apple iie checking for collisions in Applesoft basic

[Frozone212]

I'm following this guy on youtube called the coding train and was wondering:

how do I check for collisions without resorting to a subroutine?

I want to structure the code somewhat without making it too unreadable on the off chance that I decide to make my own game.

This will be an awesome opportunity for me to move from making text adventures to actual games.

 

I can get movement well enough but collision is never checked and because of that, the position of the fruit (he was doing snake) never changes.

can someone post an example of the code? I've checked google and archive.org but no luck. I even RTFM but no dice

 

 

[TheDevil'sCompass]

I have not watched the video, but I see it is on the text screen. At the simplest level, collision between the snake and food can be checked like this:

(Assuming FX, FY are the X, Y coordinates for the food and SX, SY are the coordinates for the snake head)

 

IF SY=FY AND SX=FX THEN SC=SC+10 : REM CHECK FOR COLLISION, IF DETECTED ADD 10 TO THE SCORE VARIABLE, SC

 

 

 

[jhd]

 

I am not familiar with Applesoft BASIC, specifically, but in other versions of BASIC it is possible to use a PEEK command to read a value directly from the screen RAM. 

 

For example, I once wrote a Centipede clone using text mode. As the projectile travelled up the screen, the code would PEEK the byte on the line immediately above it. If it was empty (blank space), then the projectile would continue to advance upwards. If the byte contained a character (a mushroom, part of the centipede) then other things would happen accordingly.

 

 

[TheDevil'sCompass]

Yes, doing PEEKs is also an option. You'll have to keep track of more things though because of how the screen is addressed. The screen is divided into 3 vertical sections: Lines 0-7, 8-15, 16-23.

 

Line    (Hex)         (Decimal)
00    $400-427    1024-1063
01    $480-4A7    1152-1191    
02    $500-527    1280-1319
03    $580-5A7    1408-1447
04    $600-627    1536-1575    
05    $680-6A7    1664-1703
06    $700-727    1792-1831
07    $780-7A7    1920-1959
08    $428-44F    1064-1103
09    $4A8-4CF    1192-1231
10    $528-54F    1320-1359
11    $5A8-5CF    1448-1487
12    $628-64F    1576-1615
13    $6A8-6CF    1704-1743
14    $728-74F    1832-1871
15    $7A8-7CF    1960-1999
16    $450-477    1104-1143
17    $4D0-4F7    1232-1271
18    $550-577    1360-1399
19    $5D0-5F7    1488-1527
20    $650-677    1616-1655
21    $6D0-6F7    1744-1783
22    $750-777    1872-1911
23    $7D0-7F7    2000-2039

 

Example: 0,0 - Check for <space> character
X=PEEK(1024)
IF X-128<>32 THEN POKE 1024,207 : REM IF IT'S A SPACE, POKE AN "O" FOR THE SNAKE CHARACTER
 

 

Another way to do it is to setup a two-dimensional array and use that to check for collisions. You'd essentially update the array (and check for collisions) at the same time you update the screen.

 

DIM PF$(23,39) : PLAYFIELD ARRAY IS SAME SIZE AS TEXT SCREEN

IF PF$(SY,SX)=32 THEN PF$(SY,SX)="O" : HTAB SX : VTAB SY : PRINT "O"; : REM IF A SPACE IS FOUND IN THE ARRAY, PRINT AN "O" FOR THE SNAKE CHARACTER

 

 

[The Usotsuki]

Applesoft also has SCRN() which is intended for reading off the LRG display but can be used to pick nibbles off video memory.

[Frozone212]

ok so I have the base playfield set, with J as the bit flag to check for collisions. Thanks for the help guys. I really appreciate it