<title>5. Inventory</title>

<link rel="stylesheet" href="htpataic.css" type="text/css" />

<table class="contents"><tr><td>

<h4>Contents</h4>

<div><a href="htpataic01.html">1. Introduction</a></div>

<div><a href="htpataic02.html">2. The main loop</a></div>

<div><a href="htpataic03.html">3. Locations</a></div>

<div><a href="htpataic04.html">4. Objects</a></div>

<div><b>5. Inventory</b></div>

<div><a href="htpataic06.html">6. Passages</a></div>

<div><a href="htpataic07.html">7. Distance</a></div>

<div><a href="htpataic08.html">8. North, east, south, west</a></div>

<div><a href="htpataic09.html">9. Code generation</a></div>

<div><a href="htpataic10.html">10. More attributes</a></div>

<div><a href="htpataic11.html">11. Conditions</a></div>

<div><a href="htpataic12.html">12. Open and close</a></div>

<div><a href="htpataic13.html">13. The parser</a></div>

<div><a href="htpataic14.html">14. Multiple nouns</a></div>

<div><a href="htpataic15.html">15. Light and dark</a></div>

<div><a href="htpataic16.html">16. Savegame</a></div>

<div><a href="htpataic17.html">17. Test automation</a></div>

<div><a href="htpataic18.html">18. Abbreviations</a></div>

<div><a href="htpataic19.html">19. Conversations</a></div>

<div><a href="htpataic20.html">20. Combat</a></div>

<div><a href="htpataic21.html">21. Multi-player</a></div>

<div><a href="htpataic22.html">22. Client-server</a></div>

<div><a href="htpataic23.html">23. Database</a></div>

<div><a href="htpataic24.html">24. Speech</a></div>

<div><a href="htpataic25.html">25. JavaScript</a></div>

<h1>How to program a text adventure in C</h1>

by Ruud Helderman

&lt;<a href="mailto:r.helderman@hccnet.nl">r.helderman@hccnet.nl</a>&gt;

Licensed under

<a href="https://github.com/helderman/htpataic/blob/master/LICENSE">MIT License</a>

<h2>5. Inventory</h2>

<p class="intro">

We will now make it possible for actors to &lsquo;hold&rsquo; 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 <a href="http://en.wikipedia.org/wiki/MUD">MUD</a>-style

or a single-player game where you can switch between different characters),

but it is equally useful in a solo mission.

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:

<li><b>has a location</b> (&ldquo;where am I?&rdquo;)</li>

<li><b>is a location</b> for any items the player may be holding.</li>

This makes certain common actions very easy to implement:

<table cellpadding="8"><tr>

<th>Action</th>

<th>Typical<br />command</th>

<th>Example</th>

<td class="comment">Player moves from location to location</td>

<td class="comment">go</td>

<td class="snippet"><span class="red">player</span>-&gt;location = cave;</td>

<td class="comment">List items and actors present at a location</td>

<td class="comment">look</td>

<td class="snippet">listObjectsAtLocation(<span class="red">cave</span>);</td>

<td class="comment">Player gets an item</td>

<td class="comment">get</td>

<td class="snippet">silver-&gt;location = <span class="red">player</span>;</td>

<td class="comment">Player drops an item</td>

<td class="comment">drop</td>

<td class="snippet">silver-&gt;location = <span class="red">player</span>-&gt;location;</td>

