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5. PN 1 notes that the number of cards in the player s hand has increased by one The next line prints out the name of the card that was drawn 460 PRINT YOU HAVE DRAWN THE GOSUB 3000 PRINT The subroutine at line 3000 prints the actual name of the card in our special notation The statement PRINT prints a period at the end of the sentence According to the rules if the card that the player draws from the deck is of the same denomination that he or she just requested from the other player then the player gets another turn Thus we check to see if the denomination of the card that was drawn DE is equal to the denomination of the card that was asked for PD 470 IF DE PD THEN PRINT CONGRATULATIONS YOU GET AN EXTRA TURN GOTO 170 If so we loop back to the beginning of the main loop on line 170 and the player s turn is repeated And now you know why we saved the value of the requested card in variable PD If we don t loop back we still must check to see if the player has any books in his or her hand now that a new card has been added This part of the program proceeds much as before 480 CP 1 GOSUB 4000 REM LOOK FOR BOOKS CP S HAND 490 IF 0 THEN 530 REM IF NO BOOKS SKIP AHEAD 500 PRINT YOU HAVE A BOOK OF GOSUB 5000 REM ELSE ANNOUNCE IT 510 PRINT YOU DISCARD THIS BOOK 520 PB PB 1 PN PN 4 IF 0 THEN PRINT YOU ARE OUT 37 OF CARDS
6. 950 REM IF PLAYER OUT CARDS END GAME Either way it is now the computer s turn This part of the program is a lot like the last part except that a few numbers have been changed and a couple of routines added First we must announce how many books the computer has discarded variable CB and how many cards it holds variable CN 530 PRINT PRINT THE COMPUTER HAS DISCARDED CB BOOKS 540 PRINT HAS CN CARDS When it was the human player s turn we printed out the con tents of the hand However it would be cheating to let the human player see the contents of the computer s hand Nonetheless you might want to look at the computer s hand occasionally to check if the program is working correctly so we will include instructions to print it out But we will put these instructions in REM state ments so that the computer will think they are remarks and ignore them If you want the computer to execute these state ments and show you the computer s hand remove the word REM at the beginning of each line 550 REM PRINT THEY ARE 560 REM FOR DE 1 TO 13 570 REM FOR SU 1 TO 4 580 REM IF DK SU DE 2 THEN GOSUB 2000 PRINT 590 REM NEXT SU 600 REM NEXT DE 610 REM PRINT Next we check the computer s hand for books just as we checked the human player s hand 38 620 CP 2 GOSUB 4000 REM LOOK FOR BOOKS IN CP S HAND 630 IF 0 THEN 670 REM IF NO BOOKS SKIP AHEAD 640 PRINT T
7. HOW CREATE COMPUTER GAMES CHRISTOPHER LAMPTON HOW TO CREATE COMPUTER GAMES A Computer Awareness First Book Franklin Watts New York London Toronto Sydney 1986 Library of Congress Cataloging in Publication Data Lampton Christopher How to create computer games A Computer awareness first book Includes index Summary Provides instructions for creating computer games using BASIC 1 BASIC Computer program language Juvenile literature 2 Computer games Juvenile literature 1 Computer games 2 BASIC Computer program language 3 Programming languages Computers 4 Programming Computers I Title II Series QA76 73 B3L3535 1986 794 8 2 85 26635 ISBN 0 531 10120 7 Copyright O 1986 by Christopher Lampton All rights reserved Printed in the United States of America 5 43 2 4 CONTENTS Chapter One The Ghost in the Machine 1 Chapter Two A Fishing Expedition 13 Chapter Three Playing the Game 25 Chapter Four Something Completely Different 53 Chapter Five Checkers 63 Chapter Six All the World s a Game 81 Index 83 HOW CREATE COMPUTER GAMES THE GHOST IN THE MACHINE Everybody likes to play with simulations What are simulations Usually they are scaled down models of things that are too expensive or too dangerous or too rare to play with for real A dollhouse is a simulation of a real house for instance model airplane is a simulati
8. PRINT NAME AND 2 BLANK SPACES 230 NEXT SU 240 NEXT DE 250 PRINT The two FOR NEXT loops run through all the denominations and suits Every time it finds one that is in the player s hand that is if DK SU DE is equal to 1 it calls the subroutine at line 3000 to print the name of that card on the video display It then prints two blank spaces to separate the names The PRINT statement on line 250 simply prints a carriage return at the end of the list of cards Before we proceed with the game we should check to be sure that there are no books of cards in the player s hand Although the game has not really begun it is possible that the player has been dealt four cards of the same denomination Not very likely but possible Of course we will also need to check again for a book later after the player has drawn cards from the computer s hand or the deck And we will need to check the computer s hand for books too when it is the computer s turn to play Therefore we should create a subroutine that will check for books since we will be doing it often Here is such a subroutine 29 4000 FOR DE 1 13 REM CHECK ALL 13 DENOMINATIONS 4010 NC 0 REM SET NUMBER OF CARDS IN DENOMINATION TO 0 4020 FOR SU 1 TO 4 REM CHECK ALL 4 SUITS 4030 IF DK SU DE CP THEN NC NC 1 REM COUNT NUMBER OF CARDS OF THIS DENOMINATION IN CURRENT PLAYER S HAND 4040 IF NC 4 THEN 4090 REM IF IT S A BOOK OF FOUR SKIP AHEAD 4050 NEXT SU 40
9. 2020 2040 DK SU DE CP ND ND 1 2050 RETURN 2989 REM 2990 REM NAME THE CARD 2991 REM 2992 REM A SUBROUTINE TO WRITE THE NAME 2993 REM OF A CARD ON THE VIDEO DISPLAY 2994 REM IN ABBREVIATED NOTATION WHERE 2995 REM SU IS THE SUIT AND DE IS THE 2996 REM DENOMINATION OF THE CARD 49 2997 REM 2998 3000 IF DE gt 1 AND DE lt 10 THEN PRINT 0 GOTO 3060 3010 IF DE 10 THEN PRINT 10 GOTO 3060 3020 IF DE 1 THEN PRINT GOTO 3060 3030 IF DE 11 THE PRINT J GOTO 3060 3040 IF DE 12 THEN PRINT Q GOTO 3060 3050 IF DE 13 THEN PRINT K GOTO 3060 3060 8IMI 3070 IF SU 1 THEN PRINT Ht RETURN 3080 IF SU 2 THEN PRINT RETURN 3090 IF SU 3 THEN PRINT Di RETURN 3100 IF SU 4 THEN PRINT Sp RETURN 3989 REM 3990 REM CHECK FOR A BOOK 3991 REM 3992 REM A SUBROUTINE TO CHECK THE DECK 3993 REM TO SEE IF THE CURRENT PLAYER CP 3994 REM HAS FOUR CARDS OF THE SAME 3995 REM DENOMINATION DE A BOOK IN 3996 REM THE PARLANCE OF THE GAME RETURNS 3997 REM THE DENOMINATION OR 0 IN DE 3998 REM 3999 REM 4000 FOR DE 1 TO 13 4010 0 4020 FOR SU 1 TO 4 4030 IF DK SU DE CP THEN NC NC 1 4040 IF NC 4 THEN 4090 4050 NEXT SU 4060 NEXT DE 4070 DE 0 4080 RETURN 50 4090 FOR 50 1 TO 4 4100 DK SU DE 3 4110 NEXT SU 4
10. 450 PD DE CP 1 GOSUB 2000 DK SU DE 1 PN PN 1 460 PRINT YOU HAVE DRAWN THE GOSUB 3000 PRINT 470 IF DE PD THEN PRINT CONGRATULATIONS YOU GET AN EXTRA GOTO 170 480 CP 1 GOSUB 4000 REM LOOK FOR BOOKS 490 IF DE 0 THEN 530 500 PRINT YOU HAVE A BOOK GOSUB 5000 510 PRINT YOU DISCARD THIS BOOK 520 PB PB 1 PN PN 4 IF 0 THEN PRINT YOU ARE OUT OF CARDS GOTO 950 530 PRINT PRINT THE COMPUTER HAS DIS CARDED CB BOOKS 540 PRINT IT HAS CN CARDS 550 REM PRINT THEY ARE 560 REM FOR 1 TO 13 570 REM FOR SU 1 TO 4 580 REM IF DK SU DE 2 THEN GOSUB 2000 PRINT 590 REM NEXT SU 600 REM NEXT DE 610 REM PRINT 620 CP 2 GOSUB 4000 630 IF DE 0 THEN 670 640 PRINT THE COMPUTER HAS A BOOK OF GOSUB 5000 47 650 DISCARDS THIS BOOK 660 CB CB 1 CN CN 4 IF CN 0 THEN PRINT THE COMPUT ER IS OUT OF CARDS GOTO 950 670 DE INT RND 0 13 1 680 IF DK 1 DE 3 THEN 670 690 CP 2 GOSUB 6000 700 IF FL 0 THEN 670 710 PRINT THE COMPUTER SAYS 720 PRINT GIVE ME ALL OF YOUR GOSUB 5000 730 PRINT PRESS lt RETURN gt TO CONTINUE 740 INPUT Q 750 NC 0 760 FOR SU 1 4 770 IF DK SU DE 1 THEN NC NC 1 DK SU DE 2 780 NEXT SU 790 IF NC O THEN 840 800 CN CN NC PN PN NC 810 PRINT YOU GIVE THE COMPUTER NC GOSUB 5000 820 IF 0 THEN
11. A BOOK OF GOSUB 5000 920 PRINT DISCARDS THIS BOOK All the numbers must be properly adjusted and we must check to see if the computer has run out of cards 930 t 1 CN CN 4 IF CN 0 THEN PRINT THE COMPUT ER IS OUT OF CARDS GOTO 950 Finally we return to the start of the main loop so that the human player can have another turn as the game continues 940 PRINT GOTO 170 The PRINT statement merely adds a blank line on the video dis play to make it more readable We have made several references to a routine on line 950 which ends the game and congratulates the winner Here it is 950 REM CONGRATULATE THE WINNER 960 PRINT THE GAME IS OVER 970 PRINT YOU HAVE PB BOOKS 980 PRINT THE COMPUTER HAS CB BOOKS 990 IF CB PB THEN PRINT THE COMPUTER HAS WON GOTO 1020 1000 IF PB THEN PRINT YOU HAVE WOW GOTO 1020 1010 PRINT THE GAME IS A TIE 43 We politely ask the player if or she wants to play again 1020 PRINT PLAY AGAIN 1 YES 2 1030 INPUT Q 1040 IF Q 1 THEN 30 REM IF YES PLAY AGAIN 1050 IF Q 2 THEN END REM IF NO END PROGRAM 1060 GOTO 1020 REM IF NEITHER ASK AGAIN And that is the Go Fish program complete listing is given at the end of this chapter for you to type on your computer We should mention that this is a stripped down no frills ver sion of Go Fish There is a lot that can be done
12. DEALT 2040 DK SU DE CP ND ND 1 REM IF NOT CHECK IT OFF 2050 RETURN This is a very important subroutine so we will describe in some detail what it does Line 2000 checks to make sure that there are still cards in the deck In Go Fish the deck shouldn t run out of cards so the instruction GOTO 2020 should always be executed If not some thing is very wrong and line 2010 will stop the program with a 20 message reading THE DECK IS EMPTY If this happens check your program to make sure it is correctly typed Line 2020 uses the RND function to choose a random suit which it stores in variable SU and a random denomination which it stores in variable DE How does the RND random func tion work Unfortunately it works differently in the various ver sions of BASIC so you may have to make some changes here In most versions of BASIC RND 0 will be equal to a random fraction that is larger than 0 and smaller than 1 This may be dif ficult to understand but you can simply think of the RND func tion as a way of coming up with an unpredictable number between 0 and 1 but not including 1 The first time we use RND 0 in the above subroutine we write SU INT RND 0 4 1 The RND 0 function produces a random fraction between 0 and 1 Multiplying it by 4 changes this fraction into a random number between 0 and 4 but not including 4 Adding 1 to this changes it into a random number between 1 and 5 but n
13. EVERY DENOMINATION 70 DK SU DE 0 REM SET IT EQUAL TO 0 80 NEXT DE 90 NEXT SU 19 The first loop FOR SU 1 TO 4 will execute four times The second loop FOR 1 TO 13 is inside the first loop so it will execute thirteen times every time the first loop executes for a grand total of fifty two executions once for every card in the deck The first loop steps the variable SU through all four values that a suit can take that is 1 through 4 The second loop inside the first steps the variable DE through all 13 values that a denom ination can take that is 1 through 13 Between them they step DK SU DE through every possible element in the array Line 70 sets each of these elements to 0 Now the action starts We must deal seven cards to each player from this deck Dealing cards is such an important part of the program that we will put part of this operation in a subroutine because it will be performed several times in the game subrou tine as you should already know is a routine that is called with the GOSUB statement and ends with a RETURN statement We will write a subroutine that will deal one random card from the deck of 52 That subroutine will look like this 2000 IF ND 0 THEN GOTO 2020 REM IF CARDS IN DECK DEAL EM 2010 PRINT THE DECK IS EMPTY END REM IF NOT END GAME 2020 SU INT RND 0 4 1 DE INT RND 0 13 1 REM RANDOM CARD 2030 IF DK SU DE 0 THEN 2020 REM ALREADY
14. Go Fish a player that is successful in requesting cards from the other player is automatically given a free turn Therefore we jump back to the beginning of the main loop 430 GOTO 170 REM REPEAT HUMAN PLAYER S TURN A glance at line 170 will tell you that this will cause the player s hand to be printed out again and another search to be conducted for books A search for new books should be conducted every time the player puts new cards in his or her hand What if the player was unsuccessful in requesting cards The computer will have jumped ahead to line 440 as in the instruction in line 390 This is where the computer tells the player to Fish 440 PRINT THE COMPUTER SAYS FISH Fishing is a simple matter of drawing a new card from the deck and this means that we must call our card dealing subrou tine at line 2000 to deal us a random card like this 450 PD DE 1 GOSUB 2000 DK SU DE 1 PN PN 1 What is this new variable called PD It stands for previous denomination The value of variable DE which holds the denomination that the player asked for is about to be changed by the subroutine at line 2000 However we want to remember what denomination this was so we save it in PD The reason for this will become obvious in a moment 36 The rest of line 450 sets equal to 1 and calls the subroutine at line 2000 to deal a random card to the human player The instruction PN
