list1)
print('d' in list1)

Elements can be removed from a list using the remove operator. The remove operator follows the same syntax as the append operator. The remove operator can be used to remove one time use items from the inventory. An example would be removing a potion once it has been consumed.

#remove an element from a list
list1.remove('a')
print(list1)

The following lines of code demonstrate further the use of the remove operator. It shows that if an element occurs multiple times in a list, then the remove operator only removes the first occurrence of the element. Note that if we attempt to remove an element that does not exist in the list then an error will occur. It is therefore important to test whether the element is present before attempting to remove it.

#remove only removes the first occurrence of an item from the list
list2=['a', 'b', 'a', 'c']
list2.remove('a')
print(list2)

#remove the second occurrence of 'a'
list2.remove('a')
print(list2)

Given that elements can occur multiple times in a list, it is useful to be able to determine how many times an element occurs. The count operator can be used to determine how many times a particular element occurs.

#check how many times an element occurs in a list
list3=['a','a','b', 'a', 'b', 'c']
print(list3.count('a'))
print(list3.count('b'))
print(list3.count('c'))

In some games the player may start off with no items in their inventory. To represent this as a list we can use an empty list. An empty list is represented by a pair of square brackets with nothing in between. 

#empty list
list4=[]
list4.append('a')
list4.append('b')
print(list4)

Download source code [List operations]

Implementing the inventory

The code developed in part 2 of the tutorial is updated to make use of an inventory system. There are two items that can be added to the inventory – the sword in location 2 and the key in location 3. The inventory is initialised to an empty list. 

inventory=[]

Next we update the playLocation2 function so that the sword is added to the inventory after it is located (line 70). To ensure that the inventory variable can be accessed within the function, we need to define inventory as a global variable (line 55).

def playLocation2():
    global inventory
    print("You stand atop a narrow ledge.")
    print("A rickety rope ladder dangles off the edge.")
    print("You see a pile of rubble at the far end of the ledge.")
    print("You can either (c)limb down the rope ladder or (s)earch the rubble.")
    response=input("> ")
    if (response=="c"):
        print("You carefully climb down the rope ladder.")
        location="location1"
    elif (response=="s"):
        print("You edge your way along the ledge, almost slipping a couple of times.")
        print("You search the pile of rubble, move large rocks until you discover a sword.")
        print("The sword is inscribed with runes and shimmers in the sun.")
        print("You carefully strap the sword to your back.")
        location="location2"
        inventory.append("sword")
    else:
        location="location2"
    return location

We also update the playLocation3 function so that the sword can be used to slay the Ogre if it has added to the inventory. Line 91 checks that the player response is attack and the sword is in the inventory. The two conditions are joined using the and operator, which evaluates to true if both of the conditions are true. Line 97 adds the key to the inventory. 

def playLocation3():
    global inventory
    print("You reach what looks like a rudimentary camp site.")
    print("You notice a fire pit in the middle of the camp site.")
    print("Scattered around the fire pit are the many bones picked clean of flesh.")
    print("Ominously some of the bones look human like.")
    print("Just as you finish surveying the scene you here a roar.")
    print("Running towards you, wielding a large club is an Ogre.")
    print("You can either (f)lee to the south, (a)ttack the Ogre or try to (t)alk with the Ogre.")
    response=input("> ")
    if (response=="f"):
        print("You run away from the Ogre at full speed")
        print("The Ogre pursues for a while but eventually gives up.")
        print("You survive another day.")
        print("Once you know you are safe you continue your journey southward at a slower pace.")
        location="location1"
    elif (response=="a" and "sword" in inventory):
        print("As the Ogre draws near you draw your sword.")
        print("With one hefty swing of your sword you slay the Ogre.")
        print("You search the body of the Ogre and find a key.")
        print("You take the key and head back towards the clearing with the chest.")
        location="location1"
        inventory.append("key")
    elif (response=="a"):
        print("You fight valiantly with the Ogre.")
        print("You manage to inflict several telling blows against it but it fights on.")
        print("Having dodged several sweeping blows, the Ogre brings its club down upon you with a crash.")
        print("...")
        location="finish"
    elif (response=="t"):
        print("You attempt to make conversation with the Ogre but this only makes it more angry.")
        location="location3"
    else:
        location="location3"
    return location

