Adventures in Adventuring

Please Parse the Zork

Ken Rose

The most famous scientist in the whole wide world was being interviewed just before going into retirement.

"Doctor," he was asked, "looking back over all of human history, can you tell us, what is the outstanding invention, the greatest contribution ever made by anyone during the annals of man's history?"

"The thermos bottle," replied the sage.

"The thermos bottle? Do you mean to say that the thermos bottle supersedes the use of fire . . . the thermos bottle overshadows the wheel, electricity, the harnessing of the atom, the airplane?"

"Yes. The thermos bottle."

"But why, sir?" He was pressed.

The wise scientist looked at his questioner. "Because," he said, "if you put something into it hot, it stays hot. And, if you put something into it cold, it stays cold."

"So what?" said the interviewer.

The old man looked at him and tugged on his beard. "How do it know?"

Understanding Was Never Easy. As is the case with so many of our marvelous inventions, it only knows because we tell it. And our faithful computer also only knows something because we tell it, because we communicate with it.

Take the matter of adventure games, for instance. Let's consider how an adventure game understands and interacts with us. Along the way, we'll take a look at the state of the art in adventure game understanding — Zork.

Most adventurers have seen or played the Colossal Cave adventure — what's often called the original Adventure — either on a main frame, mini, micro, or in the form of an electronic game. You'll recall that communication within the game is usually accomplished by giving two-word commands consisting of a verb and a noun.

In a typical situation, you might say, "Get snake," to which the computer might respond, "Okay," or, "The snake bites you and you're dead" or "I don't understand." The computer also understands a few one word or one letter commands, such as "East" or "E" for "Go East" or "Inv" or "I" for "Inventory," meaning "List out the items I have in my possession."

In the program, there are word and noun tables. As each word is input, it is looked up in the tables to see if it is legitimate and to see if the verb and noun "go together." An appropriate message is then selected from the text messages contained in the program and printed to the screen.

Let's tell the computer to "Get snake." How does the computer know that you have told it two words, get and snake? A moment's reflection will bring you to the conclusion that something recognizes the blank space between the two words and says, "Aha, there is a blank space. The first word must end here, and the second word must start after the blank space." If this is what you imagined, you're exactly right.

You Should've Paid Attention When They Taught You To Diagram Sentences. The breaking down of a group of words into its grammatical components is called parsing, and the mechanism—the program logic, if you will—is called a parser. Most adventure games use a very unsophisticated parser similar to the one used in the original Adventure.

Except Zork.

In 1977, David Lebling and Marc Blank decided to produce a better parser. They did this because they greatly enjoyed the original Adventure but were troubled by the limits to communication with the game. Over the next couple of years, they wrote Zork on a PDP computer, giving their game the ability to recognize adjectives and to understand conjunctions and multiple sentences.

Say you're in a room and find three buttons to push— a red button, a green button, and a yellow button. In a typical adventure game, one with a fairly unsophisticated parser, the ensuing action might go like this:

"Push button," you say.

The computer responds, "Which one?" or, "I don't understand."

After thinking it over, you go back to your original tack.

"Push red," you type, and the computer understands.

Only when you figure out that "Push red" is the two-word command the computer is set up to respond to will the computer do your bidding.

In Zork, you can say "Push red button" and the computer understands exactly what you mean. In fact, Zork allows you to string commands together. You can say, for instance, "Push red button then push green button. Go north.", and the computer would know just what to do. Once you get into the habit of using this feature, it's very difficult to go back to the old two-word mode. Another superlative command in Zork is "Get all." This allows you to get every item in a room without having to type in each item one at a time. This command is complemented with a "Drop all" command.

Phoning It In. You probably know already that a modem (modulator/demodulator) is the device that you use to hook up your computer through the phone lines to other computers. You probably also know that there are bulletin boards you can access that have all sorts of entertaining information computer folks love. One of the major uses is trading information and hints on adventure games.

In late 1979 and through 1980, bulletin boards were buzzing with comments like "How do you open the egg?" and "Is there anything worthwhile in the echo room? If there is, how can I talk to the computer over the echos?" Zork had started making the mainframe circuit and was provoking a lot of interest. In fact, a Zork user group formed in Milwaukee. In 1981, Zork I was brought out for the Apple Computer and the bulletin board networks really started humming with people looking for clues and hints.

According to Dave Lebling, about two-thirds of the original Zork was used on the Apple version of Zork I.

In the fall of 1981, Zork II, which is about the same size as Zork I, was brought out. It contained most of the remainder of the original Zork, along with additional material. Zork III is currently in the mill and will, according to Lebling, finish up the series.

Zork II contains another new feature, one found in no other currently available adventure — the ability to carry on conversations with some of the characters you meet along the way.

Epicaresque. But Zork has a larger attribute that makes it different from all others—it is truly an epic adventure. When you start out in Zork I, you are in a forest. As you wander around, you eventually come to a house in a clearing. After some trials and tribulations, you are able to enter into the Great Underground Empire. Upon entering, you must battle several nasty folks and, if you defeat them, you can continue on your journey.