<td class="comment">List the player&rsquo;s inventory</td>

<td class="comment">inventory</td>

<td class="snippet">listObjectsAtLocation(<span class="red">player</span>);</td>

<td class="comment">Player gives an item to an actor</td>

<td class="comment">give</td>

<td class="snippet">silver-&gt;location = <span class="red">guard</span>;</td>

<td class="comment">Player receives an item from an actor</td>

<td class="comment">ask</td>

<td class="snippet">silver-&gt;location = <span class="red">player</span>;</td>

<td class="comment">List another actor&rsquo;s inventory</td>

<td class="comment">examine</td>

<td class="snippet">listObjectsAtLocation(<span class="red">guard</span>);</td>

<table class="demo">

<tr><th>Sample output</th></tr>

Welcome to Little Cave Adventure.<br />

You are in an open field.<br />

You see:<br />

a silver coin<br />

a burly guard<br />

<br />

--&gt; get silver<br />

You pick up a silver coin.<br />

<br />

--&gt; inventory<br />

You see:<br />

a silver coin<br />

<br />

--&gt; look around<br />

You are in an open field.<br />

You see:<br />

a burly guard<br />

<br />

--&gt; give silver<br />

You give a silver coin to a burly guard.<br />

<br />

--&gt; inventory<br />

You are empty-handed.<br />

<br />

--&gt; ask silver<br />

You get a silver coin from a burly guard.<br />

<br />

--&gt; inventory<br />

You see:<br />

a silver coin<br />

<br />

--&gt; go cave<br />

OK.<br />

You are in a little cave.<br />

You see:<br />

a gold coin<br />

<br />

--&gt; give silver<br />

There is nobody here to give that to.<br />

<br />

--&gt; drop silver<br />

You drop a silver coin.<br />

<br />

--&gt; look around<br />

You are in a little cave.<br />

You see:<br />

a silver coin<br />

a gold coin<br />

<br />

--&gt; inventory<br />

You are empty-handed.<br />

<br />

--&gt; quit<br />

<br />

Bye!<br />

Commands <i>go</i> and <i>look</i> (the first two examples above)

were already implemented in the previous chapter.

Now we will introduce some typical <b>inventory</b> actions

for both the player and the non-player characters (commands

<i>get</i>, <i>drop</i>, <i>give</i>, <i>ask</i> and <i>inventory</i>).

<table class="code"><tr>

<th>parsexec.c</th>

<li class="old">#include &lt;stdbool.h&gt;</li>

<li class="old">#include &lt;stdio.h&gt;</li>

<li class="old">#include &lt;string.h&gt;</li>

<li class="old">#include "location.h"</li>

<li class="new">#include "inventory.h"</li>

<li class="old"></li>

<li class="old">bool parseAndExecute(char *input)</li>

<li class="old">{</li>

<li class="old"> char *verb = strtok(input, " \n");</li>

<li class="old"> char *noun = strtok(NULL, " \n");</li>

<li class="old"> if (verb != NULL)</li>

<li class="old"> {</li>

<li class="old"> if (strcmp(verb, "quit") == 0)</li>

<li class="old"> {</li>

<li class="old"> return false;</li>

<li class="old"> }</li>

<li class="old"> else if (strcmp(verb, "look") == 0)</li>

<li class="old"> {</li>

<li class="old"> executeLook(noun);</li>

<li class="old"> }</li>

<li class="old"> else if (strcmp(verb, "go") == 0)</li>

<li class="old"> {</li>

<li class="old"> executeGo(noun);</li>

<li class="new"> }</li>

<li class="new"> else if (strcmp(verb, "get") == 0)</li>

<li class="new"> {</li>

<li class="new"> executeGet(noun);</li>

<li class="new"> }</li>

<li class="new"> else if (strcmp(verb, "drop") == 0)</li>

<li class="new"> {</li>

<li class="new"> executeDrop(noun);</li>

<li class="new"> }</li>

<li class="new"> else if (strcmp(verb, "give") == 0)</li>

<li class="new"> {</li>

<li class="new"> executeGive(noun);</li>

<li class="new"> }</li>

<li class="new"> else if (strcmp(verb, "ask") == 0)</li>

<li class="new"> {</li>

<li class="new"> executeAsk(noun);</li>

<li class="new"> }</li>

<li class="new"> else if (strcmp(verb, "inventory") == 0)</li>

<li class="new"> {</li>

<li class="new"> executeInventory();</li>

<li class="old"> }</li>

<li class="old"> else</li>

<li class="old"> {</li>

<li class="old"> printf("I don't know how to '%s'.\n", verb);</li>

<li class="old"> }</li>

<li class="old"> }</li>

<li class="old"> return true;</li>

<li class="old">}</li>

<div class="explanation">

Explanation:

<li>Line 5:

another new module is included.

<li>Line 25-44:

adding these 20 lines makes the game recognize five more commands.

The new commands are implemented by the following module.