The playLocation1 function is updated in a similar way to check whether the player has the key in their inventory when they attempt to open the chest.
The full code listing is given as follows:

location="start"
inventory=[]

def playStart():
    print("You are on a quest to find the lost sippy cup.")
    print("The sippy cup is a priceless treasure that grants\nthe user eternal life")
    print("You come to a junction. You can either go (w)est or (e)ast.")
    print("What do you select?")
    response=input("> ")
    if (response=="w"):
        location="location1"
        print("You travel along the path to the west...")
    elif (response=="e"):
        location="location2"
        print("You travel along the path to the east.")
        print("The path leads gradually upwards until you finally reach a ledge.")
    else:
        location="start"
    return location


def playLocation1():
    global inventory
    print("You are in a large clearing.")
    print("In the centre of the clearing is a chest.")
    print("There is a path leading north.")
    print("To the south you can see a cliff face with a rope ladder leading to a ledge.")
    print("You can go (n)orth, (c)limb the ladder or attempt to (o)pen the chest.")
    print("Enter your selection.")
    response=input("> ")
    
    if (response=="o" and "key" in inventory):
        print("You use the key recovered from the Ogre to unlock the chest.")
        print("As you open the lid of the chest a warm glow flows through your body.")
        print("Inside the chest you see the magical sippy cup. ")
        print("As you bring the sippy cup to your mouth and take a sip you feel instantly invigorated.")
        print("You have completed your quest.")
        location="finish"
    elif (response=="o"):
        print("You are unable to open the chest.")
        print("It has a large padlock which cannot be broken.")
        location="location1"
    elif (response=="n"):
        print("You travel for several hours along a winding path.")
        location="location3"
    elif (response=="c"):
        print("You climb the rope ladder which sways dangerously as you make your way up.")
        print("At the top of a ladder you climb up to a ledge.")
        location="location2"
    else:
        location="location1"
    return location

def playLocation2():
    global inventory
    print("You stand atop a narrow ledge.")
    print("A rickety rope ladder dangles off the edge.")
    print("You see a pile of rubble at the far end of the ledge.")
    print("You can either (c)limb down the rope ladder or (s)earch the rubble.")
    response=input("> ")
    if (response=="c"):
        print("You carefully climb down the rope ladder.")
        location="location1"
    elif (response=="s"):
        print("You edge your way along the ledge, almost slipping a couple of times.")
        print("You search the pile of rubble, move large rocks until you discover a sword.")
        print("The sword is inscribed with runes and shimmers in the sun.")
        print("You carefully strap the sword to your back.")
        location="location2"
        inventory.append("sword")
    else:
        location="location2"
    return location

def playLocation3():
    global inventory
    print("You reach what looks like a rudimentary camp site.")
    print("You notice a fire pit in the middle of the camp site.")
    print("Scattered around the fire pit are the many bones picked clean of flesh.")
    print("Ominously some of the bones look human like.")
    print("Just as you finish surveying the scene you here a roar.")
    print("Running towards you, wielding a large club is an Ogre.")
    print("You can either (f)lee to the south, (a)ttack the Ogre or try to (t)alk with the Ogre.")
    response=input("> ")
    if (response=="f"):
        print("You run away from the Ogre at full speed")
        print("The Ogre pursues for a while but eventually gives up.")
        print("You survive another day.")
        print("Once you know you are safe you continue your journey southward at a slower pace.")
        location="location1"
    elif (response=="a" and "sword" in inventory):
        print("As the Ogre draws near you draw your sword.")
        print("With one hefty swing of your sword you slay the Ogre.")
        print("You search the body of the Ogre and find a key.")
        print("You take the key and head back towards the clearing with the chest.")
        location="location1"
        inventory.append("key")
    elif (response=="a"):
        print("You fight valiantly with the Ogre.")
        print("You manage to inflict several telling blows against it but it fights on.")
        print("Having dodged several sweeping blows, the Ogre brings its club down upon you with a crash.")
        print("...")
        location="finish"
    elif (response=="t"):
        print("You attempt to make conversation with the Ogre but this only makes it more angry.")
        location="location3"
    else:
        location="location3"
    return location

    
while (location!="finish"):
    if (location=="start"):
        location=playStart()
    elif (location=="location1"):
        location=playLocation1()
    elif (location=="location2"):
        location=playLocation2()
    elif (location=="location3"):
        location=playLocation3()
    else:
        break

Download source code [Stage 3 code]

Limitations

When we play through the game we notice that when the player returns to an area we are presented with the same options. For example, when returning to location 3 after having already slaying the Ogre the player encounters a new Ogre. To address this limitation we need some way of tracking the events that have occurred within a particular area.