15. PRINT KINGS RETURN This is a long but simple subroutine It does nothing but check the value of DE and print an appropriate name for the denomination Line 5120 assumes that the denomination must be KINGS if the computer has gotten that far The result of calling this subroutine is that line 280 above will print out something like this YOU HAVE A BOOK OF JACKS if the value of DE is 11 or YOU HAVE A BOOK OF FOURS if the value of DE is 4 and so forth Line 290 prints YOU DISCARD THIS BOOK so that you will know that it is no longer in your hand We must also increase the value of variable PB which counts the number of books that the player has discarded and decrease the value of PN which counts the number of cards in the player s hand If PN equals 0 we must end the game because the player is out of cards of this is done here 300 1 PN PN 4 IF 0 THEN PRINT YOU ARE OUT OF CARDS GOTO 950 32 This tells us that the player has one more book and four less cards It is impossible for the game to be over after a single book has been discarded but this routine will be executed many more times in the course of the game and so we must check to see if the player has run out of cards thus ending the game If so the com puter prints YOU ARE OUT OF CARDS then jumps to line 950 The routine beginning at line 950 will announce that the game is over then decide who has won and co
16. Subroutines and of card games 13 15 checkers Subroutines and of checkers 65 see also tic tac toe Arrays GOTO command in card Dealing cards 14 20 24 36 games 20 37 42 Graphics 3 44 see also Draw Decision making 39 ing board games Denominations 16 30 42 see also Numbering cards IBM computer instructions for DIM command 5 11 18 card games 18 44 Dimension See DIM command Improvements 44 75 76 Drawing board games 55 56 INPUT command 7 8 65 67 card games 41 44 in tic tac toe 60 Elements 6 11 18 Instruction 5ee Assignment defined 5 statement Routine Subrou in card games 18 22 29 tine and specific commands 84 Instructions for Apple comput er 8 21 22 26 28 56 60 Instructions for Atari comput er 18 22 26 28 56 Instructions for Commodore computer 18 22 60 Instructions for Radio Shack computer 18 22 44 Instructions for Timex Sinclair computer 18 28 International Business Ma chine See IBM computer in structions INT function in checkers 67 Internal memory 3 10 Interrupt function See INT function in checkers Kilobytes 10 see also Internal memory Legality of checkers program 68 72 74 LET command 4 Limitations of computer 10 Loops 8 9 20 in card games 19 20 23 29 31 34 36 37 42 43 in checkers 74 75 in tic tac toe 57 59 60 65 Maps 81 Memory See Internal memory Moving checkers 67 74 see also Forced jump Multidimens
17. in variable NB is 1 In the same way the computer will say things like THE COMPUTER GIVES YOU 1 ACES Add instructions to tell the computer that 71 aces should be 1 ace Another area you might want to improve is in the strategy by which the computer chooses the denomination it requests of the human player As explained earlier this choice is made random ly but it doesn t have to be While you play the game notice the way in which you make your own choices about which cards to ask for Do you notice which denomination the computer keeps ask ing you for and therefore must have in its hand then wait until you draw a card of that denomination so that you can get those cards from the computer Do you ever ask the computer for a card that you know it cannot possibly have Are there certain circum stances under which you must ask the computer for a card you know it does not have Once you have figured out a strategy see if you can program that strategy into the computer As you play the game of course you will see other areas where it could use improvement Make all the improvements you want As written this game is really a skeleton waiting for you to flesh it out to add the extra features that programmers call bells and whistles And when you have put the program together exactly as you want it try it out in your friends But don t be surprised if they have a few suggestions of their own Here s the complete pro
18. row and column numbers of the destination square must differ from the row and column numbers of the source square by exactly 1 We can test for this by subtracting R2 from R1 and C2 from C1 and checking to see if the result of each subtraction is either 1 or 1 negative To simplify this operation we will use the BASIC ABS or absolute value function The ABS function will change the number 1 to the number 1 but will leave the num ber 1 unchanged Thus ABS 1 will be equal to 1 and so will ABS 1 Here is how we check 230 IF ABS C1 C2 1 THEN 270 240 IF ABS R1 R2 lt gt 1 THEN 270 If the difference in either the row or column numbers is not equal to 1 1 we jump to line 270 What happens on line 270 There is one exception to the rule that we must always move one square horizontally and one square vertically If we are capturing one of our opponent s pieces we must jump over that piece diagonally and thus move our piece two squares horizontally and vertically Line 270 will check for this possibility We ll look at line 270 in a minute 70 Meanwhile if the requested move passes the first test we must also check to see if the piece was moved in the proper direc tion that is in the direction of our opponent s side of the board If we are playing black this means that we must move from the low numbered rows to the high numbered rows If we are playing red we must move from the high n
19. the class roll book The arrays that we ve been showing you are called one dimen sional arrays This is a fancy way of saying that each element of the array has one subscript after it Most versions of BASIC how ever allow us to create arrays that have more than one subscript after each element like this DK 2 9 BG 1 9 13 7 89 3 1 2 An that has two subscripts after each element is called two dimensional array An array that has three subscripts after each element is called a three dimensional array and so forth Some versions of BASIC have a limit on the number of dimen sions you can have in an array but most allow at least three dimensional arrays A more important limit is the amount of inter nal memory storage your computer has This is usually measured in kilobytes Ks as in 64K or 256K memory When an array has a lot of subscripts it eats up a lot of memory Your computer will tell you if you try to use more memory than it has Don t worry you won t lose this memory permanently It will return as soon as the program and its arrays are removed from the computer s memory We create multidimensional arrays just as we create one dimensional arrays with a DIM statement The difference is that we have to specify the extra subscripts like this 10 DIM AB 14 7 This DIM statement creates an array called AB with two dimen sions There are fourteen possible subscripts in the first dimension and se
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21. with this program and we encourage you to experiment As we noted earlier there are no graphics in this program no pictures It would be very satisfying if the player could see the cards as they are dealt A lot of computers like the Commodore 64 the Atari 800 the Radio Shack Model III 4 and the IBM PC have the heart spade club and diamond symbols built into their character sets You could use these characters to replace the cryp tic abbreviations Ht 5 and Di that we use for these suits in the subroutine at line 3000 And if you feel particularly ambitious you could even add sound effects so that the player could hear a whooshing sound as each card is dealt Many versions of BASIC have a SOUND com mand that can be used to produce such sound effects How else can you improve this program Well you will notice several rough edges in the way the game talks to the player We have left these rough edges in because it would have complicated the program to remove them However you might want to add some instructions that will get rid of them For instance if you have discarded only one book in the game the computer will continually remind you that YOU HAVE DISCARDED 1 BOOKS 44 Nobody with decent command of the English language says 1 books but the computer doesn t know any better You can add instructions telling the computer to change the word books to book when the number of books
22. 0 GOSUB 1000 REM DRAW CHECKERBOARD Finally it s time for the game to begin The following instruc tions will be executed for every move in the game until someone wins First we prompt the current player CP to make a move 110 PRINT PRINT PLAYER CP IT S YOUR MOVE The extra PRINT statement adds a space between the picture of the board and the prompt for neatness Next we ask the player which piece he or she wishes to move 120 PRINT WHICH PIECE DO YOU WISH TO MOVE The instructions that input the row and column numbers of the piece will be used a couple of times so we ll put them in a subroutine like this 67 2000 INPUT ROW 2010 IF ROW lt 1 OR ROW gt 8 THEN PRINT SUCH ROW TRY AGAIN GOTO 2000 2020 PRINT COLUMN INPUT COL 2030 IF COL lt 1 OR COL gt 8 THEN SUCH COLUMN TRY AGAIN GOTO 2020 2040 RETURN This subroutine prompts the user to type in a ROW number which is stored in variable ROW followed by a column number which is stored in variable COL Although we have used three letter variable names here for clarity most versions of BASIC will see only the first two letters of each name so that these will be treated as variables RO and CO The value of each variable is checked to make sure that it falls in the range 1 to 8 and the player is given an error message and a second chance to type the number if it does not We ll call thi
23. 0 IF CB ROW COL 2 THEN PRINT RD GOTO 1140 1120 IF CB ROW COL 3 THEN PRINT BK GOTO 1140 1130 IF CB ROW COL 4 THEN PRINT 1140 NEXT COL 1150 PRINT 1160 NEXT ROW 1170 RETURN 2000 PRINT ROW INPUT ROW 2010 IF ROW lt 1 OR gt 8 THEN PRINT SUCH ROW TRY AGAIN GOTO 2000 2020 PRINT COLUMN INPUT COL 2030 IF COL lt 1 OR C0L gt 8 THEN SUCH COLUMN TRY AGAIN GOTO 2020 2040 RETURN 3000 DATA 0 1 0 1 0 1 0 1 3010 DATA 1 0 1 0 1 0 1 0 3020 DATA 0 1 0 1 0 1 0 1 3030 DATA 0 0 0 0 0 0 0 0 3040 DATA 0 0 0 0 0 0 0 0 3050 DATA 2 0 2 0 2 0 2 0 3060 DATA 0 2 0 2 0 2 0 2 3070 DATA 2 0 2 0 2 0 2 0 79 CHAPTER SIX ALL THE WORLD S A GAME Once you know how data structures such as arrays can be used to simulate decks of cards and game boards well the sky s the limit Just think of the other games you could write programs for The same array that we used to create a checkerboard CB 8 8 could represent a chessboard except that a much larger set of numbers would be needed to represent the great variety of pieces in chess And there s no reason why you should stop with a chess board You could use a two dimensional array to represent a map of a country or an entire planet as part of a political or war game Each element of the array could be assigned a number indicating what kind of terrain is found at that point on the map The num ber 1 coul
24. 120 RETURN 4989 REM 4990 REM NAME THE DENOMINATION 4991 REM 4992 REM A SUBROUTINE TO PRINT THE 4993 REM NAME OF DENOMINATION DE TO 4994 REM THE VIDEO DISPLAY AS A WORD 4995 REM 4996 REM 5000 IF DE 1 THEN PRINT ACES RETURN 5010 IF DE 2 THEN PRINT RETURN 5020 IF DE 3 THEN PRINT THREES RETURN 5030 IF DE 4 THEN PRINT FOURS RETURN 5040 IF DE 5 THEN PRINT FIVES RETURN 5050 IF DE 6 THEN PRINT SIXES RETURN 5060 IF DE 7 THEN PRINT SEVENS RETURN 5070 IF DE 8 THEN PRINT EIGHTS RETURN 5080 IF DE 9 THEN PRINT NINES RETURN 5090 IF DE 10 THEN PRINT TENS RETURN 5100 IF DE 11 THEN PRINT JACKS RETURN 5110 IF DE 12 THEN PRINT QUEENS RETURN 5120 PRINT KINGS RETURN 5989 REM 5990 REM CHECK HAND FOR DENOMINATION 5991 REM 5992 REM A SUBROUTINE TO CHECK THE 5993 REM HAND OF THE CURRENT PLAYER CP 5994 REM DENOMINATION DE THE VARIABLE 5996 REM FL IS SET EQUAL TO 1 IF SO 5997 REM AND 0 IF NOT 5998 REM 51 5999 6000 FL 0 6010 FOR SU 1 TO 4 6020 IF DK SU DE CP THEN FL 1 6030 NEXT SU 6040 RETURN 52 CHAPTER FOUR SOMETHING COMPLETELY DIFFERENT A variable array seems ideally suited for representing a deck of cards as we saw in Chapter Two The truth is variable arrays are ideally suited for representing almost anything In this chapter we will show how the da
25. 60 NEXT DE 4070 DE 0 REM NO BOOK WAS FOUND 4080 RETURN 4090 FOR SU 1 TO 4 REM ELSE IF FOUND THEN DISCARD IT 4100 DK SU DE 3 REM 3 DISCARDED BOOK 4110 NEXT SU 4120 RETURN Before calling this routine variable CP must be set equal to 1 if you wish to search the player s hand and 2 if you wish to search the computer s hand Once again we use a pair of nested FOR NEXT loops to pro ceed through the entire deck that is one FOR NEXT loop inside a second FOR NEXT loop as in our shuffling routine The FOR NEXT loop that begins in line 4000 steps variable DE through all thirteen denominations The second FOR NEXT loop which begins in line 4020 steps variable SU through all four suits This loop executes four times as the first loop executes once Variable NC counts the number of cards of each denomination that the current player is hold ing This variable is set to 0 each time the first loop executes right before the second loop executes Line 4030 counts the number of cards of the current denomination in CP s hand Line 4040 checks to see if the count reaches 4 meaning that there is a full book of that denomination in CP s hand If such a book is found the 30 search is terminated and the computer jumps ahead to line 4090 There can never be more than one book in hand at a time so there is no need to search further If no book is found DE is set equal to 0 and the subroutine RETURNS to the main
26. AY AGAIN 430 IF Q 2 THEN END REM IF NO END PROGRAM 440 GOTO 400 REM IF NEITHER ASK AGAIN Here is the complete tic tac toe program 60 10 REM Add an instruction here to clear the video display of your com puter See Chapter Two for details 20 DIM TB 3 3 30 0 0 40 FOR ROW 1 50 FOR COL 1 TO 3 60 TB ROW COL 0 70 NEXT COL 80 NEXT ROW 90 GOSUB 1000 REM DRAW TIC TAC TOE BOARD 100 NU NM 1 1F NM gt 9 THEN PRINT PRINT TIE GAME GOTO 400 110 CP CP 1 120 IF CP gt 2 THEN 1 130 MOVE 140 ROW 150 INPUT ROW 155 IF ROW 1 THEN PRINT ABORTED GOTO 400 160 PRINT COLUMN 170 INPUT COL 180 PRINT 190 IF COL lt 1 OR COL gt 3 OR ROW 1 OR ROW gt 3 THEN GOTO 130 200 IF TB ROW COL 0 THEN TB ROW COL CP GOTO 230 210 PRINT THAT SQUARE IS TAKEN TRY AGAIN 220 GOTO 130 230 LL 0 240 FOR 1 1 250 IF I8 I C0L lt gt CL THEN FL 1 260 NEXT 270 IF FL 0 THEN 370 280 FL 0 290 FOH 1 TO 3 300 IF TB ROW lI lt gt CP THEN FL 1 310 NEXT I 61 320 IF FL 0 THEN 370 340 IF TB 1 1 TB 2 2 TB 3 3 CP 3 THEN 370 350 IF TB 3 1 TB 2 2 TB 1 3 CP 3 THEN 370 360 GOTO 90 370 GOSUB 1000 REM DRAW TIC TAC TOE BOARD 380 PRINT CONGRATULATIONS PLAYER CP YOU HAVE WON 390 PRINT 400 PRINT PLAY AGAIN 1 YES 2 410 INPUT Q 420 IF Q 1 THEN GO