<table class="code"><tr>

<th>inventory.h</th>

<li class="new">extern void executeGet(const char *noun);</li>

<li class="new">extern void executeDrop(const char *noun);</li>

<li class="new">extern void executeAsk(const char *noun);</li>

<li class="new">extern void executeGive(const char *noun);</li>

<li class="new">extern void executeInventory(void);</li>

<th>inventory.c</th>

<li class="new">#include &lt;stdio.h&gt;</li>

<li class="new">#include "object.h"</li>

<li class="new">#include "misc.h"</li>

<li class="new">#include "noun.h"</li>

<li class="new">#include "move.h"</li>

<li class="new"></li>

<li class="new">void executeGet(const char *noun)</li>

<li class="new">{</li>

<li class="new"> OBJECT *obj = getVisible("what you want to get", noun);</li>

<li class="new"> if (obj == NULL)</li>

<li class="new"> {</li>

<li class="new"> // already handled by getVisible</li>

<li class="new"> }</li>

<li class="new"> else if (obj == player)</li>

<li class="new"> {</li>

<li class="new"> printf("You should not be doing that to yourself.\n");</li>

<li class="new"> }</li>

<li class="new"> else if (obj-&gt;location == player)</li>

<li class="new"> {</li>

<li class="new"> printf("You already have %s.\n", obj-&gt;description);</li>

<li class="new"> }</li>

<li class="new"> else if (obj-&gt;location == guard)</li>

<li class="new"> {</li>

<li class="new"> printf("You should ask %s nicely.\n", obj-&gt;location-&gt;description);</li>

<li class="new"> }</li>

<li class="new"> else</li>

<li class="new"> {</li>

<li class="new"> moveObject(obj, player);</li>

<li class="new"> }</li>

<li class="new">}</li>

<li class="new"></li>

<li class="new">void executeDrop(const char *noun)</li>

<li class="new">{</li>

<li class="new"> moveObject(getPossession(player, "drop", noun), player-&gt;location);</li>

<li class="new">}</li>

<li class="new"></li>

<li class="new">void executeAsk(const char *noun)</li>

<li class="new">{</li>

<li class="new"> moveObject(getPossession(actorHere(), "ask", noun), player);</li>

<li class="new">}</li>

<li class="new"></li>

<li class="new">void executeGive(const char *noun)</li>

<li class="new">{</li>

<li class="new"> moveObject(getPossession(player, "give", noun), actorHere());</li>

<li class="new">}</li>

<li class="new"></li>

<li class="new">void executeInventory(void)</li>

<li class="new">{</li>

<li class="new"> if (listObjectsAtLocation(player) == 0)</li>

<li class="new"> {</li>

<li class="new"> printf("You are empty-handed.\n");</li>

<li class="new"> }</li>

<li class="new">}</li>

<div class="explanation">

Explanation:

<li>Line 9, 34, 39, 44:

functions <i>getVisible</i> and <i>getPossession</i>

are used to determine which object belongs to a given noun;

see <i>noun.c</i> below.

<li>Line 28, 34, 39, 44:

function <i>moveObject</i>

is used to transport the object to its new location;

see <i>move.c</i> below.

<li>Line 39, 44:

due to the simplicity of our verb-noun parser,

commands <i>ask</i> and <i>give</i> have only one argument: the item.

The actor involved is implied by using function <i>actorHere</i>

(defined below).

<li>Line 49:

function <i>listObjectsAtLocation</i>

was already defined in the previous chapter (<i>misc.c</i>).

Its return value tells us how many objects were found.

<li>Line 51:

the user expects some response, even when the list of objects is empty.

Essentially, commands <i>get</i>, <i>drop</i>, <i>give</i> and <i>ask</i>

do little more than move an item from one place to another.

A single function <i>moveObject</i>

can perform that action for all four commands.

<table class="code"><tr>

<th>move.h</th>

<li class="new">extern void moveObject(OBJECT *obj, OBJECT *to);</li>

<th>move.c</th>

<li class="new">#include &lt;stdio.h&gt;</li>

<li class="new">#include "object.h"</li>

<li class="new"></li>

<li class="new">static void describeMove(OBJECT *obj, OBJECT *to)</li>

<li class="new">{</li>

<li class="new"> if (to == player-&gt;location)</li>

<li class="new"> {</li>

<li class="new"> printf("You drop %s.\n", obj-&gt;description);</li>

<li class="new"> }</li>

<li class="new"> else if (to != player)</li>

<li class="new"> {</li>

<li class="new"> printf(to == guard ? "You give %s to %s.\n" : "You put %s in %s.\n",</li>

