I started a post pointing out the errors in your code, Jason, but gave up. Let's just say, it's quite a mess
Given this, It might be better if we start over from the beginning. From what I could gather, you want to make a program that: 1) Randomly places a spider graphic on the screen, 2) assigns it a random direction, and 3) shows that spider bouncing off the walls, right? IF I've missed anything or stated something different from what you wanted, please let me know.
So in order to do this we need a spider graphic. I see that you've got a simple one already in your program, so we'll use that. Your code for that is:
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CIRCLE (x, y - 5), 5, 4
PAINT (x, y - 5), 4, 4
CIRCLE (x + 30, y - 5), 5, 4
PAINT (x + 30, y - 5), 4, 4
LINE (x, y)-(x + 30, y + 30), 14, BF
I also notice that when you "undraw" the spider back to the black background, you undraw each part of him in turn. A simpler and quicker way to do that would be to simply draw a black box over the entire thing like so:
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LINE (x - 5, y - 10) - (x + 35, y + 30), 0, BF
Your way worked fine, and you can still keep it if you want, but for my example, I'm going to use the above. You also want to try to blank the spider out at the very last possible moment before it is drawn in at the new location. For this, we will use another couple of variables called
oldxpos and
oldypos and we will blank the spider at the old position out right before drawing the spider in at the new position.
Now, I note that we are already using the variables
x and
y for the x and y position of the spider. Those variable names are again okay, but they would be better if they were a bit more descriptive. I will change them to
xposition and
yposition.
Two more values that we should keep track of for the spider are its direction along the x axis and its direction along the y axis. We will use the variables
xdirection and
ydirection for these. The values of these variables will be either -1 or 1. A -1 in the x direction means the spider moves to the left, and a 1 means it moves to the right. A -1 in the y direction means the spider moves up, while a 1 means the spider moves down. We will pick random values for these variables as well as for the x ans y position variables at the beginning of the program.
Whenever you are using random numbers in your programs, however, you need to include the statement RANDOMIZE TIMER at the beginning of your program. The reason is that QBASIC generates random numbers by way of a formula from a certain starting value or "seed". Unfortunately, this seed is the same everytime you run your program, so all of the "random" numbers will always be the same. When you include the RANDOMIZE TIMER statement, it changes this seed to the value of TIMER, the internal clock. This has a different value every second of the day, so it mixes the numbers up good enough for us.
During our program loop, we simply add the values of
xdirection and
ydirection to
xposition and
yposition, and then check to see if they've hit the edge of the screen. As Zim mentioned earlier, if the spider does hit an edge, you just multply the value in either
xdirection or
ydirection (or both if it hits right in a corner) by -1 to make the spider go the other way.
I notice that you used an empty FOR ... NEXT loop to control how fast the spider moves. The problem with this is different machines may be able to go through those loops much slower or much faster than yours. To keep the timing exact, we will use the TIMER again, this time to count the hundredths of a second between movements.
Finally, we will make it so that the user can press the escape key whenever they like and exit the program. You had another condition for exiting in your program example that I didn't really understand. That's okay, we can add that condition later if you'd like.
So now our program should look something like this:
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SCREEN 12
CLS
RANDOMIZE TIMER
xposition = INT(RND * 640)
yposition = INT(RND * 480)
oldxpos = xposition
oldypos = yposition
escapekey$ = CHR$(27)
xdirection = 1
IF INT(RND * 2) = 0 then xdirection = -1
ydirection = 1
IF INT(RND * 2) = 0 then ydirection = -1
DO
LINE (oldxpos - 5, oldypos - 10) - (oldxpos + 35, oldypos + 30), 0, BF
CIRCLE (xposition, yposition - 5), 5, 4
PAINT (xposition, yposition - 5), 4, 4
CIRCLE (xposition + 30, yposition - 5), 5, 4
PAINT (xposition + 30, yposition - 5), 4, 4
LINE (xposition, yposition)-(xposition + 30, yposition + 30), 14, BF
pausetime = TIMER
DO
LOOP UNTIL TIMER >= pausetime + .01
oldxpos = xposition
oldypos = yposition
xposition = xposition + xdirection
yposition = yposition + ydirection
IF xposition <0 or xposition > 640 THEN xdirection = xdirection * -1
IF yposition <0 or yposition > 480 THEN ydirection = ydirection * -1
LOOP UNTIL INKEY$ = escapekey$
END
I didn't set the edges of the spider's movements very well, so it will go partly off the screen before it bounces back, but hopefully you will fix that. If you have any other questions about what my code above does, let me know.