Your trip takes you down an underground river, through a mine with strange machinery, and to weird and mysterious temples and dungeons. Always lurking in the dark are the mysterious grues, waiting to pounce on you if you give them the slightest opportunity. There is one not-too-difficult maze that must be thoroughly explored if you are to complete the adventure successfully. The object of this adventure is to collect treasures and store them. When you have done this, you will find another opening into the empire that puts you in position to tackle Zork II.

Zork II is subtitled "The Wizard of Frobozz." While you can play it independently of Zork I, the practice you get in Zork I will stand you in good stead, because Zork II is tough. Not impossible, but tough.

A Fierce and Funny Fellow. As you journey down the corridors of the Great Underground Empire, you will meet and talk to the great Wizard of Frobozz himself. Unfortunately, he doesn't like you very much, and with his affinity for "F" words such as "float," "fall," and "fence," he'll give you fits in completing his assignment. There is also a balloon ride that takes you briefly out of the underground, a beautiful princess with an affinity for unusual animals„ and a visit to the Empire Savings Institution where you may meet the gnome of Zurich.

Humor, adventure, multiple plot lines, and a really good parser commend this adventure as the best there is at this stage of the game.

Early on, we talked about the computer's recognizing words in a table. Answering the question "How do it know?" by referring to tables and parsing and all that jazz just may not satisfy your curiosity. So let's go on.

Stringing Along. In Basic, there is a function called string handling. A string variable differs from a numeric (or real or integer) variable in that it's able to store letters of the alphabet as well as representations of numbers and most characters found on a keyboard. A string variable is designated by a dollar sign ($) following a variable name, and the information in the variable must be enclosed by quotes. VI$ is a string variable named VI. VI$ = "piglet" would tell the computer to store the word piglet in the variable VI$. A function called len allows you to find out how many characters are stored in a string variable. If we ask the computer to print len (VI$) and VI$ = "piglet" the computer will print the number six.

Another function, called val, allows you to find out what the number value in a string variable is. If VI$ = "piglet" and we say print val (VI$), then the computer will print 0. But if VI$ = "72" then the computer will print 72 when asked for the value of VI$. This doesn't seem like a big deal, except that 72 in a string variable ain't really a number in the same fashion as 72 in a number variable.

Another function of string handling in some Basics is the ability to pluck out letters or blocks of letters from a string. These functions are not available in all Basics, but there is usually some way to emulate them.

They are left$, right$ and mid$. Respectively, they can select characters starting at the left side, right side, or the middle of a string variable. If V1$ = "piglet" and we say print left$(VI$,2), the computer will print pi. Asking the computer to print right$ (VI$,2) will cause the computer to print et. And saying print mid$ (VI$, 3,2) will have the computer print gl, because we have told it to print two letters starting at the third letter.

What Happened to the Frog? With all this in mind, it's time to take a look at the program listing that illustrates these principles. Note that if your computer doesn't have the left$, right$, or mid$ functions, lines 235, 240, and 270 will have to be modified. Since what these lines do is look through the string to find a blank, you can modify a get routine or other string inspection routine, which your Basic does understand, to do the same thing. In line 930, we check for a yes or no answer by looking at the first character at the left side of the string. This is also easily modifiable. As always, if you have a question, drop me a note.

This little program also illustrates how a nine-word vocabulary can be used in a lot of ways.