<li class="new"> obj-&gt;description, to-&gt;description);</li>

<li class="new"> }</li>

<li class="new"> else if (obj-&gt;location == player-&gt;location)</li>

<li class="new"> {</li>

<li class="new"> printf("You pick up %s.\n", obj-&gt;description);</li>

<li class="new"> }</li>

<li class="new"> else</li>

<li class="new"> {</li>

<li class="new"> printf("You get %s from %s.\n",</li>

<li class="new"> obj-&gt;description, obj-&gt;location-&gt;description);</li>

<li class="new"> }</li>

<li class="new">}</li>

<li class="new"></li>

<li class="new">void moveObject(OBJECT *obj, OBJECT *to)</li>

<li class="new">{</li>

<li class="new"> if (obj == NULL)</li>

<li class="new"> {</li>

<li class="new"> // already handled by getVisible or getPossession</li>

<li class="new"> }</li>

<li class="new"> else if (to == NULL)</li>

<li class="new"> {</li>

<li class="new"> printf("There is nobody here to give that to.\n");</li>

<li class="new"> }</li>

<li class="new"> else if (obj-&gt;location == NULL)</li>

<li class="new"> {</li>

<li class="new"> printf("That is way too heavy.\n");</li>

<li class="new"> }</li>

<li class="new"> else</li>

<li class="new"> {</li>

<li class="new"> describeMove(obj, to);</li>

<li class="new"> obj-&gt;location = to;</li>

<li class="new"> }</li>

<li class="new">}</li>

<div class="explanation">

Explanation:

<li>Line 6-23:

various messages to confirm that the move was successful.

Alternatively, we could have given a simple &lsquo;OK&rsquo;.

<li>Line 28-39:

various conditions under which the move will not take place.

<li>Line 32-35:

there is only one possibility for the item&rsquo;s destination to be NULL:

when trying to &lsquo;give&rsquo; something

at a location where the player is alone.

<li>Line 36-39:

some objects are not meant to be picked up;

for example <i>field</i> and <i>cave</i>.

The condition to recognize those objects, is still rather crude here.

For example, it is currently possible to pick up the guard.

This will be improved in chapter 10.

<li>Line 43:

the core of the function; here the object is actually moved.

Command &lsquo;get&rsquo; uses function <i>getVisible</i>

to translate a noun to an object,

just like command &lsquo;go&rsquo; does; see the previous chapter.

But for commands that operate on objects the player (or some other actor)

is already holding (<i>drop</i>, <i>ask</i>, <i>give</i>),

we need something slightly different.

We will add a function <i>getPossession</i> to <i>noun.c</i>.

<table class="code"><tr>

<th>noun.h</th>

<li class="old">extern OBJECT *getVisible(const char *intention, const char *noun);</li>

<li class="new">extern OBJECT *getPossession(OBJECT *from, const char *verb, const char *noun);</li>

<th>noun.c</th>

<li class="old">#include &lt;stdbool.h&gt;</li>

<li class="old">#include &lt;stdio.h&gt;</li>

<li class="old">#include &lt;string.h&gt;</li>

<li class="old">#include "object.h"</li>

<li class="old"></li>

<li class="old">static bool objectHasTag(OBJECT *obj, const char *noun)</li>

<li class="old">{</li>

<li class="old"> return noun != NULL &amp;&amp; *noun != '\0' &amp;&amp; strcmp(noun, obj-&gt;tag) == 0;</li>

<li class="old">}</li>

<li class="old"></li>

<li class="old">static OBJECT *getObject(const char *noun)</li>

<li class="old">{</li>

<li class="old"> OBJECT *obj, *res = NULL;</li>

<li class="old"> for (obj = objs; obj &lt; endOfObjs; obj++)</li>

<li class="old"> {</li>

<li class="old"> if (objectHasTag(obj, noun))</li>

<li class="old"> {</li>

<li class="old"> res = obj;</li>

<li class="old"> }</li>

<li class="old"> }</li>

<li class="old"> return res;</li>

<li class="old">}</li>

<li class="old"></li>

<li class="old">OBJECT *getVisible(const char *intention, const char *noun)</li>

<li class="old">{</li>

<li class="old"> OBJECT *obj = getObject(noun);</li>

<li class="old"> if (obj == NULL)</li>

<li class="old"> {</li>

<li class="old"> printf("I don't understand %s.\n", intention);</li>

<li class="old"> }</li>

<li class="old"> else if (!(obj == player ||</li>

<li class="old"> obj == player-&gt;location ||</li>

<li class="old"> obj-&gt;location == player ||</li>

<li class="old"> obj-&gt;location == player-&gt;location ||</li>

<li class="old"> obj-&gt;location == NULL ||</li>

<li class="old"> obj-&gt;location-&gt;location == player ||</li>

<li class="old"> obj-&gt;location-&gt;location == player-&gt;location))</li>

<li class="old"> {</li>

<li class="old"> printf("You don't see any %s here.\n", noun);</li>

<li class="old"> obj = NULL;</li>

<li class="old"> }</li>

<li class="old"> return obj;</li>

<li class="old">}</li>

<li class="new"></li>

<li class="new">OBJECT *getPossession(OBJECT *from, const char *verb, const char *noun)</li>