27. C 2 4 3 2 7 Ifa E INDA OER D eR da 5 eC DLS on 8 29 FLORE RET A MEE 4 75 Torren aurea vary VISCUSOWO 3 2 SO 255 iS cap nce EA 2 22 Lp WARDS bide UE OL LPE 9 9 A egos on Pee A CN VEST XL 9 boo A 5 BIB X ap up Le ANYI 21 P San Fe RED la 4 IS IRE oW 1 LTS Re 3 CCE 8 T A DA V Cu HA A Hig Po par DOR S Vos UA AES Sup TR Ei AREA O ELA EEL Natal 46 25 usce EIE idn eC T NUN UNS US ER TU PET E PCS cst esie Nite Bay d S ESSEN SOLE Re ROAD U UT POS se PS Ti E PRA ye RTT EB UT ASIE eas mest i 2 55 AT SAS m as CLES Re OES USGI Nir
28. EN 270 240 IF ABS R1 R2 lt gt 1 THEN 270 250 IF R2 R1 lt gt LM AND CB R1 C1 lt 3 THEN 460 260 GOTO 330 270 IF ABS C1 C2 2 THEN 460 280 IF ABS R1 R2 lt gt 2 THEN 460 290 IF R2 R1 lt gt LM 2 AND CB R1 C1 lt 3 THEN 460 300 IF CB R1 R2 R1 2 C1 C2 C1 2 lt gt OP THEN 460 310 PRINT YOU HAVE CAPTURED A PIECE 320 CB R1 R2 R1 2 C1 C2 C1 2 0 77 330 CB R2 C2 CB R1 C1 340 CB R1 C1 0 350 IF R2 lt gt KR OR CB R2 C2 gt 2 THEN 370 360 PRINT YOU HAVE GAINED A CB R2 C2 CB R2 C2 2 370 FL 0 380 FOR ROW 1 8 390 FOR COL 1 8 400 IF CB ROW COL OP THEN FL 1 410 NEXT COL 420 NEXT ROW 430 IF FL 0 THEN 470 440 IF CP 1 THEN CP 2 1 GOTO 100 450 CP 1 2 GOTO 100 460 PRINT ILLEGAL MOVE GOSUB 1000 GOTO 120 470 GOSUB 1000 480 PRINT CONGRATULATIONS PLAYER CP 490 PRINT YOU HAVE WON 500 PRINT WOULD YOU LIKE TO PLAY AGAIN 510 1 YES 2 NO 520 INPUT Q 530 IF Q 1 THEN 30 540 IF Q 2 THEN END 550 GOTO 520 1000 PRINT 1010 FOR COL 1 TO 8 1020 COL 1030 NEXT 1040 PRINT 1050 FOR ROW 1 8 1060 PRINT ROW 1070 FOR COL 71 8 1080 IF INT ROW 4 COL 2 2 ROW COL THEN GOTO 1140 1090 IF CB ROW COL 0 THEN PRINT 5 GOTO 1140 78 1100 IF CB ROW COL 1 THEN PRINT BL GOTO 1140 111
29. HE COMPUTER HAS A BOOK OF GOSUB 5000 REM ELSE ANNOUNCE IT 650 PRINT DISCARDS THIS 660 1 4 PRINT THE COMPUT ER IS GOTO 950 The last line adjusts variable CB the number of books discarded by the computer and variable CN the number of cards in the computer s hand If the computer is out of cards we jump to line 950 which announces the end of the game Now it is time for the computer to ask the human player for cards of a certain denomination This is a very important part of the program It is here that we give the computer its game playing strategy its method of decision making Strategy Decision making Those are pretty heavy concepts for a computer program They smack of artificial intelligence computer programs that actually think like human beings Maybe so but we will beg this issue by giving the computer a very simple strategy for choosing the denomination it asks for We will have it choose a denomination at random This might sound like a pretty poor strategy but you ll be sur prised at how well it works In fact the computer may beat you the first few times you play against it unless you are a very expe rienced Go Fish player random strategy in Go Fish is fairly effective though a clever human player will learn to beat the machine most of the time anyway Of course the computer will occasionally do stupid things like ask for card
30. NEXT loops and a READ statement like this 40 FOR ROW 1 8 REM LOOP THROUGH EACH ROW 50 FOR COL 1 8 REM AND EACH COLUMN 60 READ A REM READ THE DATA FOR THAT SQUARE 70 CB ROW COL A REM INITIALIZE THE SQUARE 80 NEXT COL 90 NEXT ROW As in the tic tac toe program we ll include a special subrou tine to draw the board Black squares will be represented by a pair of hyphens red squares by a pair of equals signs black 65 pieces the letters BL red pieces by the letters RD black kings by the letters BK and red kings by the letters RK Here s the sub routine 1000 PRINT 1010 FOR COL 1 TO 8 REM FOR ALL 8 COLUMNS 1020 PRINT COL 5 PRINT THE COLUMN NUMBERS 1030 NEXT 1040 PRINT 1050 FOR ROW 1 TO 8 REM FOR ALL 8 ROWS 1060 PRINT ROW PRINT THE ROW NUMBERS 1070 FOR 1 TO8 REM AND PRINT EACH SQUARE 1080 IF INT ROW COL 2 2 ROW COL THEN PRINT GOTO 1140 REM PRINT RED SQUARE 1090 IF CB ROW COL 0 THEN PRINT GOTO 1140 REM PRINT BLACK SQUARE 1100 IF CB ROW COL 1 THEN PRINT BL GOTO 1140 1110 IF CB ROW COL 2 THEN PRINT RD GOTO 1140 1120 IF CB ROW COL 3 THEN PRINT BK GOTO 1140 1130 IF CB ROW COL 4 THEN PRINT 1140 NEXT COL 1150 PRINT 1160 NEXT ROW 1170 RETURN This is similar to the subroutine that drew the tic tac toe board but more com
31. PRINT YOU ARE OUT OF CARDS GOTO 950 830 GOTO 530 840 PRINT YOU TELL THE COMPUTER GO FISH 850 PD DE 2 GOSUB 2000 DK SU DE 2 CN OCN 1 860 PRINT THE COMPUTER DRAWS FROM THE DECK IF PD lt gt DE THEN 890 870 PRINT THE COMPUTER HAS DRAWN THE GOSUB 3000 PRINT 880 PRINT GETS AN EXTRA TURN GOTO 530 890 CP 2 GOSUB 4000 REM LOOK FOR BOOKS 900 IF DE 0 THEN 940 910 PRINT THE COMPUTER HAS A BOOK OF GOSUB 5000 920 PRINT DISCARDS THIS BOOK 930 CB CB 1 CN CN 4 IF CN 0 THEN PRINT THE COMPUT ER IS OUT OF CARDS GOTO 950 940 PRINT GOTO 170 48 950 REM CONGRATULATE THE WINNER 960 PRINT GAME IS OVER 970 PRINT YOU HAVE PB BOOKS 980 PRINT THE COMPUTER HAS CB BOOKS 990 IF CB PB THEN PRINT THE COMPUTER HAS WON GOTO 1020 1000 IF PB CB THEN PRINT YOU HAVE WON GOTO 1020 1010 PRINT THE GAME IS A TIE 1020 PRINT PLAY AGAIN 1 YES 2 NO 1030 INPUT Q 1040 IF Q 1 THEN 30 1050 IF Q 2 THEN END 1060 GOTO 1020 1989 REM 1990 REM DEAL A CARD 1991 REM 1992 REM SUBROUTINE TO DEAL ONE CARD 1993 REM FROM A DECK OF 52 CARDS IN 1994 REM ARRAY DK SUIT DENOMINATION 1995 REM 1996 REM 2000 IF ND gt 0 THEN GOTO 2020 2010 PRINT THE DECK IS EMPTY END 2020 SU INT RND 0 4 1 DE INT RND 0 13 1 2030 IF DK SU DE lt gt 0 THEN
32. TB 3 3 CP 3 THEN 370 350 IF TB 3 1 TB 2 2 TB 1 3 CP 3 THEN 370 The square bracket symbol in lines 340 and 350 is a special arithmetic operator It raises a number to a power that is multi 59 plies a number by itself a certain number of times On some ver sions of BASIC such as Applesoft this should be replaced by a circumflex symbol and on others such as Commodore BASIC by a symbol that looks like an upward pointing arrow T If player 1 has placed markers on all squares in the diagonal then the result of multiplying the values in all of those squares together as we have done above will be 1 multiplied by itself three times or 1 If player 2 has placed markers in all three squares then the result of multiplying the values together will be 2 multiplied by itself three times or 8 Thus we can see if any player has occupied all squares in the diagonal by checking if the result of this multi plication is 1 or 8 Finally we must loop back draw the board again and let the other player make a move 360 GOTO 90 REM DO IT AGAIN When the game is over we must print a picture of the board and congratulate the winner 370 GOSUB 1000 REM DRAW TIC TAC TOE BOARD 380 PRINT CONGRATULATIONS PLAYER CP YOU HAVE WON 390 PRINT As before we will let the players decide whether to play again 400 PRINT PLAY AGAIN 1 YES 2 410 INPUT Q 420 IF Q 1 THEN GOTO 30 REM IF YES PL
33. TO 30 430 IF Q 2 THEN END 440 GOTO 400 1000 REM DRAW TIC TAC TOE BOARD 1010 PRINT 1020 FOR COL 1 TO3 1030 PRINT COL 1040 NEXT COL 1050 PRINT 1060 FOR ROW 1 3 1070 PRINT ROW 1080 FOR COL 1 TO 3 1090 IF TB ROW COL 0 THEN PRINT 1100 IF TB ROW COL 1 THEN X 1110 IF TB ROW COL 2 THEN PRINT 07 1120 NEXT COL 1130 PRINT 1140 NEXT ROW 1150 PRINT 1160 RETURN 62 CHAPTER FIVE CHECKERS Now that we know how to set up a game board it s a simple well relatively simple matter to write programs for more plex board games In this chapter we ll write a program for the game checkers Like the tic tac toe program in the last chapter our checkers program will not play the game against a human player rather it will provide a playing board that two human players can use to play against each other However it will check for legal moves and detect when one player or the other has won Presumably you ve played a game of checkers at some time in your life To refresh your memory of the rules here s a brief sum mary Checkers is played on a board of sixty four red and black squares arranged in eight rows of eight squares each in alternat ing colors Each player has twelve pieces either red or black in color At the beginning of the game these pieces are placed in the black squares of the first three rows on either side of the board black pieces on
34. URN to the main program There is one problem with line 3000 as it is now written We mentioned earlier that most versions of BASIC print extra blank spaces before and after a number If this happens in line 3000 it will mess up the card name by adding spaces to it If you are using one of the versions of BASIC that does not add these extra spaces such as Applesoft or Atari BASIC you can use this sub routine as is However in the final version of this program we will make a change to this line to remove the extra spaces like this 3000 IF DE gt 1 AND DE lt 10 THEN PRINT CHR DE ASC 0 GOTO 3060 If you are using Timex Sinclair BASIC you will need to change the letters ASC to CODE You don t need to understand 28 how this line works Just take our word for it that it does Unfor tunately it won t work properly if the card is 10 which is why we have deliberately singled out 10 for special treatment on line 3010 Now that we have a subroutine that prints out the name of the card we can call this subroutine to print out the cards in the human player s hand We determine which cards are in the player s hand by looking through the entire deck and checking to see which elements of array DK are equal to 0 like this 200 FOR DE 1 TO 13 REM CHECK ALL 13 DENOMINATIONS 210 FOR SU 1 TO 4 REM CHECK ALL 4 SUITS 220 IF DK SU DE 1 THEN GOSUB 3000 PRINT REM IF THE PLAYER IS HOLDING THE CARD
35. ady know few things about computer programming For instance you should know how to write computer programs using the programming language called BASIC If you don t know anything about BASIC or if you don t even know what a pro gramming language is then this book will probably be too tough for you We suggest that you read at least one introductory book on BASIC programming before you continue The program examples in this book are intended to run on any computer equipped to understand BASIC that is a computer that is running a BASIC interpreter the program that translates BASIC instructions into an electronic form that the computer understands Of course this means that we ve left out features such as graphics pictures that you might expect to find in a game program because the way in which graphics are programmed varies widely from one computer to another However we ll make suggestions as to how you can add these features if you wish The key to simulating games on a computer is choosing the proper data structure What is a data structure It is the way in which information or data is organized within the internal memory storage of the computer The BASIC programming language does not offer us as many possible data structures as some more sophisticated languages like Pascal do Nonetheless with a little creative imagination we can make fine use of the data structures that BASIC does offer By arranging information properly
36. ation we would draw pictures of the cards on the video dis play to show the player what cards he or she is holding Howev er as we said earlier we are going to write this program without any pictures since different computers draw pictures using differ ent BASIC instructions If you want to add some instructions to the program that will draw pictures of the seven cards we have just dealt go right ahead In fact this would be a terrific way of exploring the graphics capabilities of your computer Of course you will have to consult the user s manual for your computer to find out what sort of graphics instructions are available if you don t know already Instead of drawing a picture here we will print out a list of the cards that the human player is holding We will also print out other important information the number of cards that the player 25 is holding and the number of books that the player has dis carded Printing out the number of books that have been discarded is easy This number you will remember is contained in variable PB player s books so we can print it out like this 170 PRINT YOU HAVE DISCARDED PB BOOKS Notice that we don t leave any blank spaces around the number of books This is because most versions of BASIC will automatically place blank spaces before and after a number If you are using a version of BASIC that does rot do this such as Applesoft or Atari BASIC you will need to add s
37. bove you are not just creating a single variable you are creating ten different variables Here are their names 6 2 8 4 9 5 10 Actually there is an eleventh variable in this array called 0 However to avoid confusion we are not going to get into that Each of the ten variables in this array is called an element of the array Each element is a completely separate variable and can be used to store a different item of data The number in paren theses after the name is called a subscript It tells us which element 5 of the array we are dealing with For instance the element A 6 has a subscript of 6 so we know it is element 6 of the array Since the array above is a numeric array that is an array made up of numeric variables we can use each element to store a number We can assign a value to an element of an array just like we assign a value to a regular variable like this A 2 45 This assigns a value of 45 to the second element of array A We can print that value on the video display like this PRINT A 2 In fact we can do exactly the same things with an element of an array as we can do with an ordinary variable including assigning the value of the array element to another variable B A 2 or performing arithmetic on the array element C A 2 15 and so forth Wait a minute you exclaim If we can do exactly the same things wit