Developing a text-based adventure game (Part 2)

Adding and linking multiple locations

In the first part of the text-based adventure game development tutorial we described how print, input and conditional statements can be used to implement an interactive encounter. We also noted that although nested conditional statements could be used to link multiple encounters, this approach will quickly lead to the development of spaghetti code.

In this tutorial we investigate a solution to developing linked encounters using functions and loops.

Game synopsis

We begin by providing a brief synopsis for our proposed game.

The player quests to retrieve the magical sippy cup, which is believed to grant eternal life whoever has it in their possession. However the sippy cup has been stolen, and
it is the responsibility of our brave adventurer to rescue it. The game consists of three location.

  • Location 1 contains a locked chest. The chest cannot be opened without the key which is not in this location.
  • Location 2 contains a magical sword hidden on a ledge at the top of a steep cliff face.
  • Location 3 contains an evil Ogre who happens to have the key to the chest in his possession. The Ogre can only be slain if the player has found the magical sword.

Game layout

Having come up with an overall synopsis for the game, the next step is to consider the game layout and how the locations connect to each other. This can be done with pencil and paper, or a simple digital tool such as OneNote or Paint.

In the layout design shown below there are three locations in addition to a start and finish location. Directed arcs show the possible transitions between locations; for example the start location links to location 1 and location 2.

The diagram shows the layout for a simple non-linear text based adventure game. The nodes of the diagram represent the game locations, while the edges represent connections between these locations.

Initial layout for a simple non-linear text based adventure game

Location details

Having designed the layout of the adventure, we next need to define the narrative for each location. This should include the contextual information as well as the options available to the player. For example, the narrative for the start location would be as follows.

You are on a quest to retrieve the find the lost sippy cup. The sippy cup is a priceless treasure that grants the user eternal life. You come to a junction. You can either go west or east.

Linking locations

The final step before we can begin code is to define the links between the different locations. We need to provide details of the actions or events that need to occur to move from one location to another.

Shows how locations can be linked with events.

Game layout with linking events

Initial coding step

To code the flowchart shown earlier we firstly add a variable to represent the location that the player is currently in. Line 1 of the code listing below
initialises the location variable to the start location. Lines 20-24 contain a while loop which is run repeatedly until the condition is true.
In this case, the code inside the while loop is repeated until the player is in the finish location. The != operator checks that the first argument is not equal to the second argument. A conditional statement is used to check the current location and call a function corresponding to the location the player is currently in. The else branch at the end uses a break statement to exit the loop. This is used to ensure that the game does not get stuck in an infinite loop.

location="start"

def playStart():
    print("You are on a quest to find the lost sippy cup.")
    print("The sippy cup is a priceless treasure that grants\nthe user eternal life")
    print("You come to a junction. You can either go (w)est or (e)ast.")
    print("What do you select?")
    response=input("> ")
    if (response=="w"):
        location="location1"
        print("You travel along the path to the west...")
    elif (response=="e"):
        location="location2"
        print("You travel along the path to the east.")
        print("The path leads gradually upwards until you finally reach a ledge.")
    else:
        location="start"
    return location

while (location!="finish"):
    if (location=="start"):
        location=playStart()
    else:
        break

Download source code [Stage 1 code]

Defining functions

To avoid monolithic code and to aid with the understandability of the code we employ functions to break up the code. In lines 3-18 a function that handles the starting location is defined. Line 3 of the function declares the name of the function, in this case playStart, together with any inputs that are passed to the function. The first line of a function definition must begin with the def keyword and end with a colon. Input arguments are specified inside of brackets – note that if the function does not have any inputs then an empty of brackets are used. Lines 4-18 contain the body of the function – this is the code that runs when the function is called. Lines 4-7 output the narrative text using print statements. Line 8 prompts the user for their action and assigns the value to the response variable. If the player enters ‘w’, then “location1” is assigned to a local variable used to store the location. Otherwise, if the player enters ‘e’, then “location2” is stored in the local variable. In all other cases the local variable is assigned the “start” location. On line 18 the value stored in the local variable is returned as the result of the function.

