using for-next statements

[+atari2600land]

Suppose I've got a set of 2 data like this:

 data xdata

 28, 52, 76, 100, 124

end

data ydata
 12, 50, 86
end

 

How would I then access all 5 data in xdata and all 3 data in ydata at the same time? Would it be something like this:

 

if x=xdata[o] && y=ydata[p] then ----whatever

 

then how would I set o to equal 0, 1, 2, 3, and 4; and p to equal 0, 1, and 2? Using a for statement? I've looked through the help file and didn't understand anything about for-next statements, and I don't even know what a "step" is. Help!

Edited June 1, 2007 by atari2600land

[Gorf]

Suppose I've got a set of 2 data like this:

 data xdata

 28, 52, 76, 100, 124

end

data ydata
 12, 50, 86
end

 

How would I then access all 5 data in xdata and all 3 data in ydata at the same time? Would it be something like this:

 

if x=xdata[o] && y=ydata[p] then ----whatever

 

then how would I set o to equal 0, 1, 2, 3, and 4; and p to equal 0, 1, and 2? Using a for statement? I've looked through the help file and didn't understand anything about for-next statements, and I don't even know what a "step" is. Help!

 

 

 

 

 

Why would you have 5 in the x and only 3 in the y?

[+atari2600land]

Why would you have 5 in the x and only 3 in the y?

This is the playfield I'm working with:

[SeaGtGruff]

I've looked through the help file and didn't understand anything about for-next statements, and I don't even know what a "step" is.

 

To answer your last question first, a "for...next" statement is a kind of loop that lets you perform a set of instructions a certain number of times, based on the value of a particular variable. Each time the program goes through the loop, that particular variable will have a particular value, and its value will be incremented or decremented by a particular amount after each pass through the loop, based on the "step" value specified. For example, consider the following "for...next" loop:

 

   for i = 1 to 10 step 2
  pfpixel i i on
  next

 

This is functionally equivalent to the following:

 

   i = 1
loop
  pfpixel i i on
  i = i + 2
  if i <= 10 then goto loop

 

Both of these code snippets will do the same thing-- they will turn on the playfield pixels at coordinates (1,1), (3,3), (5,5), (7,7), and (9,9). So the generalized format of a "for...next" loop, and the equivalent "goto" loop, would be as follows:

 

   for variable = starting_value to ending_value step increment_or_decrement_amount
  rem do something here
  next
  rem continue program

 

   variable = starting_value
loop_label
  rem do something here
  if variable = ending_value then goto exit_label
  variable = variable + increment_or_decrement_amount
  goto loop_label
exit_label
  rem continue program

 

Suppose I've got a set of 2 data like this:

 data xdata

 28, 52, 76, 100, 124

end

data ydata
 12, 50, 86
end

 

How would I then access all 5 data in xdata and all 3 data in ydata at the same time? Would it be something like this:

 

if x=xdata[o] && y=ydata[p] then ----whatever

 

then how would I set o to equal 0, 1, 2, 3, and 4; and p to equal 0, 1, and 2? Using a for statement?

 

That depends on how you want to access the two data tables-- i.e., how you want to use them. Typically, you wouldn't want to use a "for...next" loop unless you want to perform some code a particular number of times, or you want to vary an index or other variable from one (starting) value through another (ending) value. For example, consider the following code:

 

   for o = 0 to 4
  for p = 0 to 2
	 if x = xdata[o] && y = ydata[p] then gosub do_whatever
  next
  next

 

That would be equivalent to doing the following:

 

   if x = xdata[0] && y = ydata[0] then gosub do_whatever
  if x = xdata[0] && y = ydata[1] then gosub do_whatever
  if x = xdata[0] && y = ydata[2] then gosub do_whatever
  if x = xdata[1] && y = ydata[0] then gosub do_whatever
  if x = xdata[1] && y = ydata[1] then gosub do_whatever
  if x = xdata[1] && y = ydata[2] then gosub do_whatever
  if x = xdata[2] && y = ydata[0] then gosub do_whatever
  if x = xdata[2] && y = ydata[1] then gosub do_whatever
  if x = xdata[2] && y = ydata[2] then gosub do_whatever
  if x = xdata[3] && y = ydata[0] then gosub do_whatever
  if x = xdata[3] && y = ydata[1] then gosub do_whatever
  if x = xdata[3] && y = ydata[2] then gosub do_whatever
  if x = xdata[4] && y = ydata[0] then gosub do_whatever
  if x = xdata[4] && y = ydata[1] then gosub do_whatever
  if x = xdata[4] && y = ydata[2] then gosub do_whatever

 

So whether you'd want to use a "for...next" statement for that particular example you posted would depend on whether or not you wanted it to behave as shown in the equivalent code.

 

Michael

[SeaGtGruff]

Why would you have 5 in the x and only 3 in the y?

This is the playfield I'm working with:

Okay, so you are thinking about using two data tables to define which rows and columns of the maze you can place a player in, so the player doesn't end up being stuck inside one of the contained areas? That makes sense, but a pair of nested "for...next" loops might take up more execution time than you'd prefer, and I don't think you'd need to check both tables at once, unless you were trying to find the intersections in the maze.

 

   dim is_this_a_safe_spot = a
  is_this_a_safe_spot = 0 : rem initialize it to "no" ("false," or 0)
  for b = 0 to 4
  if x = xdata[b] then is_this_a_safe_spot = 1 : rem yes it is
  next
  for b = 0 to 2
  if y = ydata[b] then is_this_a_safe_spot = 1 : rem yes it is
  next

 

Michael

[+atari2600land]

I ended up using the first example, as I was pressed for space (was trying to make a 2k game.) The game I made will be in my blog later today. Thank you, Michael.

[Nukey Shay]

It's better to find a formula that accomplishes the checks (in this case, checking for valid spots where 90 deg. direction changes are possible). How about changing 86y to be 88y? Then you can check for remainders (//) to see if the direction wanted is allowed...

 

temp2 = (X+20)//24

if temp1 = 0 then gosub y_ok

temp2 = (Y+26)//38

if temp1 = 0 then gosub x_ok

 

The subroutine x_ok should check the stick only for horizontal movement...as it's OK to move in that direction.

Likewise, y_ok checks the stick only for vertical movement.

 

 

Bb docs mention that you need to include div_mul16.asm for the remainder function to work.

 

Not versed in Bb logic, so this could probably be simpler.