38. ce we ve assigned a value to a variable we can retrieve that value just by using the variable s name For instance if we write the instruction PRINT A it will cause the computer to print the value of variable A on the video display If variable A is still equal to 178 then the computer will print the number 178 Because the computer treats the variable almost as though it actually were the number we have assigned to it we often say that we have set the variable equal to the number Once again you should know all of this already because you ve written BASIC programs before However you may still be surprised at a few of the things we can do with numeric vari 4 ables For instance there is special kind of data structure found in most programming languages called the array An array is a list of variables that can be used to hold a list of data items In BASIC an array variable looks like an ordinary variable except that the name of the variable is followed by one or more numbers in pa rentheses like this A 1 BG 14 Y6 10 1 All of these are names that can be used for array variables Before you can use an array variable in a BASIC program however you must dimension the array using a DIM statement like this DIM A 10 This tells the BASIC interpreter that you are establishing an array called A and that this array will have ten elements What is an element Well when you create an array like the one a
39. d can be played on a computer Of course some games lend themselves better to simulation than others For instance while it is possible to simulate a swimming meet by depicting the pool and the swim mers on the computer s video display the computer can never duplicate the tingling thrill of plunging into cool water or the exhilaration of physical exercise Something is definitely lost in the translation On the other hand games and sports such as auto racing and skydiving are best played on computer at least for those of us who would rather not risk our lives to have fun A computer is a number crunching machine a high class cal culator that adds and subtracts numbers at high speeds How can it simulate games that have nothing to do with numbers The ability of the computer to play games is an illusion but it is a wonderful illusion When a computer simulates a game or anything else for that matter it converts the elements of that game into numbers and performs special operations on those numbers When you play games on a computer though you don t see these numbers because they are inside the computer A clever computer programmer found a way to make those numbers look like something else a deck of playing cards or a game board race track or the surface of an alien planet In this book we will show you how to create simple comput erized simulations of familiar games We will assume that you 2 alre
40. d represent grassland the number 2 could represent arid desert 3 could represent ocean 4 could represent mountains and special numbers could represent major cities such as New York Washington London Paris or Moscow assuming that the plan et is Earth 81 three dimensional array could represent a portion of outer space or a scaled down model of the entire universe with the subscripts indicating how far up down left right or sideways each element is from some starting point Elements could be assigned numbers to indicate whether they represent stars plan ets or just empty space Players could move through the universe simply by changing elements in the array Of course it would take an imaginative programmer to devise pictures to do justice to such an array You could fill the video display with stars and planets and spaceships to show the player what kind of universe the array represents And that s what com puter game simulations are all about converting your imagina tion into numbers inside the computer then converting those numbers back into something that a person can sit down at the computer and play with In this book we ve given you a starting point for creating sim ulations of your own Now go ahead and think of new games to simulate and write the programs yourself Even the sky isn t the limit 82 INDEX ABS function in checkers 70 Artificial intelligence 39 72 Assignment state
41. e the initial jump That rule is not included in our game but can be added with some work 75 Another feature that is missing here is the forced jump Once again standard checkers rules require that a player make a jump if it is available even if the jump will place his or her piece in an undesirable position This rule is not included here but could be added In fact you could add both of these features with the addition of a single subroutine What would this subroutine do It would look for jumps that the current player could make First it would scan the board looking for the current player s pieces Then when it found a square containing such a piece it would scan the squares attached diagonally to that square to see if the opposing player has pieces on those squares Finally it would scan the squares diagonally past the opposing player s pieces to see if they are empty which would mean that the opposing player s piece could be jumped If a possible jump is found this information would be recorded perhaps by setting flag variable such as the FL variable we use to detect a winning move equal to a spe cial code number This subroutine could be called to see if the current player should be forced to make a jump If a jump or more than one jump is available the program would not accept any move that isn t a jump After a jump is made the subroutine could be called a second time to see if a multiple jump is poss
42. e of instructions prompting the current player to input a 71 move If the move is legal we must jump ahead to the routine that makes the move like this 260 GOTO 330 REM MAKE THE MOVE Now we must consider those moves that failed the test in lines 230 and 240 because they moved more than one square As we said before the only case in which a player may move more than one square either horizontally or vertically is when an opponent s piece is being jumped and captured In this case the player s piece may be moved two squares horizontally and two squares vertical ly Once again we will use the ABS function to see if C1 C2 and R1 R2 are equal to either 2 or 2 270 IF ABS C1 C2 2 THEN 460 280 IF ABS R1 R2 lt gt 2 THEN 460 If the move fails either test we jump ahead to line 460 which tells the player that he or she has made an illegal move Next we must check again to see if the player was moving in a legal direction As before we test for the direction by subtracting R1 from R2 But now the result can be either 2 or 2 so we must multiply variable LM by 2 290 IF R2 R1 lt gt LM 2 AND CB R1 C1 lt 3 THEN 460 Even if the move passes these tests we must still make sure that one of the opponent s pieces has been jumped and captured Therefore we must check to see what was in the square that the player has jumped over But how do we determine the number of the square that was jumped The squa
43. e second position repre sents the king We could also write DK SU DE with SU representing any possible suit from 1 to 4 and DE repre senting any possible denomination from 1 to 13 By changing the values of these two variables DK SU DE can represent any pos sible card in the deck Why do we want to represent the deck as a two dimensional array Because at any time in the game any of the fifty two cards will be in one of four places in the deck in the human player s hand in the computer s hand or in a discarded book We can represent each of these as a number like this the deck in the player s hand the computer s hand in a discarded book In this way we keep track of each card setting the appropriate element of array DK equal to one of these numbers For instance if we want to put the 8 of diamonds in the player s hand we would write DK 3 8 1 Remember the 3 represents the suit of diamonds and the 8 repre sents the denomination 8 By setting this element equal to 1 we are making note of the fact that this card is in the player s hand a position that is represented by the number 1 We don t actually 17 have to move the card anywhere rather we use array DK to keep track of it in a way that the computer can understand This data structure the array DK is the key to our entire game Everything we do in the actual program will be based on t
44. e set to 1 if the human player s hand is to be searched and 2 if the computer s hand is to be searched and DE must equal the denomination to be searched for The subroutine loops through all cards of that denomination in array DK to see if any are equal to CP which indicates that the card is in that player s hand Since the denomination requested by the human player is already in DE we can now call this subroutine like this 330 CP 1 GOSUB 6000 REM SCAN PLAYER S HAND 340 IF FL 0 THEN PRINT YOU DON T HAVE ANY IN YOUR HAND GOTO 310 If no cards of that denomination are found in the player s hand then we have caught the player cheating Of course it was prob ably an accident We then loop back and ask the player again which denomination he or she would like On the other hand if the player is holding at least one card of that denomination we must search the computer s hand to see if it is holding cards of that denomination and if so how many The next few lines of our program do all that as well as move the cards from the computer s hand to the player s hand 350 NC 0 REM START COUNT AT 0 360 FOR SU 1 TO 4 REM CHECK ALL FOUR SUITS 370 IF DK SU DE 2 THEN 1 DK SU DE 1 REM IF IN COMPUTER S HAND 2 COUNT IT AND MOVE IT TO PLAYER S HAND 1 380 NEXT SU 34 These four lines do a lot They check through all four suits looking for cards of the requested denomination The numbe
45. e variable L X in line 30 will be equal to a different element of array L each time the loop executes The first time it will be L 1 because X will be equal to 1 The second time it will be L 2 and so on Each time the user will be prompted with a question mark to type a num ber and that number will be assigned to an element of the array If you don t already know how a FOR NEXT loop works you might find this example confusing Once again you should read an introductory book on BASIC if you find these program exam ples too difficult In short this program does exactly what the earlier program did but it does it with four instructions instead of ten And if we suddenly decided that we wanted to input 100 numbers instead of 10 we would need to make only one small change to the pro gram like this 10 DIM L 100 20 FOR 1 TO 100 30 INPUT L X 40 NEXT X Now the program will input 100 numbers To change the earlier version so that it would input 100 numbers we would need to add 9 ninety new instructions and ninety new variable names See how much easier the job becomes when we use an array Why would you want to put a list of numbers in an array There are almost as many reasons as there are computer pro grams Maybe you are writing a program to keep track of the bat ting averages of your favorite players Or maybe your program contains the grade averages of all the students in a class in order of their names in
46. ed deck However we must make sure that this card has not already been drawn from the deck Line 2030 reads IF DK SU DE lt gt 0 THEN 2020 DK SU DE is the element of array DK that represents the card we have just drawn from the deck that is the card with suit SU and denomination DE This line checks to see if this element is not equal to 0 which would mean that the card is no longer in the deck If so this line sends the computer back to the previous line to choose another random card The computer will continue to choose random cards until it finds one that has not already been dealt The first instruction in line 2040 reads DK SU DE CP The variable CP stands for current player Before calling this subroutine with a GOSUB instruction we must set CP equal to either 1 or 2 depending on whether we are dealing a card to the human player or to the computer If we are dealing a card to the player we will set CP equal to 1 If we are dealing a card to the computer we will set CP equal to 2 This instruction simply 22 records the whereabouts of card DK SU DE so that we will know whether it is in the player s hand or the computer s The next instruction reads ND ND 1 The variable ND you will recall is equal to the number of cards in the deck Since we have just removed a card from the deck we now subtract 1 from variable ND Finally line 2050 ends the subroutine At this point remem ber SU wi
47. emaining squares The job of drawing this board is important enough that we will put it in a subroutine like this 1000 REM DRAW TIC TAC TOE BOARD 1010 PRINT REM BLANK SPACE 1020 FOR COL 1 TO 3 REM FOR ALL COLUMNS 1030 PRINT COL REM PRINT NUMBERS ALONG 1040 NEXT COL 1050 PRINT REM ADD A BLANK LINE AT TOP 1060 FOR ROW 1 TO 3 REM FOR ALL 3 ROWS 1070 PRINT ROW REM PRINT NUMBERS ALONGSIDE 1080 FOR COL 1 TO 3 REM FOR ALL COLUMNS 1090 IF TB ROW COL 0 THEN PRINT REMFILL IN 1100 IF TB ROW COL 1 THEN PRINT X REM EACH SQUARE 1100 IF TB ROW COL 2 THEN O REM WITH SYM BOLS 1120 NEXT COL 55 1130 REM CARRIAGE RETURN NEXT LINE 1140 NEXT ROW 1150 PRINT REM BLANK LINE AT BOTTOM 1160 RETURN As you will see this routine prints out numbers at the head of each column and at the beginning of each row so that we can easily identify the location of each square The spacing of the characters is very important here which may present a problem if you are using certain versions of BASIC As we have mentioned before some versions of BASIC print extra blank spaces around numbers and others don t This subroutine assumes that the spaces are printed If you are using Applesoft BASIC or Atari BASIC or any other version that does not perform this automatic spacing the board will look very cramped and the column n
