A Monthly Column
The Beginner's Page
Writing Your First Game
Richard Mansfield
Senior Editor
If you are tempted to write your own games, go ahead. It's a good way to learn to program. Games are basically the same as any other kind of programming.
Computer games fall into two broad categories: 1. imitations of old standards (checkers, Othello) and 2. games (Space Invaders, PacMan) which could not be played without a computer. This second category is more difficult to program for several reasons. For one thing, you've got to think up a whole new, and entertaining, concept and then adjust the action until it is just hard enough to be challenging but not so difficult that people want to give up.
This category (basically "arcade" games) is especially hard to program precisely because a good computer-only game exploits all of the computer's special attributes: speed, color, sound. To do this well, to make things look and respond just the way you imagine them, requires a good bit of programming experience. Usually, too, several things are happening at once in an arcade game. This often means that such a program must be written in machine language, which is far faster than BASIC.
High Card Slice
Old standards, on the other hand, can often be the best way to get started programming games. You already know the game concept, and cards or dice or game boards are fairly easily constructed and manipulated on your computer screen. To illustrate, let's take a look at a simple simulation of one of the oldest card games, "High Card." The rules are simple: you place a bet, and then you draw a card from the deck. The computer, your opponent, draws a card too, and the highest card wins the money.
One simplification here is that there is no attempt to represent the cards on the screen. The entire game relies simply on words ("Ace of Spades," for example) when cards are drawn.
Like most computer programs, the program can be visualized as having four distinct zones: initialization, main loop, subroutines, data tables. We can go through the steps in programming this game by looking at each zone separately.
Initialization
From lines 10 through 80 we are "teaching" the computer some basics about this game. Initialization is the activity which must take place before any of the action can begin. Computers are so fast that they will zip up through these lines and start things off in the main loop at line 100 in a flash. However, as programmers, we are aware that several preliminary events took place inside before anything else.
In line 20, the computer discovers that there is a variable called "dollars" which is to equal 500. It sets aside a section (like a small box) in its memory which it labels "dollars." When the game is running, it will add or subtract from this "box", (lines 230–240) to keep a running total of how much money you have left to bet. From time to time (line 110), it will check the box and report to the player how much he has. The box labelled "dollars" is called a variable because during the game the amount in it will vary.
Lines 30 through 60 are simple enough – they ask the player to give his or her name. The computer "memorizes" it in another "box" called "name$" and can now speak more personally to the player in lines 140 and 230. Also, the computer prints the rules of the game in line 60.
Line 70 "reads" four names (the face cards) from the data tables in lines 510 on. It also makes a "mental note" that it already READ four items. So, when it's asked to READ again (line 80), it will start with the next unread item of data which will be "clubs." By now, the computer has "memorized" a variety of important facts: the player's name, the amount of his or her betting purse, the names of the face cards, and the suits of a standard deck. In less than a second, the computer has grasped and filed away the necessary facts to go on to the main loop where all the action takes place.
The Main Loop
After checking that the player has money to bet, the computer asks for the bet, checks again that the bet is possible, and then runs through one cycle of the game starting in line 160. At this point, a programmer might find it worthwhile to visualize the steps involved in the game: 1. draw a card for the player; 2. draw for the computer; 3. decide who won; 4. adjust the player's purse.
Since both draws are essentially identical actions (the only difference will be that we say "Bob draws a…" instead of "The computer draws"), we don't need to program the draw twice. This is where subroutines come in handy.
The Subroutine
Twice in the main loop, we GOSUB 300. First the player, then the computer, draws. Line 310 randomly picks two numbers, the card and the suit. If line 320 finds that this selection matches the one drawn just before by the player, it goes back for another draw. Line 330 makes the name of the card be the number if it wasn't a number higher than 11 (a face card).
Then line 340 announces the draw using three variables. The first variable (player$) is set up in either line 160 or 190 as appropriate. Then the card$ and suit$ variables are selected from the lists that were "memorized" back in the initialization phase (lines 70–80). The subroutine then RETURNs to the main loop.
