(idea) by ferrouslepidoptera Mon Oct 02 2000 at 4:50:12

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The objects in an adventure game, like the rooms, are represented by an array of structures. Each object has a structure which contains the following information:
  • A short name for the object.
  • A longer name for the object.
  • The object's location (room number).
  • Whether or not the object may be carried.

The following C structure may be used for this: 

struct item_t {         /* one for each object in the game */ 
        char *short_name;
        char *description;
        int location;   /* index into room array */
        int portable;   /* can be carried? */
};

The short name should be the single word by which the player will refer to the object.

In a more sophisticated game, there might be a "type" field which identifies if the object of some special type, such as a "door", which may be opened, closed, locked and prevent or allow travel between rooms, or perhaps a container which may contain other objects. Additionally a void * pointer may be used here to point to a special purpose structure to hold the properies associated with these special objects. For ordinary objects that pointer would be null. For "door" type objects, it would point to a kind of structure that contains information peculiar to doors, (opened or closed, locked or unlocked, what rooms is it between, etc.) and for "container" objects it would point to a structure that contained the information specific to that container, etc.).

I'm not really going to go into too much more detail about those types of things here though, just a taste.

So all this is pretty straightforward, just strings for the descriptions, and an integer for the location. So, what about objects which the player is carrying? What room are they considered to be in? Well, just pick a number outside the range of room numbers. Traditionally, the value of zero is used, and the zeroth element of the room array is just burned up for that purpose. The player cannot usually accidentally wander into room zero (his own pocket) due to the simple fact that there are no travel table entries that lead to room zero.

However, there's nothing magic about zero. It could easily be argued that the value -1 makes more sense. One advantage of using zero over -1 is that negative values may be conveniently used to indicate relative position of two objects such as "insideness", or "on-top-of-ness". For example to represent that an apple is "on the table", the apple might have the negative value of the table's object number as it's location. The program would know that the apple has some relation to the table because the location is first of all, negative, and therefore not a room number, and it is the table's object number. It would know that the apple is "on" the table and not "inside" the table because it would know that the table is a "surface" type object, and not a "container". This simple scheme breaks down for objects which may be multiple types. (E.g. a large sea chest could serve as both a table and a container.)

That limitation may be overcome by assigning each relationship type a small integer, starting with zero, and advancing upwards by one. For example, Suppose we have: 


Object          Object number
-----------------------------
apple           21
table           37

total number of objects in the game: 154

relationship    relationship number
-----------------------------------
"inside"        0
"tied to"       1
"bolted to"     2
"on top of"     3

Now, to represent that the apple is on the table, what value could we use for the apple's location?

Well, How about the negative object number of the table minus relationship number times the total number of objects in the room?

something like: 

  int apple=21, table=37, on_top_of=3, total_objs=154 ;

  object[apple].location = -table - (total_objs*on_top_of);

This formula gives a unique value for the location for each combination of the two objects and a relationship, taking into account that the order of the two objects is important. ("the apple is on the table" is not equivalent to "the table is on the apple"). It's a fairly straightforward operation to decode this value to find what it means. Use the modulus operator ('%' in C) with the total number of objects to find the object number, and truncating integer division to get the relationship number.

Ok, that's it for the objects in our adventure game. Next up, tying it all together with a parser.

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I like it!