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49. f array TB will represent one square of the tic tac toe board in this pattern If we think of the tic tac toe board as being made up of three horizontal left to right rows and three vertical up and down columns then the first subscript after TB stands for the row and the second stands for the column If the value of an element of TB is 0 then the square where that row and column meet is empty If it is equal to 1 then the first player has his or her mark the X in that square If it is equal to 2 then the second player has his or her mark the O in that square We must now set up some variables 30 CP 0 0 CP equals the number of the current player 1 for player one and 2 for player 2 Player 1 will use the X to mark squares and player 2 54 will use the NM equals the number of markers both Xs and Os that have been placed on the board At the start of the game we must set every element of the TB to 0 to indicate that no squares have been marked with either Xs or Os 40 FOR ROW 1 TO 3 REM GET EACH ROW 20 FOR COL 1 TO 3 REM AND EACH COLUMN 60 TB ROW COL 0 REM SET THEM TO 0 70 NEXT COL 80 NEXT ROW Then we must draw the tic tac toe board Although we are not using graphics in this game we will depict the board as a series of letters and symbols Hyphens will represent blank squares that is squares that have not yet been marked Xs and Os will fill the r
50. g It then assigns that number to vari able A We can also put array variables in INPUT statements like this INPUT A 7 This lets the user assign a numeric value to element 7 of array If we were writing program that would let the user assign 7 values to ten non array variables we would need to write ten separate input statements like this 10 INPUT A 20 INPUT B 30 INPUT C 40 INPUT D 50 INPUT E 60 INPUT F 70 INPUT G 80 INPUT H 90 INPUT 100 INPUT J You don t need to type and RUN this program but if you did it would prompt you to type ten numbers pressing RETURN or ENTER after each Each number would be assigned to a separate variable from A through J though you could change these to any variable names that you might prefer This program would be much simpler if we used array vari ables rather than ordinary variables We would dimension the array in the first line of the program like this 10 DIM L 10 We would then set up a FOR NEXT loop that would repeat ten times 20 FOR 1 TO 10 In the middle of the loop we would place an INPUT statement 30 INPUT L X Then we would end the loop 8 40 The entire program would look like this 10 DIM L 10 20 FOR 1 TO 10 30 INPUT L X 40 NEXT X Do you see how this program works The FOR NEXT loop will be executed ten times Each time the variable X will be equal to a different number from 1 to 10 Therefore th
51. game over to the other player Lines 440 and 450 swap the values of CP and OP and loop back to the beginning of the main game loop at line 100 74 440 IF 1 THEN 2 1 GOTO 100 450 1 2 GOTO 100 We ve made several references to line 460 which tells the player that he or she has made an illegal move and repeats the instructions that prompt for the move Here is line 460 460 PRINT ILLEGAL MOVE GOSUB 1000 GOTO 120 The GOSUB 1000 instruction reprints the game board We ve also made several references to line 470 which begins the routine that congratulates the winner and ends the game Here is that routine 470 GOSUB 1000 480 PRINT CONGRATULATIONS PLAYER CP 490 PRINT YOU HAVE WON 500 PRINT WOULD YOU LIKE TO PLAY AGAIN 510 PRINT 6 2 NO 520 INPUT Q 530 IF Q 1 THEN 30 540 IF Q 2 THEN END 550 GOTO 520 As in the tic tac toe game we offer the players a chance for another game If the offer is declined the program ends If it is accepted we loop back to line 30 to begin again And if the response makes no sense we ask the question again There are lots of improvements you can add to this game For instance this program does not allow multiple jumps In standard checkers a player that has just jumped an opponent s piece may jump a second piece without waiting for the next turn if such a jump is available and can be made by the same piece that mad
52. gram listing 45 10 REM Add an instruction on this line to clear the video display of your computer See Chapter Two for details 20 DIM DK 4 13 30 ND 52 40 PRINT DEALING 50 FOR SU 1 4 60 FOR DE 1 13 70 DK SU DE 0 80 NEXT DE 90 NEXT SU 100 FOR 1 1 TO 7 110 GOSUB 2000 DK SU DE 1 120 GOSUB 2000 DK SU DE 2 130 NEXT 150 PN 7 CN 7 160 0 CB 0 170 YOU HAVE DISCARDED PB BOOKS 180 YOU HAVE PN CARDS 190 THEY 200 FOR DE 1 13 210 FOR SU 1 TO 4 220 IF DK SU DE 1 THEN GOSUB 3000 PRINT 230 NEXT SU 240 NEXT DE 250 PRINT 260 1 GOSUB 4000 REM LOOK FOR BOOKS 270 IF DE 0 THEN 310 280 PRINT YOU HAVE A BOOK OF GOSUB 5000 290 PRINT YOU DISCARD THIS BOOK 300 PB PB 1 PN PN 4 IF PN 0 THEN PRINT YOU ARE OUT OF CARDS GOTO 950 310 PRINT WHICH DENOMINATION WOULD YOU LIKE 1 13 320 INPUT DE 330 CP 1 GOSUB 6000 340 IF FL 0 THEN PRINT YOU DON T HAVE ANY IN YOUR HAND 46 GOTO 310 350 0 360 FOR SU 1 4 370 IF DK SU DE 2 THEN MC MC 1 DK SU DE 1 380 NEXT SU 390 IF NC 0 THEN 440 400 CN CN NC 410 IF CN 0 THEN PRINT THE COMPUTER IS OUT OF CARDS GOTO 950 420 PRINT THE COMPUTER GIVES GOSUB 5000 430 GOTO 170 440 PRINT THE COMPUTER SAYS GO FISH
53. h an array element as we can with an ordinary variable what s the point of using an array Good question The answer is that an array can be used to put information in order If we have a list of numbers and we need to keep that list organized we can store that list in an array Further more it is easier to use an array of ten elements to store data than it is to use ten separate variables with different names Why Because we can use a second variable to represent the subscript of the array like this 6 We said before that the subscript identifies which element of the array we are dealing with But which element is this Well that depends on the current value of B If the variable B is equal to 2 then we are dealing with ele ment 2 of the array just as though we had written A 2 On the other hand if the variable B is equal to 7 we are deal ing with element 7 of the array as though we had written A 7 And why is this an advantage Suppose we wanted to write a program that would allow the user that is the person who runs the program to assign numeric values numbers to ten vari ables In BASIC we let the user assign numbers to variables with the INPUT statement like this INPUT A As you know this instruction prints a question mark on the video display and stops the program until the user types a number and presses the key marked RETURN or ENTER depending on the computer you are usin
54. h the number of cards in the player s hand and the number of cards in the computer s hand because these numbers will keep changing throughout the game and we must keep track of them We will use the variable PN player s num ber to store the number of cards in the player s hand and the variable CN computer s number to store the number of cards in the computer s hand At the beginning of the game both hands have seven cards in them so we will set both variables equal to 7 like this 150 PN 7 CN 7 We also need to keep track of the number of books that each player has discarded since this number will determine who wins the game We will use the variable PB player s books to store the number of books the player has discarded and the variable CB computer s books to store the number of books the computer has discarded At the beginning of the game neither will have discarded any books so we will set both variables equal to 0 like this 160 0 0 of the basic elements of the game are now in place Sharp en your Go Fish skills it s time to play 24 PLAYING THE GAME We have now completed all of the preparation required for a game The cards have been dealt and the variables have been ini tialized It is time for the main loop to begin Since the computer dealt the cards we will let the human player make the first move In a fancy computerized card game simul
55. hat s getting ahead of the game so to speak The best way to learn to write a computer simulation of a card game is to write one This chapter will show you how to set up the data structure for the game The next chapter will show you how to play the game with and possibly beat the computer What game should we choose Well it should be a game with simple rules and one that can be played by two players so that the user of the program can pit himself or herself directly against the computer with the computer playing one hand of cards and the user playing the other It should also be a game that is familiar to most people or that is so simple it can be explained easily 13 There several card games that match this description One of these is Go Fish You ve probably played this game before though perhaps not recently In case you haven t played it or have forgotten the rules we ll describe it here briefly Go Fish like most card games is played with a deck of fifty two cards We ll assume you know what a deck of cards looks like For our purposes the most important thing about a deck is that each card has a suit and a denomination The suit for each card is either hearts diamonds clubs or spades The denomination is either ace two through ten jack queen or king The game of Go Fish is played by two to six players In this book we will restrict ourselves to two players When the game begins one of the two player
56. he fact that DK represents a deck of cards that the first subscript of DK is the suit that the second subscript of DK is the denomi nation and that the value of an element represents the where abouts of that card Now let s begin writing the program Once again you don t need to type the program until we show you the complete listing at the end of this chapter but you should follow along as we build the program line by line and routine by routine A routine is a set of program instructions that accomplish a single task or related group of tasks A subroutine is a special kind of routine that is called or activated by the GOSUB instruction We ve already written line 20 which DIMs the array DK Line 10 should clear the video display of your computer but the instruction for this will vary from one version of BASIC to anoth er For instance if you are using a Radio Shack IBM or Timex Sinclair computer you should use the instruction 10 CLS If you are using an Apple computer you should use the instruc tion 10 HOME If you are using a Commodore computer use the instruction 10 PRINT CHR 147 and if you are using an Atari computer use the instruction 10 PRINT CHR 125 18 If you not using one of the above computers and don t know what instruction will clear the video display just leave this line out It isn t really necessary it just makes things look neater The rest of the program will be pret
57. ible If so the player would be called on to make the jump or the program could make the jump automatically However if more than one jump is available the player would have to choose which jump to make And of course you could also add graphics to this program and to any of the programs in this book Since the drawing of the checkerboard is done entirely by the subroutine at line 1000 you would need only to replace this subroutine with a subroutine of your own which would use your computer s graphics capabilities to draw a picture of the board Of course you would also need to rewrite all statements that PRINT sentences on the screen so that they don t ruin your neatly planned graphics display 76 is the complete program 20 DIM CB 8 8 30 1 2 40 ROW 1 TO 8 50 FOR COL 1 TO 8 60 READ A 70 CB ROW COL A 80 NEXT COL 90 NEXT ROW 100 GOSUB 1000 110 PRINT PRINT PLAYER CP IT S YOUR MOVE 120 PRINT WHICH PIECE DO YOU WISH TO MOVE 130 GOSUB 2000 140 R1 ROW C1 COL 150 IF CB R1 C1 CP OR CB R1 C1 CP 2 THEN 170 160 PRINT YOU DON T HAVE A PIECE THERE GOSUB 1000 GOTO 120 170 PRINT WHERE DO YOU WISH TO MOVE THE PIECE 180 GOSUB 2000 190 R2 ROW C2 COL 200 IF CB R2 C2 0 THEN PRINT THAT SQUARE IS OCCUPIED GOSUB 2000 GOTO 170 210 IF CP 1 THEN 1 8 220 IF 2 THEN LM 1 1 230 IF ABS C1 C2 lt gt 1 TH
58. ide The game continues until one player runs out of cards The winner is the player who has dis carded the most books If both players create the same number of books the game is a tie How do we turn a game like Go Fish into a computer program We start by looking at the essential parts of the game that is the actual physical structures with which the game is played and deciding what sort of data structures can be used to represent these physical structures What are the essential parts of Go Fish The most obvious is the deck of fifty two cards In fact this is an essential part of almost every card game We will need a data structure to represent a deck of cards The second element is the players hands that is the cards that each player holds We can use a second data structure to rep resent the cards or we can simply make the hands part of the same data structure that represents the deck More about this in a min ute Finally there are books the sets of four cards that the players set aside periodically throughout the game Once again we can use a separate data structure for the books or we can use the same data structure that represents the deck Next we must look at the actions that take place during the game shuffling and dealing cards finding and discarding books moving cards back and forth between hands drawing cards off the top of the deck etc and invent program instructions that will be the equivalent t
