Color BASIC Attract Screen – part 3

See also: part 1 and part 2.

Previously, I took a target from the attract screen code to talk more on using arrays in BASIC than individual variables. I gave an example of moving ghosts around a screen, and then did modifications to use an array. This let the user select how many ghost they wanted to see randomly display on the screen.

I mentioned that arrays were slower, but allowed flexibility. With that in mind, here is that “wandering ghosts” example turned in to a simple game demo. The player will appear as a yellow block in the top left corner of the screen. The ghosts will randomly appear around the screen as white blocks. The goal is to navigate to the bottom right corner of the screen without hitting a ghost, or being hit by one.

To hopefully make this compatible with the MC-10 computer, it uses the keyboard letters “WASD” — A for left, D for right, W for up and S for down.

You can choose one ghost, and get a single fast moving ghost to avoid. Or you can choose 100 ghosts, and get 100 slow moving ghosts to avoid, making it more like navigating a random maze that slowly moves walls.

Ghost Run in Color BASIC

10 ' ghostrun.bas
15 INPUT "NUMBER OF GHOSTS";G:G=G-1:IF G<0 THEN 15
20 CLS0:DIM G(G):DIM C(G):C=207:B=128:FOR I=0 TO G
30 G(I)=1023+RND(512)
41 NEXT
45 D(0)=1:D(1)=-1:D(2)=-32:D(3)=32
46 PL=1056:PC=159:POKE PL,PC
50 ' DISPLAY GHOSTS
60 FOR I=0 TO G:POKE G(I),C
65 D=INSTR(" DAWS",INKEY$):IF D>1 THEN GOSUB 190
70 ' RANDOM MOVE G(I)
80 NL=G(I)+D(RND(4)-1)
130 IF NL<1024 THEN 170
140 IF NL>1535 THEN 170
150 ' ERASE G(I) AND UPDATE LOCATION
160 POKE G(I),B:POKE NL,C:G(I)=NL
165 IF NL=PL THEN 480
170 NEXT:GOTO 60
180 ' PLAYER MOVED D
190 NL=PL+D(D-2)
200 IF NL<1024 THEN RETURN
210 IF NL>1535 THEN RETURN
220 IF NL=1535 THEN 510
230 IF PEEK(NL)=C THEN 530
240 POKE PL,B:POKE NL,PC:PL=NL
250 RETURN
470 GOTO 60
480 ' GHOST GOT PLAYER
490 PRINT "WE GOT YA!"
500 END
510 PRINT "YOU MADE IT!"
520 END
530 PRINT "YOU HIT A GHOST!"
540 END

To reduce instant death, I made the ghosts spawn no higher than one line below the player. But, with a small amount of faster ghosts, something could spawn then randomly move towards the player quickly. It’s surprisingly challenging (or frustrating).

To make the game responsive to the player, as the code it updating the position of the ghost, the player can move. For example, if drawing 100 ghosts, the player can move as each ghost being drawn. I found this much more fun than doing a turn-by-turn game like the old 1976 CHASE game (also known as Robots for Unix, Daleks for Mac, and a zillion spinoffs in the 70s and 80s). Check out the first published listing Creative Computing 1976 or the wikipedia entry for more details.

But I, as I say, digress.

Maybe we can revisit this in future installments of this series.

Until then, here is another size optimization of the attract screen code, this time removing all the hard-coded array initializations and turning them in to DATA statements loaded by the READ command:

10 ' ATTRACT3.BAS
20 FOR I=0 TO 3
30 READ L(I),LD(I),CL(I),CD(I)
40 NEXT
50 Z=143
60 CLS 0:PRINT @268,"ATTRACT!";
70 Z=Z+16:IF Z>255 THEN Z=143
80 FOR I=0 TO 3
90 POKE L(I),Z
100 L(I)=L(I)+LD(I)
110 FOR C=0 TO 3
120 IF L(I)=CL(C) THEN LD(I)=CD(C)
130 NEXT
140 NEXT
150 GOTO 70
160 ' L,LD,CL,CD
170 DATA 1024,1,1024,1
180 DATA 1047,1,1055,32
190 DATA 1535,-1,1535,-1
200 DATA 1512,-1,1504,-32

We have no changed the original 30 line version in to a 20 line version… but that is actually two lines longer than the previous one due to adding some extra lines for DATA and READ. But, if we were dealing with 50 objects instead of just 4, we’d likely be quite ahead at this point.

Next time, we’ll try to reduce this even further by packing lines together.

To be continued…

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