Looking at line 22 of the code again, on this line the playStart function is called. Since this function has no inputs we do not pass any values, however we still need to include the empty pair of brackets. The result returned by the function is assigned to the location variable, which will in turn be used in the next iteration of the loop to determine the next function to call.

Coding the other locations

Once we have got the first location coded we follow a similar process to add the additional locations. A function needs to be defined for each of the additional locations and extra cases need to be added in the while loop.

location="start"

def playStart():
    print("You are on a quest to find the lost sippy cup.")
    print("The sippy cup is a priceless treasure that grants\nthe user eternal life")
    print("You come to a junction. You can either go (w)est or (e)ast.")
    print("What do you select?")
    response=input("> ")
    if (response=="w"):
        location="location1"
        print("You travel along the path to the west...")
    elif (response=="e"):
        location="location2"
        print("You travel along the path to the east.")
        print("The path leads gradually upwards until you finally reach a ledge.")
    else:
        location="start"
    return location


def playLocation1():
    print("You are in a large clearing.")
    print("In the centre of the clearing is a chest.")
    print("There is a path leading north.")
    print("To the south you can see a cliff face with a rope ladder leading to a ledge.")
    print("You can go (n)orth, (c)limb the ladder or attempt to (o)pen the chest.")
    print("Enter your selection.")
    response=input("> ")
    if (response=="o"):
        print("You are unable to open the chest.")
        print("It has a large padlock which cannot be broken.")
        location="location1"
    elif (response=="n"):
        print("You travel for several hours along a winding path.")
        location="location3"
    elif (response=="c"):
        print("You climb the rope ladder which sways dangerously as you make your way up.")
        print("At the top of a ladder you climb up to a ledge.")
        location="location2"
    else:
        location="location1"
    return location

def playLocation2():
    print("You stand atop a narrow ledge.")
    print("A rickety rope ladder dangles off the edge.")
    print("You see a pile of rubble at the far end of the ledge.")
    print("You can either (c)limb down the rope ladder or (s)earch the rubble.")
    response=input("> ")
    if (response=="c"):
        print("You carefully climb down the rope ladder.")
        location="location1"
    elif (response=="s"):
        print("You edge your way along the ledge, almost slipping a couple of times.")
        print("You search the pile of rubble, move large rocks until you discover a sword.")
        print("The sword is inscribed with runes and shimmers in the sun.")
        print("You carefully strap the sword to your back.")
        location="location2"
    else:
        location="location2"
    return location

def playLocation3():
    print("You reach what looks like a rudimentary camp site.")
    print("You notice a fire pit in the middle of the camp site.")
    print("Scattered around the fire pit are the many bones picked clean of flesh.")
    print("Ominously some of the bones look human like.")
    print("Just as you finish surveying the scene you here a roar.")
    print("Running towards you, wielding a large club is an Ogre.")
    print("You can either (f)lee to the south, (a)ttack the Ogre or try to (t)alk with the Ogre.")
    response=input("> ")
    if (response=="f"):
        print("You run away from the Ogre at full speed")
        print("The Ogre pursues for a while but eventually gives up.")
        print("You survive another day.")
        print("Once you know you are safe you continue your journey southward at a slower pace.")
        location="location1"
    elif (response=="a"):
        print("You fight valiantly with the Ogre.")
        print("You manage to inflict several telling blows against it but it fights on.")
        print("Having dodged several sweeping blows, the Ogre brings its club down upon you with a crash.")
        print("...")
        location="finish"
    elif (response=="t"):
        print("You attempt to make conversation with the Ogre but this only makes it more angry.")
        location="location3"
    else:
        location="location3"
    return location

    
while (location!="finish"):
    if (location=="start"):
        location=playStart()
    elif (location=="location1"):
        location=playLocation1()
    elif (location=="location2"):
        location=playLocation2()
    elif (location=="location3"):
        location=playLocation3()
    else:
        break

Download source code [Stage 2 code]

Limitations

If you download and run the code you may notice that collecting the sword in location 2 still does not allow you to kill the Ogre. We will address this issue in the next tutorial using a simple inventory solution.