59. inside the computer we can simulate anything from a deck of cards to a checkerboard to the surface of an entire planet In this book we ll stick with cards and board games but by the time you are fin ished you will probably have a good notion of how just about anything can be simulated using BASIC data structures As you know from your previous programming experience BASIC stores data in variables variable is a symbolic label usually one or two letters of the alphabet or a letter and a num ber that identifies a place inside the computer where a piece of 3 information is stored There are several types of variables but we will concentrate on numeric variables variables that store num bers We will use these numbers to represent all sorts of things playing cards positions on a game board even well numbers We store a number using a numeric variable like this A 178 This tells the computer to store the number 178 in an internal storage location called An instruction like this is called an assignment statement because it assigns a value to the variable or rather to the place in the computer s internal storage represented by the variable A very few versions of BASIC require that we place the word LET in front of an assignment statement like this LET A 178 If you are using one of these versions of BASIC you will need to make this change to all assignment statements in this book On
60. ional arrays 10 11 Naming cards 26 27 31 32 Nested loops 19 20 30 65 74 Numbering cards 26 27 see also Denominations Numeric variables See Vari ables numeric One dimensional arrays 10 11 Pascal language 3 Pause in the program 41 Pictures See Graphics PRINT command 4 in card games 26 43 in checkers 67 in tic tac toe 58 Program Checkers 77 79 Program Go Fish 45 51 Program Tic tac toe 61 62 Programming 1 2 4 11 Radio Shack computer instruc tions card games 18 22 44 Random function See RND function READ command and checkers 65 REM command 38 85 RETURN command 7 and card games 20 RND function in card games 21 22 39 Routine card game 18 33 see also Subroutine RUN command 8 SOUND command 44 Stopping a game 58 Storage computer See Memo ry Strategy card 39 45 Subroutines and card games 16 18 20 23 27 34 37 38 40 43 Subroutines and checkers 65 68 75 Subroutines and tic tac toe 55 56 Subscript 6 11 definition of 5 Three dimensional arrays 10 82 Tic tac toe 53 62 complete program of 60 Timex Sinclair computer in structions card games 18 28 Two dimensional arrays 10 81 in card games 16 17 in tic tac toe game 53 54 64 Variables definition of 3 Variables in card games 19 22 23 30 32 35 36 39 41 Variables in checkers 64 68 69 71 73 74 Variables in tic tac toe 54 57 59 Variab
61. les numeric 4 6 8 Value 6 69 74 86 ABOUT THE AUTHOR Christopher Lampton s interests and knowledge extend to many areas of science He is the author of sixteen computer books for Franklin Watts including the eleven book series Computer Liter acy Skills Among his other published works for Watts are Dino saurs and the Age of Reptiles Fusion The Eternal Flame and Mete orology An Introduction Mr Lampton has just finished a book on mass extinctions the latest theory on why the dinosaurs disap peared Christopher Lampton lives outside of Washington D C with three computers and stacks and stacks of books AQ pala 57 ONSE Per om PS ES EM e e NR 2 2 n A A z M 3 of TN 277 7 na SUM ERA ET T 3n i 5 use don 5 e fax 25252 S NIS 23 7 b 1 a 1 23257 1 m 7 AR pe scie W u d 7 4 2 E 1 75 7 Tm
62. ll be equal to the suit of our random card DE will be equal to the denomination and DK SU DE will be equal to 1 or 2 depending on who got the card Now that we have a subroutine for dealing a random card we must call this subroutine with a GOSUB As we said above the first use for this subroutine will be to deal seven cards each to the human player and the computer Therefore we will need to call the card dealing subroutine fourteen 7 cards X 2 players times The best way to do that is with a FOR NEXT loop like this 100 I 1 TO 7 REM REPEAT 7 TIMES 110 1 GOSUB 2000 DK SU DE 1 REM DEAL RANDOM CARD TO PLAYER 1 120 CP 2 GOSUB 2000 DK SU DE 2 REM DEAL RANDOM CARD TO PLAYER 2 130 NEXT I The loop that starts on line 100 repeats seven times Each time it calls the card dealing subroutine twice once with CP equal to 1 and once with CP equal to 2 dealing one card to the human player and one card to the computer for a total of seven cards in each hand of this dealing and shuffling takes time and the player may wonder what the computer is up to To calm the player s 23 fears that the computer might abandoned him or her we ll print a message on the video display like this 40 PRINT DEALING Notice that this message goes way back in line 40 so it will be printed before the dealing starts After the dealing we must take care of some important busi ness We must establis
63. ment 4 Absolute value function See Atari computer instructions ABS function in checkers card games 18 22 26 28 Apple computer instructions 44 card games 18 21 22 26 Atari computer instructions 28 tic tac toe 56 Apple computer instructions tic tac toe game 56 60 BASIC language 3 11 see also Applesoft BASIC See Apple specific commands computer instructions card Board games See Checkers games Apple computer in Tic Tac Toe structions tic tac toe games Arrays 6 11 Card deck empty 20 21 definition of 5 Card games 13 52 in card games 16 17 22 Cheating card 34 38 42 29 34 Checkerboard array 64 67 in checkers 64 65 Checkers 63 79 in tic tac toe 53 55 program for 77 79 83 Checking programs checkers in checkers 64 68 72 74 in tic tac toe 54 55 see Checking programs tic tac also Data structure toe 56 60 68 ENTER command 7 8 Commands See DIM ENTER FOR NEXT GOSUB FOR NEXT command 8 9 INPUT LET in card games 19 20 23 PRINT READ REM RE 29 TURN RUN SOUND in checkers 65 see also Commodore computer instruc Loops 4 tions card games 18 24 44 Forced jump 76 Commodore computer instruc Functions See ABS function tions tic tac toe 60 INT function RND function Computer definition of 2 Chessboard 81 Go Fish 14 52 complete program for 45 Data statements See Data structure GOSUB See Subroutines and Data structure 3 18 card games
64. n DE The player doesn t really need to respond because the com puter can check his or her hand automatically and transfer the 40 cards However it would be nice to pause the program and give the player a chance to rest a minute 730 PRINT PRESS lt RETURN gt TO CONTINUE 740 INPUT Q This will pause the program until the RETURN or ENTER key is pressed If the key on your computer is called ENTER rather than RETURN you can change this program line accordingly Next we must search the human player s hand for cards of this denomination 750 NC 0 REM SET COUNT TO 0 760 FOR SU 1 TO 4 REM CHECK ALL FOUR SUITS 770 IF DK SU DE 1 THEN MC MC 1 DK SU DE 2 REM IF CARD FOUND COUNT IT AND TRANSFER IT 780 NEXT SU Just as before any cards of the requested denomination are auto matically transferred this time from the player s hand to the com puter s Variable NC will be set equal to the number of cards of denomination DE that are transferred If this is 0 then we must skip ahead and tell the computer to Go Fish 790 IF 0 THEN 840 REM IF NO CARDS THEN FISH Otherwise we adjust the values of CN number of cards in com puter s hand and PN number of cards in player s hand 800 CN CN NC PN PN NC REM ADD TO COMPUTER S HAND SUBTRACT FROM PLAYER S HAND and we announce the transfer 41 810 PRINT YOU GIVE THE GOSUB 5000 Then we check to
65. ngratulate the win ner We will show you this routine shortly Now the player must ask the computer for all of its cards of a certain denomination Of course the player can t speak to the computer directly unless the computer is equipped with voice recognition equipment Instead the program asks the player which denomination he or she desires like this 310 PRINT WHICH DENOMINATION WOULD YOU LIKE 1 13 320 INPUT DE The player must type a number from 1 to 13 indicating the desired denomination with 1 representing the ace 2 representing the two and so forth Line 320 uses an INPUT statement to obtain this number and stores it in variable DE The player requesting a denomination must already have at least one card of that denomination in hand Therefore we must scan through the player s hand to see if it contains cards of this denomination Here is a subroutine that will look through the hand of player CP for a card of denomination DE It reports its findings in vari able FL for since it flags the presence of the cards If one or more cards of denomination DE are found the subroutine will set FL to 1 If no cards of that denomination are found FL will be set to 0 Here is the subroutine 6000 FL 0 REM CARD NOT FOUND YET 6010 FOR SU 1 TO 4 REM CHECK ALL FOUR SUITS 33 6020 IF DK SU DE CP THEN FL 1 REM IF CARD FOUND FLAG IT 6030 NEXT SU 6040 RETURN Before calling this subroutine CP must b
66. o see if the destination square is on the king row but also checks to see if the piece on that square is already a king that is if it has a value greater than 2 If we are not on the king row or if the piece is already a king we skip line 360 which announces the conversion of the piece to a king and increases the piece s value by 2 to indicate its new status As a last order of business before we go on to the next player s move we must check to see if we have a winner yet Obviously this won t happen on the first time through this loop but we will check for it after every move just to be safe 370 0 380 FOR ROW 1 TO 8 390 FOR COL 1 8 400 IF CB ROW COL OP THEN FL 1 410 NEXT COL 420 NEXT ROW 430 IF FL 0 THEN 470 This routine first sets variable FL flag equal to 0 Then it loops through all eight rows and all eight columns checking every square to see if the current player s opponent has a piece on any square If one of the opponent s pieces is found FL is set equal to 1 If FL is still equal to 0 after these loops have executed then the opponent has no pieces left on the board and the current player has won We jump to the routine on line 470 to congratulate the winner and end the game Notice that we don t check to see if the current player has any pieces on the board since it is impossible for a player to lose while it is his or her turn to move Assuming that no winner is found we must turn the
67. o these actions Remember a computer works with numbers We must find a way to represent all the parts of this game as numbers And when we perform actions like moving cards from one hand to another or rearranging the order of cards we must perform these actions with numbers 15 This isn t quite as hard as it sounds Honest In our game of Go Fish we will represent the deck of fifty two cards as a two dimensional array called DK deck This array will be created at the beginning of the game like this 20 DIM DK 4 13 Don t type this line or any of the rest of the program yet At the end of Chapter Three we ll give you the complete program You can type it and play with it then This line tells the computer that the first subscript can be any of four possible numbers and that the second subscript can be any of thirteen possible numbers Do these numbers sound familiar We are going to use the first subscript to represent the four possi ble suits hearts clubs diamonds spades I II And we will use the second subscript to represent the thirteen possible denominations of each suit ace 2 10 two through ten 11 jack 12 queen 13 king In this way we can represent any card in the deck as an element of array DK For instance if we wrote DK 2 13 16 this would represent the king of clubs because the 2 in the first position represents clubs and the 13 in th
68. ome spaces inside the quotes until this sentence looks right on your video display Printing out the number of cards that the player is holding is also easy This number is contained in the variable PN player s number and can be printed out like this 180 PRINT YOU HAVE PN CARDS Finally we must list the cards themselves 190 PRINT THEY But wait Naming the cards is trickier than numbering them First we will need a special notation for the cards that is a way of naming them on the video display Because there is a limited amount of space on the video display for printing the card names this notation should be short We will use only one or two char acters to represent the denomination ace 8 eight 2 two 9 nine 3 three 10 ten 4 four J jack 26 5 queen 6 six king 7 seven We will use two letters to represent the suit Ht hearts Cl clubs Di diamonds Sp spades 47 We will put a hyphen between the denomination and the suit The jack of clubs to give one example would be represented like this J CI The five of spades would be represented like this 5 Sp The ace of hearts would be represented like this A Ht and so forth Now we need a subroutine that will take a suit number from 1 to 4 and a denomination number from 1 to 13 and print the card name on the video display using this notation Here is
69. on of a real airplane Toy soldiers are simulated soldiers that can fight simulated battles And so on Simulations aren t just for kids Airplane pilots train on flight simulators before they fly real planes Investors test their skills with stock market games before risking real cash IL is even possible to simulate things that have never hap pened Toy rocket ships simulate battles in space that may not take place for hundreds of years if ever toy animals simulate creatures that exist only in someone s imagination A computer is a simulation machine When we program a computer we are teaching it to simulate some process that may or may not exist in the real world This book for instance was typed on a computer running a program called a word processor which simulates a typewriter but includes some features that a real typewriter could never offer 1 Anyone who has played games a computer has some idea of the power of computer simulation Arcade games for instance simulate events that have never taken place on the planet Earth and maybe nowhere else in the universe computer can show us what it is like to be a space pilot defending a planet from vicious invaders or a mythological knight jousting with sword in hand on the back of a giant bird This is why a computer is such a marvelous game machine Any game or sport ever played in the real world and many games and sports that could never be played in the real worl
