Find out if coordinate has a text character there>>100 points

Posted on 1998-06-08
Last Modified: 2010-04-16
I'm having some trouble with a small program i'm writing for fun.  You see, i have this little ascii character that moves around.  What i would like is that i can load a text file, which is displayed onscreen, and if a text character is there, it acts as a wall.  Basically i need some sort of boolean thing that just tells me if a character is writtne there.  THANKS
Question by:hansell
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Accepted Solution

kellyjj earned 100 total points
ID: 1217098
YOu can do it like this

if mem[b800:0000]='C' then dothis;

the offset inside the mem[] is the memory location for video.  You can read it just like that. However, and I don't remember off the top of my head which is,  even addresses are for attributes, odd for the char. (Could be the other way).

Expert Comment

ID: 1217099
he's right about the B800 address, but to access it use this formula:

offset := ((y * 80) + x) * 2

where x and y are the coordinates on the screen (zero based - ie 0-79, 0-24 not 1-80,1-25).  The reason for the *2 is that there is one byte for the attribute of the character (ie colour, flashing, etc) and the other is the ASCII code.

I can't quite remember which way round the bytes are though - if that doesnt work just add 1 to the offset.

then use this:

ch := mem[B800:offset];

or to make it faster, embed the equation in the mem[] call.

This is the best way to do this - can be up to 100 times faster than going through the BIOS - let alone a Pascal call.

Author Comment

ID: 1217100
Thanks for your help, I'll try to work with that...

But if you have any small snippets of code where this is done, that would be much appreciated, considering I don't quite understand all of what you have explained  ):

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Author Comment

ID: 1217101
Well, i think i'm doing everything as I was told...but I get an 'unknown identifyer' for 'b800' and i'm not sure what to make it? Here is my snippet of code I was using...

program chartest;

uses crt;

offset:integer; {I assume offset would be an integer}

offset:=((1*80)+1)*2; {the "1"'s are the coordinates}

if mem[b800:offset] then  {the b800 is an "unknown identifyer"}
writeln('A character is there!');

NOTE:  I just want the simplest way of doing this, no speed efficiency is required.

Author Comment

ID: 1217102
Wait!!  I think i know SOMETHING i did wrong...i need to use $b800, so that's one thing solved...but i have another problem then..look at the comments...

Part of above code:
     if mem[b800:offset] then {I get boolean expression expected?}
     writeln('A character is there!');



Expert Comment

ID: 1217103
You need to tell the program what it is looking for:
you either of these as I don't recall of what type mem is.
In the first I use typecasting (turning a variable into another, char)

if char(mem[b800:offset]) = " " then ......
if mem[b800:offset] = " " then ......
if mem[b800:offset] = 32 then ......

I think 1 and 3 results in the same, however I'm not sure.



Expert Comment

ID: 1217104
Sorry..... the above will trigger if there is NOT a character there. Simply add a NOT after IF or change = to <>


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