5 HOME : REM CLEAR SCREEN
10 REM HOW DO IT KNOW
15 REM DIM VARIABLES
20 DIM V1$(10),N1$(10)
25 REM LOAD DATA INTO VARIABLES
30 FOR A = 1 TO 4
35 READ V1$(A)
40 NEXT A
45 FOR A = 1 TO 5
50 READ N1$(A)
55 NEXT A
100 REM THE STORY STARTS HERE
105 PRINT "YOU'RE STANDING IN A ROOM. IN THE "
110 PRINT "MIDDLE OF THE ROOM ARE THESE ITEMS:
115 PRINT
170 REM LISTING OUT THE ITEMS
130 PRINT N1$(A)
135 NEXT
140 PRINT : PRINT
145 GOTO 200: REM GOING TO THE PARSER
150 REM BACK FROM THE PARSER. CARRYING OUT THE INSTRUCTIONS
155 IF V$ = "EAT" THEN GOTO 505
160 IF V$ = "SMELL" THEN GOTO 545
165 IF V$ = "DRINK" THEN GOTO 565
170 IF V$ = "GIVE" THEN GOTO 580
200 REM HERE'S HOW IT'S DONE
205 V$ = "":N$ = ""
210 PRINT "WHAT DO YOU WANT TO DO? ";
215 INPUT "";A$
220 REM NOW WE'LL BREAK DOWN THE INPUT
INTO VERB AND NOUN
225 REM FIRST GET THE VERB
230 FOR A = 1 TO LEN (A$)
235 IF MID$ (A$,A,1) = " " THEN X = A + 1:A = 0: GOTO 260: REM THIS LOOKS FOR THE BLANK SEPARATING THE WORDS
240 V$ = V$ + MID$ (A$,A,1)
245 NEXT A
250 X = A — 1: REM IN CASE THERE'S ONLY ONE WORD. IF X = THE WHOLE LENGTH OF THE STRING, THE NEXT LINE WILL
SEND YOU BACK TO THE BEGINNING
255 IF X = LEN (A$) THEN PRINT : PRINT "I CAN'T ";V$;" NOTHING.": PRINT : GOTO 100
260 REM NOW GET THE NOUN
265 IF X = LEN (A$) THEN PRINT : PRINT "I CAN'T ";V$;" NOTHING.": PRINT : GOTO 100
270 N$ = MID$ (A$,X, LEN (A$) — (X — 1))
275 REM CHECK TO SEE IF VERB IS GOOD
280 FOR A = 1 TO 5
285 IF V$ = V1$(A) THEN A = 0: GOTO 305: REM VERB IS O.K.
290 NEXT A
295 PRINT
300 PRINT "I CAN'T ";V$;" SOMETHING.": PRINT : GOTO 100
305 REM CHECK TO SEE IF NOUN IS GOOD
310 FOR A = 1 TO 5
315 IF N$ = N1$(A) THEN A = 0: GOTO 155: REM EVERYTHING CHECKS AND NOW WE'LL DO SOMETHING.
320 NEXT A
325 PRINT : PRINT "SORRY, THERE'S NO ";N$;" HERE.": PRINT : GOTO 100: REM SOMETHING'S WRONG SO WE HAVE TO START OVER
500 REM HERE'S WHERE THINGS HAPPEN
505 REM EATING THINGS
510 IF N$ = N1$(3) THEN CO = 1: PRINT : PRINT "I'M IN LOVE.": PRINT : GOTO 100
515 PRINT : PRINT "I CAN'T EAT A ";N$: PRINT : PRINT "BURP! I GUESS I CAN.": PRINT
520 FOR A = 1 TO 4
525 IF N$ = N1$(A) THEN N1$(A) = "":A = 0:GOTO 535
530 NEXT A
535 X = 0: FOR A = 1 TO 4:X = LEN (N1$(A)) + X: NEXT : IF X = 0 THEN GOTO 900: REM CHECKING TO SEE IF EVERYTHING EATEN
540 GOTO 100
545 REM SMELLING THINGS
550 IF N$ = N1$(1) THEN N$ = N1$(5)
555 IF N$ = N1$(3) THEN N$ = "SKUNK CABBAGE"
560 PRINT : PRINT "THAT SMELLS LIKE A"; N$;"" : PRINT : GOTO 100
565 REM DRINKING
570 IF N$ = N1$(4) THEN PRINT : PRINT "THAT WAS THE LAST BOTTLE OF SACRED ALE.": GOTO 900
575 PRINT : PRINT "YOU CAN'T.": PRINT : GOTO 100
580 REM GIVING THINGS A WAY
585 IF N$ = N1$(1) THEN PRINT : PRINT "WHAT? ";: GOTO 595
590 PRINT "YOU CAN'T GIVE A ";N$; "ANYTHING." : PRINT : GOTO 100
595 INPUT "";N$ : IF LEN (n1$(2)) > 0 AND N$ = N1$(2) AND CO > 0 THEN GOTO 800
600 PRINT : GOTO 100
800 REM AND HERE'S THE WINNING OUTCOME OF THE GAME
805 PRINT : PRINT "THERE IS A CLAP OF THUNDER AND THE "
810 PRINT "HIGH PRIEST OF YORE APPEARS, MARRIES"
815 PRINT "YOU AND THE PRINCESS, GIVES YOU HALF"
820 PRINT "A KINGDOM AND LEAVES. YOU HAVE WON.
825 PRINT "UNFORTUNATELY, THE PRINCESS HAS JUST"
830 PRINT "TURNED BACK INTO A GOAT."
835 END
900 REM AND HERE'S THE LOSING OUTCOME
905 PRINT : PRINT "OH OH. YOU HAVE OFFENDED MARVIN, CHIEF"
910 PRINT "GOD OF ADVENTURE PLAYERS AND BARBERS."
915 PRINT "HOWEVER, HE MAY GIVE YOU JUST ONE OR"
920 PRINT "MORE CHANCES TO PLAY AGAIN."
925 PRINT
930 PRINT "DO YOU WANT TO? ";: INPUT "";A$: IF LEFT$ (A$,1) = "Y" THEN RESTORE : GOTO 30
935 END
950 DATA "EAT","DRINK","SMELL","GIVE "
955 DATA "GOAT","FLOWER", "BOTTLE","PRINCESS"

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