70. one side and red pieces on the other Players alter nate moves Pieces may be moved one square diagonally but may 63 not moved onto occupied square movement must toward the opposing player s side of the board When a piece makes it to the far side of the board it becomes a king and may then move toward either side of the board though it must still move diagonally A player may jump diagonally over an oppon ent s piece if the square beyond that piece is empty thereby capturing the piece and removing it from the board The winner is the player who captures all of the opposing player s pieces We ll call our checkerboard array CB It can be dimensioned exactly as we dimensioned the tic tac toe board except that the rows and columns will be longer with eight squares apiece instead of three 20 DIM CB 8 8 Once again the first subscript will indicate the row of a square and the second subscript will indicate the column If there is no piece on a square the element representing that square will be set to 0 If player 1 has a piece on that square the element will be set to 1 If player 2 has a piece on that square the element will be set to 2 If player 1 has a king on that square the element will be set equal to 3 If player 2 has a king on that square the element will be set equal to 4 The black player always moves first hence we ll call this player player 1 As usual the variable CP will hold
71. ot including 5 The INT integer function with which we have surrounded all of these other operations chops off any fractional value of the result so that what we end up with will be either the number 1 the number 2 the number 3 or the number 4 chosen at random You ll remember that the four suits in a deck of cards are rep resented in this program by the numbers 1 through 4 This instruction sets the variable SU equal to one of the four suits Since the number is chosen at random there is an equal chance of SU being equal to any of the four suits just as we have an equal chance of drawing any of the four suits when we deal a card from a shuffled deck This instruction will work in most versions of BASIC but the number in parentheses following may have to be changed If you are using Applesoft BASIC for instance you must change RND 0 to RND 1 The instruction will work as writ 21 ten Radio Shack BASIC Atari BASIC and Commodore BASIC Consult your computer s manual for details if necessary The next instruction in line 2020 is DE INT RND 0 13 1 This works pretty much like the last instruction setting DE equal to a random number in the range 1 to 13 Since the denomination of a card can be any number from 1 ace to 13 king this sets DE equal to a random denomination of card We have now produced both a random suit and a random denomination as though we had drawn a random card from a shuffl
72. plicated Once again we print out the column num bers at the top of each column and the row numbers at the start of each row adjust the spacing if necessary Lines 1080 and 1090 print out the red and black symbols for the unoccupied squares The trickiest task performed by this subroutine is figuring out which squares are red and which squares are black by their row and column numbers This is accomplished in line 1080 by adding together the values of ROW and COLUMN If the sum of these two variables is odd then the square is red If the sum is 66 even then the square is black But how do we tell the difference between an odd and even sum The simplest way is to divide the sum by 2 chop off any fractional value with the INT function and multiply the result by 2 If the result of these operation is equal to the original sum then the sum was even If it is not equal to the original sum then the sum was odd Line 1080 does all of this to see if the square is red printing the red symbol if it is If it is not red and there is no piece on the square line 1090 assumes it is black and prints the black symbol If the square is occupied lines 1100 through 1130 print the symbol for the piece that is on the square Notice that line 1080 does not check to see if the square is unoccupied before printing the red symbol since red squares can never be occupied We ll call this subroutine to draw the initial picture of the board like this 10
73. program If a book is found the loop begin ning on line 4090 sets all cards of that denomination DK SU DE equal to 3 indicating that they have been discarded from the hand The variable DE now contains the number of the denomination for which we found a book We first call this subroutine immediately after we print out the contents of the human player s hand like this 260 1 GOSUB 4000 REM LOOK FOR BOOKS IN PLAYER S HAND We then check to see if a book was found 270 IF 0 THEN 310 REM IF NO BOOK SKIP AHEAD If the book was found that is if DE is not equal to 0 we announce the fact 280 PRINT YOU HAVE A BOOK OF GOSUB 5000 290 PRINT YOU DISCARD THIS BOOK What is this mysterious subroutine on line 5000 It will print out the name of the denomination that we have discarded based on the value of variable DE Here is that subroutine 5000 IF DE 1 THEN PRINT ACES RETURN 5010 IF DE 2 THEN PRINT TWOS RETURN 5020 IF DE 3 THEN PRINT THREES RETURN 5030 IF DE 4 THEN FOURS RETURN 5040 IF DE 5 THEN PRINT FIVES RETURN 5050 IF DE 6 THEN PRINT SIXES RETURN 31 5060 IF DE 7 THEN 5 5 RETURN 5070 IF DE 8 THEN PRINT EIGHTS RETURN 5080 IF DE 9 THEN NINES RETURN 5090 IF DE 10 THEN PRINT TENS RETURN 5100 IF DE 11 THEN PRINT JACKS RETURN 5110 IF DE 12 THEN PRINT QUEENS RETURN 5120
74. r of cards is counted in variable NC When a card is found it is moved from the computer s hand 2 to the player s hand 1 At the end of the loop NC is equal to the number of cards that were moved If NC is still equal to zero then the computer was holding no cards of that denomination Effectively the computer has told the player to Go Fish and so we skip ahead to the Go Fish routine like this 390 IF NC 0 THEN 440 REM IF NO CARDS THEN FISH Otherwise we adjust the number of cards in the player s hand PN and the computer s hand CN to reflect the transfer of NC cards 400 PN PN NC CN CN NC This could cause the computer to run out of cards in which case the game is over We check for that like this 410 IF CN 0 THEN PRINT THE COMPUTER IS OUT OF CARDS GOTO 950 The routine beginning on 950 you ll recall will announce the end of the game and check to see who won If the game isn t over we continue First we have to announce how many cards were transferred 420 PRINT THE COMPUTER GIVES YOU NC GOSUB 5000 At this point variable DE still contains the number of the denom ination the player requested The subroutine at line 5000 which we saw earlier prints out the name of the denomination con 35 tained in DE If the denomination was 7 and the number of cards transferred NC was 3 this would print THE COMPUTER GIVES YOU 3 SEVENS If you ll recall the rules of
75. re will be halfway between the source square and the destination square Thus we can take the difference between R1 and R2 and the difference between C1 and C2 72 divide each by 2 and add the results to the row and columns numbers of the source square This gives us the number of the square that was jumped Then we must check this square to see if it contains the opponent s number like this 300 IF CB R1 R2 R1 2 C1 C2 C1 2 lt gt OP THEN 460 If not we jump to line 460 to declare an illegal move Otherwise we tell the player that he or she has captured a piece 310 PRINT YOU HAVE CAPTURED A PIECE And we must remove the piece from the square that was jumped by setting that square equal to 0 320 CB R1 R2 R1 2 C1 C2 C1 2 0 As we mentioned earlier the routine that actually moves the piece begins on line 330 330 CB R2 C2 CB R1 C1 340 CB R1 C1 0 These lines set the destination square equal to the number of the piece on the source square then set the source square equal to 0 If the current player s piece has reached the opposite side of the board for the first time on this move we must make it into a king Here we check to see if we have reached the king row the number of which is contained in variable KR 350 IF R2 lt gt KR OR CB R2 C2 gt 2 THEN 370 360 PRINT YOU HAVE GAINED A KING CB R2 C2 2 2 2 73 Notice that line 350 not only checks t
76. s is chosen to be the dealer In our computerized version the computer will always deal The dealer shuffles the deck and hands seven cards to each player These cards become the players hands The players then alternate turns usually beginning with the player who did not deal the cards The player whose turn it is asks the other player for all of the cards of a certain denomination that he or she holds For instance one player may say to the other Give me all of your nines If the other has cards of this denom ination he or she must give all of these cards to the player who requests them Before a player can ask for cards of a certain de nomination however that player must have at least one card of that denomination in hand For instance the player who asks for nines must have at least one nine in hand already If the other player does not have any cards of that denomina tion in hand or she says fish The player asking for the cards must then draw a card from the top of the deck and add it to the cards in his or her hand If the player gets the denomination of card asked for either from the other player or from the deck that player gets an extra turn and can continue asking for cards until he or she fails to receive the requested denomination 14 When player has four cards of the same denomination hand that player is said to have a book and must discard those cards that is put them as
77. s subroutine like this 130 GOSUB 2000 REM GET ROW AND COLUMN NUMBERS We ll be calling the subroutine again in a second which will change the values of variables ROW and COL We ll need to save the values of these variables in two other variables like this 140 R1 ROW C1 COL Variables R1 and C1 now contain the row and column numbers of the square that holds the piece the current player wants to move We ll refer to this square as the source square because it is the source of the piece to be moved What if the player doesn t have a piece on the source square That s easy enough to find out by checking to see if CB R1 C1 is equal to CP the current player s identifying number However at some later point in the game player CP may also have some kings on the board that is 68 pieces that have made it all the way across the board and gained the power to move in all directions Kings have an identifying number that is 2 greater than the player s usual identifying num ber hence we also have to check to see if CB R1 C1 is equal to CP 2 Line 150 does both 150 IF CB R1 C1 CP OR CB R1 C1 CP 2 THEN 170 160 PRINT YOU DON T HAVE A PIECE THERE GOSUB 1000 GOTO 120 If the source square is not occupied by the current player we redraw the board in case the last picture of the board has scrolled off the top of the screen and repeat the lines that asked the cur rent player which piece to move Once we ge
78. s that you have just given it and couldn t possibly have drawn in the meantime but human players do stupid things too Here is how the computer picks the random denomination 670 DE INT RND 0 13 1 39 This sets DE equal to random denomination betwen 1 and 13 Remember if you needed to change the RND 0 function in the subroutine at line 2000 to fit your version of BASIC you will need to make a similar change here If this denomination has already been discarded then the computer should choose a different denomination This is easy enough to check 680 IF DK 1 DE 3 THEN 670 REM IF IN A BOOK TRY AGAIN We need only check one card of that denomination to see if it is equal to 3 that is if it is in a book because if any card of that denomination is in a book then all four are in that book Also the computer is not allowed to ask for a denomination unless it already has at least one card of that denomination in its hand Hence we must use the subroutine at line 6000 to search the computer s hand 690 CP 2 GOSUB 6000 REM ANY IN THE COMPUTER S HAND If there are none in the computer s hand we must go back for another random denomination 700 IF FL 0 THEN 670 REM IF NONE THEN TRY AGAIN Once we come up with a valid denomination we must ask the player for it 710 PRINT THE COMPUTER SAYS 720 PRINT GIVE ME ALL OF YOUR GOSUB 5000 The subroutine at line 5000 prints the name of denominatio
79. see if the human player ran out of cards 820 IF PN 0 THEN PRINT YOU ARE OUT OF CARDS GOTO 950 If not we loop back to give the computer another turn because it was successful in its request 830 GOTO 530 If the computer was unsuccessful in its request 840 PRINT YOU TELL THE COMPUTER GO FISH We deal the computer a random card from the deck 850 PD DE 2 GOSUB 2000 DK SU DE 2 2 CN CN 1 We then announce that the computer has drawn a card but not what card it has drawn That would be cheating 860 PRINT THE COMPUTER DRAWS FROM THE DECK IF PD lt gt DE THEN 890 Notice that we skip ahead to line 890 if the computer does not get the denomination it requests that is if PD does not equal DE If it does get the denomination it requests it gets another turn just as the human player did but it must show what card it drew to prove it got the requested denomination 870 PRINT THE COMPUTER HAS DRAWN THE GOSUB 3000 PRINT 880 PRINT GETS AN EXTRA TURN GOTO 530 42 The subroutine at line 3000 prints out the name of the denomina tion Of course these lines are skipped if the computer does not get the denomination it asked for We must now search the computer s hand a second time for books and announce any books we find 890 CP 2 GOSUB 4000 REM LOOK FOR BOOKS 900 IF DE 0 THEN 940 REM IF NOT FOUND SKIP AHEAD 910 PRINT THE COMPUTER HAS