<li class="new">{</li>

<li class="new"> OBJECT *obj = NULL;</li>

<li class="new"> if (from == NULL)</li>

<li class="new"> {</li>

<li class="new"> printf("I don't understand who you want to %s.\n", verb);</li>

<li class="new"> }</li>

<li class="new"> else if ((obj = getObject(noun)) == NULL)</li>

<li class="new"> {</li>

<li class="new"> printf("I don't understand what you want to %s.\n", verb);</li>

<li class="new"> }</li>

<li class="new"> else if (obj == from)</li>

<li class="new"> {</li>

<li class="new"> printf("You should not be doing that to %s.\n", obj-&gt;description);</li>

<li class="new"> obj = NULL;</li>

<li class="new"> }</li>

<li class="new"> else if (obj-&gt;location != from)</li>

<li class="new"> {</li>

<li class="new"> if (from == player)</li>

<li class="new"> {</li>

<li class="new"> printf("You are not holding any %s.\n", noun);</li>

<li class="new"> }</li>

<li class="new"> else</li>

<li class="new"> {</li>

<li class="new"> printf("There appears to be no %s you can get from %s.\n",</li>

<li class="new"> noun, from-&gt;description);</li>

<li class="new"> }</li>

<li class="new"> obj = NULL;</li>

<li class="new"> }</li>

<li class="new"> return obj;</li>

<li class="new">}</li>

<div class="explanation">

Explanation:

<li>Line 45-75:

Just like function <i>getVisible</i>,

the new function <i>getPossession</i> is a wrapper around <i>getObject</i>

(see line 52)

that either returns a matching object,

or NULL if no appropriate object matches the noun.

Function <i>actorHere</i> is used in commands <i>give</i> and <i>ask</i>,

but it could be useful for other commands as well.

So we define it in <i>misc.c</i>.

<table class="code"><tr>

<th>misc.h</th>

<li class="new">extern OBJECT *actorHere(void);</li>

<li class="old">extern int listObjectsAtLocation(OBJECT *location);</li>

<th>misc.c</th>

<li class="old">#include &lt;stdio.h&gt;</li>

<li class="old">#include "object.h"</li>

<li class="old"></li>

<li class="new">OBJECT *actorHere(void)</li>

<li class="new">{</li>

<li class="new"> OBJECT *obj;</li>

<li class="new"> for (obj = objs; obj &lt; endOfObjs; obj++)</li>

<li class="new"> {</li>

<li class="new"> if (obj-&gt;location == player-&gt;location &amp;&amp; obj == guard)</li>

<li class="new"> {</li>

<li class="new"> return obj;</li>

<li class="new"> }</li>

<li class="new"> }</li>

<li class="new"> return NULL;</li>

<li class="new">}</li>

<li class="new"></li>

<li class="old">int listObjectsAtLocation(OBJECT *location)</li>

<li class="old">{</li>

<li class="old"> int count = 0;</li>

<li class="old"> OBJECT *obj;</li>

<li class="old"> for (obj = objs; obj &lt; endOfObjs; obj++)</li>

<li class="old"> {</li>

<li class="old"> if (obj != player &amp;&amp; obj-&gt;location == location)</li>

<li class="old"> {</li>

<li class="old"> if (count++ == 0)</li>

<li class="old"> {</li>

<li class="old"> printf("You see:\n");</li>

<li class="old"> }</li>

<li class="old"> printf("%s\n", obj-&gt;description);</li>

<li class="old"> }</li>

<li class="old"> }</li>

<li class="old"> return count;</li>

<li class="old">}</li>

<div class="explanation">

Explanation:

<li>Line 4-15:

function <i>actorHere</i> returns a pointer to the actor

present at the same location as the player,

or NULL if the player is alone.

In line 9 there is an exhaustive, hard-coded list of non-player characters

(up until now, just one: the <i>guard</i>).

In chapter 10, we will start using an attribute as a more elegant way

to distinguish actors from items and other non-actor objects.

The other modules (<i>main.c</i>, <i>location.c</i>, <i>object.c</i>)

remain unchanged,

you can see them in the previous chapters.

In chapter 12, you will see us add more commands.

But first, we will be making some improvements to command <i>go</i>.

<hr />

<table class="download"><tr><td>

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Next chapter: <a href="htpataic06.html">6. Passages</a>