Lines 210–240 decide and announce the winner of this round. First, if the variable "card" (the computer's card) is greater than (>) "yourcard," the computer is declared the winner in line 240, the purse is adjusted, and the main loop is restarted (GOTO 100). If the cards are equal, nothing happens to the purse and the next round begins. Notice that we don't need to say "IF YOURCARD > CARD" at the start of line 230 to test if the player has won. It's the only possible thing if the computer has gotten this far.
Once you've solved a particular problem, you'll find you can use the solution in many future games. This subroutine which draws cards, for instance, would work just as well for Poker, or Blackjack, or dozens of other games. Subroutines are handy not only because they can be used repeatedly within a program, but because they can be saved and used repeatedly in future programs. So think up a simple, traditional game and teach it to your computer. There is probably no more pleasurable way to learn programming than to write a game.
Program 1: Apple, PET, VIC, OSI, Radio Shack
10 REM *** NECESSARY INITIAL INFORMATION *** 20 DOLLARS = 500 30 PRINT" WITH WHOM DO I HAVE THE PLEASURE" 40 PRINT" OF PLAYING HIGH CARD SLICE?" 50 INPUT NAME$ 60 PRINT" HIGH CARD WINS IN THIS GAME!" 70 DIM CARD$(14):FORI = 11 TO 14 : READ CARD$(I): NEXTI 80 FORI = 1TO4:READ SUIT$(I):NEXTI 90 REM 100 REM *** MAIN PROGRAM LOOP *** 110 PRINT:PRINT" YOU HAVE $" DOLLARS 120 IF DOLLARS < = 0 THEN PRINT" THE GAME IS OVER. YOU ARE OUT OF CASH." : END 130 PRINT"WHAT IS YOUR BET"; : INPUT BET 140 IF DOLLARS < BET THEN PRINT" YOU ONLY HAVE $"DOLLARS" TO BET,"NAME$ : GOTO130 150 YOURCARD = 0 : YURSUIT = 0 160 PLAYER$ = NAME$ 170 GOSUB300 180 YOURCARD = CARD : YURSUIT = SUIT 190 PLAYER$ =" THE COMPUTER" 200 GOSUB300 210 IF CARD > YOURCARD THEN GOTO 240 220 IF CARD = YOURCARD THEN PRINT" A TIE!" : GOT0100 230 PRINT NAME$ " WINS": DOLLARS = DOLLARS + BET : GOTO100 240 PRINT" THE COMPUTER WINS": DOLLARS = DOLLA RS - BET : GOTO100 290 REM 300 REM *** SUBROUTINE TO DRAW THE CARDS *** 310 CARD = INT(RND(5)*13) + 2 : SUIT = INT(RND(5)*4) + 1 320 IF CARD = YOURCARD AND SUIT = YURSUIT THEN 300 : REM NO IDENTICAL DRAWS 330 IF CARD < 11 THEN CARDS(CARD) = STR$(CARD) 340 PRINTPLAYER$ " DRAWS THE " CARDS (CARD) " ~ OF " SUITS (SUIT) 350 RETURN 490 REM 500 REM *** DATA TABLE *** 510 DATA JACK, QUEEN, KING, ACE 520 DATA CLUBS, DIAMONDS, HEARTS, SPADES
Program 2: For Atari, make these substitutions to Program 1.
20 DOLLARS = 500 : DIM NAMES(20), PLAYERS(20) 70 DIM CARDS (14*5), T$ (10) : FORI = 11 TO 14 : READT$ : CARDS(I*5 - 4, I*5) = T$ : NEXTI 80 DIM SUITS(8*4) : FORI = 1TO4 : READT$ : SUIT$(1*8-7, I*8) = T$ : NEXTI 330 IF CARD < 11 THEN T$ = STR$(CARD): GOTO340 335 T$ = CARD$(CARD*5 - 4, CARD*5) 340 PRINTPLAYERS " DRAWS THE "; T$; " OF "; SUIT$ (8*SUIT - 7, SUIT*8) 510 DATA JACK, QUEEN, KING, ACE 520 DATA CLUBS , DIAMONDS, HEARTS , SPADES