80. such a sub routine 3000 IF DE gt 1 AND DE lt 10 THEN PRINT CHR DE ASC 0 GOTO 3060 REM NUMBER CARDS 3010 IF DE 10 THEN PRINT 10 GOTO 3060 REM TEN 3020 IF DE 1 THEN PRINT GOTO 3060 REM ACE 27 3030 IF DE 11 THEN J GOTO 3060 REM JACK 3040 IF DE 12 THEN Q GOTO 3060 REM QUEEN 3050 IF DE 13 THEN PRINT K GOTO 3060 REM KING 3060 PRINT REM PRINT HYPHEN IN THE MIDDLE 3070 IF SU 1 THEN PRINT RETURN REM HEARTS 3080 IF SU 2 THEN PRINT CI RETURN REM CLUBS 3090 IF SU 3 THEN PRINT Di RETURN REM DIAMONDS 3100 IF SU 4 THEN Sp RETURN REM SPADES This is a pretty simple subroutine and doesn t need as detailed an explanation as the last one did Before you call the subroutine you must set DE equal to the denomination of the card you wish to name and SU equal to the suit of the card you wish to name The first six lines 3000 3050 check the value of DE and print out the first part of the name If the value is between 2 and 9 just the number is printed Otherwise it prints A 10 J Q or K whichever is appropriate You might wonder why 7107 is singled out here since it could be printed as any of the other num bers are We ll explain why in a moment Line 3060 prints the hyphen Lines 3070 through 3100 check the value of SU and print out the correct suit name then RET
81. t a good value for R1 and C1 we must ask which square the current player wishes to move to 170 PRINT WHERE DO YOU WISH TO MOVE THE PIECE Once again we call the subroutine at line 2000 to input the row and column numbers 180 GOSUB 2000 REM GET ROW AND COLUMN NUMBERS This time we ll transfer these numbers to variables R2 and C2 190 R2 ROW C22 COL Variables R2 and C2 now contain the row and column numbers of the square the current player wishes to move to We ll refer to this square as the destination square At this point we have a complete description of the move that the current player wishes to make The piece at R1 C1 is to be moved to R2 C2 Now we begin the most complex part of the program checking to see if this is a legal move within the rules of 69 checkers And what are those rules Well the easiest one terms of checking for a legal move is that the destination square may not already be occupied by a piece We can check for that possi bility like this 200 IF CB R2 C2 lt gt 0 THEN PRINT THAT SQUARE IS OCCUPIED GOSUB 2000 GOTO 170 If CB R2 C2 is not equal to 0 an error message is printed the board is printed out again and the player is asked a second time for the row and column numbers of the destination square Now things get tougher The next rule to consider is that we must under most circumstances move our piece one square diag onally and one square horizontally Thus the
82. ta structure that we call a two dimen sional array can be used to represent something completely differ ent the game structure that we call a tic tac toe board Actually a tic tac toe board is nothing more than a pattern of lines drawn on a sheet of paper and the pieces with which the game is played are nothing more than Xs and Os drawn with a pencil or pen Nonetheless we can represent these things much as we represented the playing cards in Go Fish We ll assume that everyone reading this book has played tic tac toe at some time or another To loosely summarize the game two players take turns placing Xs and Os in the nine squares of a crosshatched board drawn on a sheet of paper The first player who can get three Xs or Os in a row either diagonally vertically or horizontally is the winner The strategy of tic tac toe is more complex than it might at first seem so we are not going to write a program that pits the 53 player directly against the computer Rather we are going to write a program that will let two human players play against each other using the computer screen as a playing field instead of a piece of paper Of course if you feel that you are up to the job you can rewrite the program so that the game is actually played against the computer Good luck The tic tac toe board will be simulated by a two dimensional array called TB which will be dimensioned like this 20 DIM TB 3 3 Each element o
83. the interesting part We must examine the board to see if the current player has gotten three markers in a row First we ll check the horizontal row 230 FL 0 REM ASSUME THE BEST 240 FOR 1 TO 3 REM CHECK THREE SQUARES 58 250 IF TB I COL lt gt CP THEN FL 1 REM IF MARKER NOT FOUND ON ANY SQUARE ASSUME THE WORST 260 NEXT 270 IF FL 0 THEN 370 REM WE VE GOT WINNER The loop beginning in line 240 takes us through all three squares on the row where player CP put his or her marker Variable FL tells us if any of those squares are rot occupied by CP s marker If FL equals 1 after the loop CP did not have a marker on at least one of the squares and therefore cannot have three markers in a horizontal row If FL is still equal to 0 after the loop then CP does have three markers in a row so we jump to line 370 to offer our congratulations We can do the same thing for the vertical column on which CP placed his or her marker 280 FL 0 REM ASSUME THE BEST 290 FOR 1 1 REM CHECK THREE SQUARES 300 IF TB ROW I lt gt CP THEN FL 1 REM IF MARKER NOT FOUND ON ANY SQUARE ASSUME THE WORST 310 NEXT I 320 IF L 0 THEN 370 REM WE HAVE A WINNER Once again variable FL tells us if CP has three markers in a row vertically Now we have to check to see if player CP has three markers in a diagonal row We could do this in a similar way but we ll try a different method this time 340 IF TB 1 1 TB 2 2
84. the number of the current player A new feature of this program is that the variable OP for other player or opponent will hold the number of the opposing player In the beginning CP is player 1 and OP is player 2 30 1 2 Now we must initialize the checkerboard array In the tic tac toe game this was a simple matter of setting every element of the array equal to 0 In checkers however we start with twenty four 64 pieces already on the board and so we must set the appropriate elements of the array equal to 1 and 2 to indicate which squares are occupied by each player s pieces To ease our task we will create a set of DATA statements containing the opening setup of the board and place these DATA statements at the very end of our program like this 3000 DATA 0 1 0 1 0 1 0 1 3010 DATA 1 0 1 0 1 0 1 0 3020 DATA 0 1 0 1 0 1 0 1 3030 DATA 0 0 0 0 0 0 0 0 3040 DATA 0 0 0 0 0 0 0 0 3050 DATA 2 0 2 0 2 0 2 0 3060 DATA 0 2 0 2 0 2 0 2 3070 DATA 2 0 2 0 2 0 2 0 Each set of eight data items in these statements represents a single row of the board Zeroes represent empty squares ones represent squares occupied by player 1 and twos represent squares occu pied by player 2 If you look closely at these data statements in fact you can almost see a representation of the board as it will look at the beginning of the game We can move this representa tion into the actual array CB with a pair of nested FOR
85. ty much the same no matter which computer you are using Next we need to create a variable that will tell us how many cards are currently in the deck so that we will know when we run out of cards In Go Fish we should never run out of cards in the deck by the time we reach the bottom of the deck one of the players has to run out of cards ending the game Nonetheless we will check for this event in case something goes wrong with our program We create the variable like this 30 ND 52 The variable name ND stands for number in deck We set it equal to 52 because the game begins with fifty two cards in the deck Next we must shuffle the deck and deal the cards Shuffling the deck is easy Since we will be dealing out cards from the deck at random as you will see it is not necessary to shuffle the deck into random order It is only necessary to set all elements of array DK to 0 indicating that all cards are in the deck as explained above Although BASIC will automatically set the elements of a newly created array equal to 0 we should nonetheless deliberately set every element equal to 0 especially if we plan to play more than one round of the game in which case this routine will need to be repeated at the beginning of each round The easiest way to set the entire array equal to 0 is with a pair of FOR NEXT loops each inside the other like this 50 FOR SU 1 TO 4 REM STEP THROUGH EVERY SUIT 60 FOR 1 TO 13 REM AND
86. um bers will be in the wrong positions You will have to rewrite this routine so that spaces will be printed between the squares of the board and between the column numbers We ll call the subroutine like this 90 GOSUB 1000 REM DRAW TIC TAC TOE BOARD In tic tac toe tie games are very common In fact if two very good tic tac toe players are pitted against one another every game will end in a tie unless one player makes a mistake So we have to be careful to check to see if the board has been filled up with markers and stop the game if it is The number of markers is contained in variable NM If this number goes over 9 the number of squares on the board and nobody has won yet the game is a tie Line 100 watches for a tie game and increases the value of NM by 1 100 NM NM IF NM gt 9 THEN PRINT PRINT TIE GAME GOTO 400 56 Of course this isn t going to happen on the first move but this line will be repeated before every move in the game Now it is time for a player to put his or her marker on the board The current player 1 or 2 is represented by variable CP We ini tially set CP equal to 0 which is not the number of either player So we will now add 1 to that and make it the first player s turn 110 1 The next time this loop executes this statement will add another 1 to CP making the current player 2 But the next time we will want the current player to be 1 again so we must make s
87. umbered rows to the low numbered rows We can check to see which direction we have moved by subtracting the source row R1 from the destination row R2 For black player 1 the result should be 1 For red player 2 the result should be 1 Since we will be performing this particular check a couple of times as you will see we will first set up a variable called LM legal move that will hold the proper result for the current player like this 210 IF CP 1 THEN LM 1 8 220 IF CP 2 1 1 If the current player is 1 black then LM is 1 If the current player is 2 red then LM is 2 And what is this variable KR that we have also created in this line It stands for king row It is the row that the current player must reach in order for a piece to become a king We will see in a moment how this variable is used Here is how we check to see if our move was legal 250 IF R2 R1 lt gt LM AND CB R1 C1 lt 3 THEN 460 The first part of this statement checks to see if R2 R1 is equal to LM However we must also check to make sure that the piece is not king in which case it may move in either direction This check is made in the second part of the statement Since kings are numbered 3 and 4 we simply check to see if the piece on the source square has a number less than 3 If the move is illegal we jump to line 460 which will announce that the move is not allowed and then repeat the sequenc
88. ure that the value of CP never becomes greater than 2 120 IFCP gt 2 THEN CP 1 Once we have decided whose turn it is we must prompt the player to make a move 130 PRINT PLAYER S MOVE First we will ask the player to choose the row on which his or her marker is to be placed 140 PRINT ROW 150 INPUT ROW Then we will ask what column 160 PRINT 170 INPUT COL 180 PRINT 57 The PRINT statement in line 180 adds a blank line to keep the video display neat It would also be nice to give the players a way to end a game in progress in case it is obvious that nobody is going to win If the player types 1 when asked for a ROW we will stop the game like this 155 IF RO 1 THEN PRINT GAME ABORTED GOTO 400 We should make sure that the row and column that the player asked for fall within the legal bounds that is that neither is greater than 3 or less than 1 190 IF COL lt 1 OR COL gt 3 OR ROW 1 OR ROW gt 3 THEN GOTO 130 If an illegal column or row has been asked for we skip back to line 130 and ask again Finally we must mark TB ROW COL as having the current player s marker on it First though we must check to see that it is not already occupied by a marker 200 IF TB ROW COL 0 THEN TB ROW COL CP GOTO 230 210 PRINT SQUARE IS TAKEN TRY AGAIN 220 GOTO 130 Should it already be occupied we jump back to line 130 and ask again Now comes
89. ven possible subscripts in the second Actually there are fifteen and eight possible subscripts in each dimension but we are still ignoring the 0 subscript Here are a few possible elements in this array How many elements are there in this array Well at a guess you might think that there would be twenty one elements because 21 is the sum of 14 and 7 But if you guessed this you would be wrong Actually there are ninety eight elements in this array because 98 is 14 times 7 And that is why multidimensional arrays eat computer memory they have a lot of elements in them We use one dimensional arrays to hold lists of data items such as numbers Why would we use multidimensional arrays Once again there are lots of reasons In this book we will show you several ways in which both one and two dimensional arrays can be used in game programs Once you have finished reading this book you will probably have a better understanding of how these data structures can be used in other kinds of programs 11 FISHING Card games lend themselves especially well to computer simula tion Why Because card games have very precise fixed rules and because cards are usually numbered With a little imagination we can come up with simple data structures to describe all of the parts of a card game For instance a deck of cards can become nu meric with each suit But t

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