5. Inventory

We will now make it possible for persons to ‘hold’ items.

In the previous chapter, we made an array to store all objects, including the player himself. By considering the player as an object, we make the player an integrated part of the game, instead of a spectator watching the virtual world from a safe distance.

The advantage of this approach becomes most obvious in a game with multiple player characters (either MUD-style or a single-player game where you can switch between different characters), but it is equally useful when there is only one character. Player attributes no longer have to be stored in separate variables; we can use the same data structure as used for any other object. So the player, being an object:

has a location (“where am I?”)

is a location for any items the player may be holding.

This makes certain common actions very easy to implement:

Action	Typical command	Example
Player moves from location to location	go	player->location = cave;
List items and persons present at a location	look	listObjectsAtLocation(cave);
Player gets an item	get	silver->location = player;
Player drops an item	drop	silver->location = player->location;
List the player's inventory	inventory	listObjectsAtLocation(player);
Player gives an item to a person	give	silver->location = guard;
Player receives an item from a person	ask	silver->location = player;
List another person's inventory	examine	listObjectsAtLocation(guard);

Action

Typical
command

Example

Player moves from location to location

player->location = cave;

List items and persons present at a location

look

listObjectsAtLocation(cave);

Player gets an item

get

silver->location = player;

Player drops an item

drop

silver->location = player->location;

List the player's inventory

inventory

listObjectsAtLocation(player);

Player gives an item to a person

give

silver->location = guard;

Player receives an item from a person

ask

silver->location = player;

List another person's inventory

examine

listObjectsAtLocation(guard);

Commands go and look (the first two examples above) were already implemented in the previous chapter. Now we will introduce some typical inventory actions for both the player and the non-player characters (commands get, drop, give, ask and inventory).

Sample output

Welcome to Little Cave Adventure. You are in an open field. You see: a silver coin a burly guard --> get silver OK. --> inventory You see: a silver coin --> look around You are in an open field. You see: a burly guard --> give silver OK. --> inventory You are empty-handed. --> ask silver OK. --> inventory You see: a silver coin --> go cave OK. You are in a little cave. You see: a gold coin --> give silver There is nobody here to give that to. --> drop silver OK. --> look around You are in a little cave. You see: a silver coin a gold coin --> inventory You are empty-handed. --> quit Bye!

main.c

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#include <stdio.h> #include <string.h> #include "location.h" #include "inventory.h" static char input[100]; static int getInput() { printf("\n--> "); return fgets(input, sizeof(input), stdin) != NULL; } static int parseAndExecute() { char *verb = strtok(input, " \n"); char *noun = strtok(NULL, " \n"); if (verb != NULL) { if (strcmp(verb, "quit") == 0) { return 0; } else if (strcmp(verb, "look") == 0) { executeLook(noun); } else if (strcmp(verb, "go") == 0) { executeGo(noun); } else if (strcmp(verb, "get") == 0) { executeGet(noun); } else if (strcmp(verb, "drop") == 0) { executeDrop(noun); } else if (strcmp(verb, "give") == 0) { executeGive(noun); } else if (strcmp(verb, "ask") == 0) { executeAsk(noun); } else if (strcmp(verb, "inventory") == 0) { executeInventory(); } else { printf("I don't know how to '%s'.\n", verb); } } return 1; } int main() { printf("Welcome to Little Cave Adventure.\n"); executeLook("around"); while (getInput() && parseAndExecute()); printf("\nBye!\n"); return 0; }

Explanation:

Line 4: another new module is included.
Line 32-51: adding these 20 lines makes the game recognize five more commands.

The new commands are implemented by the following module.

inventory.h

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extern void executeGet(const char *noun); extern void executeDrop(const char *noun); extern void executeGive(const char *noun); extern void executeAsk(const char *noun); extern void executeInventory(void);

inventory.c

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#include <stdio.h> #include "object.h" #include "misc.h" static void moveObject(const char *noun, OBJECT *from, OBJECT *to) { OBJECT *obj = parseObject(noun); if (obj == NULL) { printf("I don't understand what item you mean.\n"); } else if (from != obj->location) { printf("You can't.\n"); } else if (to == NULL) { printf("There is nobody here to give that to.\n"); } else { obj->location = to; printf("OK.\n"); } } void executeGet(const char *noun) { moveObject(noun, player->location, player); } void executeDrop(const char *noun) { moveObject(noun, player, player->location); } void executeGive(const char *noun) { moveObject(noun, player, personHere()); } void executeAsk(const char *noun) { moveObject(noun, personHere(), player); } void executeInventory(void) { if (listObjectsAtLocation(player) == 0) { printf("You are empty-handed.\n"); } }

Explanation:

Line 5-25: moveObject is a generic function to move an object from one location to another; it is used to get, drop, give and ask an item.
Line 39, 44: due to the simplicity of our verb-noun parser, commands give and ask have only one argument: the item. The person involved is implied by using function personHere (defined below).
Line 49: function listObjectsAtLocation is defined in misc.*, see the previous chapter. Its return value tells us how many objects were found.
Line 51: the user expects some response, even when the list of objects is empty.

Function personHere is used in commands give and ask, but it could be useful for other commands as well. So we define it in misc.c.

misc.h

1 2 3

extern OBJECT *parseObject(const char *noun); extern OBJECT *personHere(void); extern int listObjectsAtLocation(OBJECT *location);

misc.c

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#include <stdio.h> #include <string.h> #include "object.h" #include "misc.h" OBJECT *parseObject(const char *noun) { OBJECT *obj, *found = NULL; for (obj = objs; obj < endOfObjs; obj++) { if (noun != NULL && strcmp(noun, obj->tag) == 0) { found = obj; } } return found; } OBJECT *personHere(void) { OBJECT *obj; for (obj = objs; obj < endOfObjs; obj++) { if (obj->location == player->location && obj == guard) { return obj; } } return NULL; } int listObjectsAtLocation(OBJECT *location) { int count = 0; OBJECT *obj; for (obj = objs; obj < endOfObjs; obj++) { if (obj != player && obj->location == location) { if (count++ == 0) { printf("You see:\n"); } printf("%s\n", obj->description); } } return count; }

Explanation:

Line 19-31: function personHere returns a pointer to the person present at the same location as the player, or NULL if the player is alone. In line 24 there is an exhaustive, hard-coded list of non-player persons (in our case, just one: the guard). In chapter 10, we will start using an attribute as a more elegant way to distinguish persons from non-person objects.

The other modules (object.* and location.*) remain unchanged, you can see them in the previous chapter.

In chapter 12, you will see us add more commands. But first, we will be making some improvements to command go.

How to program a text adventure in C

5. Inventory