CONTENTS
1. Introduction
2. The main loop
3. Locations
4. Objects
5. Inventory
6. Passages
7. Distance
8. North, east, south, west
9. Code generation
10. More attributes
11. Conditions
12. Open and close
13. The parser

How to program a text adventure in C

6. Passages

It's time to draw a map - and implement it!

The best tools for drawing a map will always be: a pencil and a piece of paper. A basic map consists of locations (the rectangles), connected by passages (the arrows). We already created locations in chapter 3, now we will start adding the passages.

In the virtual world, a ‘passage’ may be anything connecting two locations: a road, a door, a stretch of sand in a desert. Basically, a passage has the following properties:

A starting point (location).
A destination (location).
The narrative description, for example “a forest path”.
The tag by which the passage is referred to in the go command.

Considering these properties, it may not come as a surprise that the struct object defined in chapter 4 is very suitable to store a passage. In fact, a passage is not that different from an item or person; it is present at a certain location as a ‘visible exit’ (this location is the starting point). It just behaves differently to certain commands. In particular the command ‘go’: applied to a passage, go will change the player's location. The target location (the ‘destination’) can be stored in the struct object as a new attribute.

struct object { const char *description; const char *tag; struct object *location; struct object *destination; };

Notes:

Obviously, destination is unused in most other objects (items, persons), but I consider this to be a minor waste of space.
A passage runs in one direction only; bi-directional passages are implemented by defining two separate uni-directional passages. Again, try not to bother too much about the apparent waste of space.

So we expand the array of objects:

Sample output

Welcome to Little Cave Adventure. You are in an open field. You see: a silver coin a burly guard a cave entrance --> go entrance OK. You are in a little cave. You see: a gold coin a way out --> go out OK. You are in an open field. You see: a silver coin a burly guard a cave entrance --> go cave OK. You are in a little cave. You see: a gold coin a way out --> quit Bye!

	object.h
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19	typedef struct object { const char description; const char tag; struct object location; struct object destination; } OBJECT; extern OBJECT objs[]; #define field (objs + 0) #define cave (objs + 1) #define silver (objs + 2) #define gold (objs + 3) #define guard (objs + 4) #define player (objs + 5) #define intoCave (objs + 6) #define exitCave (objs + 7) #define endOfObjs (objs + 8)
	object.c
1 2 3 4 5 6 7 8 9 10 11 12 13	#include <stdio.h> #include "object.h" OBJECT objs[] = { {"an open field" , "field" , NULL , NULL }, {"a little cave" , "cave" , NULL , NULL }, {"a silver coin" , "silver" , field, NULL }, {"a gold coin" , "gold" , cave , NULL }, {"a burly guard" , "guard" , field, NULL }, {"yourself" , "yourself", field, NULL }, {"a cave entrance", "entrance", field, cave }, {"a way out" , "out" , cave , field } };

Next, we have to make some changes to function executeGo.

	location.h
1 2	extern void executeLook(const char noun); extern void executeGo(const char noun);
	location.c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46	#include <stdio.h> #include <string.h> #include "object.h" #include "misc.h" void executeLook(const char noun) { if (noun != NULL && strcmp(noun, "around") == 0) { printf("You are in %s.\n", player->location->description); listObjectsAtLocation(player->location); } else { printf("I don't understand what you want to see.\n"); } } void executeGo(const char noun) { OBJECT *obj = parseObject(noun); if (obj == NULL) { printf("I don't understand where you want to go.\n"); } else if (obj == player->location) { printf("You are already there.\n"); } else if (getPassageTo(obj) != NULL) { printf("OK.\n"); player->location = obj; executeLook("around"); } else if (obj->location == player->location && obj->destination != NULL) { printf("OK.\n"); player->location = obj->destination; executeLook("around"); } else { printf("You can't go there.\n"); } }

Explanation:

Line 30: we no longer allow the player to jump freely from one location to another: there must be a passage connecting the two. We use a separate function getPassageTo (defined below) to check this.
Line 36-41: besides go <location> (lines 30-35), we now offer an alternative way for the player to move: go <passage>. For example, when in the field, go cave and go entrance will have the same effect.

Function executeGo uses a separate function to check if two locations are connected by a passage. Because such a function will prove to be useful in the next chapter as well, I made it a general function getPassageTo as part of misc.c.

	misc.h
1 2 3 4	extern OBJECT getPassageTo(OBJECT targetLocation); extern OBJECT parseObject(const char noun); extern OBJECT personHere(void); extern int listObjectsAtLocation(OBJECT location);
	misc.c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62	#include <stdio.h> #include <string.h> #include "object.h" #include "misc.h" OBJECT getPassageTo(OBJECT targetLocation) { OBJECT obj; for (obj = objs; obj < endOfObjs; obj++) { if (obj->location == player->location && obj->destination == targetLocation) { return obj; } } return NULL; } 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 9-12: we scan through all objects until we encounter a passage running from player->location (the player's current location) to targetLocation (the function's parameter).
Line 14: if a matching passage is found, we return a pointer to the passage object.
Line 17: if no such passage exists, we return NULL.

The other modules (main.c and inventory.*) remain unchanged, you can see them in the previous chapter.

Obviously, the map in this sample is trivial: there are only two locations, and they are connected in both directions. I will leave it up to the reader to draw a more complicated map and turn it into a list of objects (locations and passages).

Many responses in the game are still kind of dull. For example, “go guard” returns “You can't go there.” Exactly the kind of output I criticized in the previous chapter! This is something we will be working on in the next chapter.

Next chapter: 7. Distance