Quick yes or no question on variable arrays

[jbs30000]

It's been a while since I've done this so I figured I'd better double check because my memory is crap.

 

If I go

dim MyArray=a

 

Then MyArray[0] is a and MyArray[1] is b, correct?

[+Random Terrain]

I don't know for sure, but I thought arrays were read-only with bB:

 

http://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#dataarrays

[SeaGtGruff]

It's been a while since I've done this so I figured I'd better double check because my memory is crap.

 

If I go

dim MyArray=a

 

Then MyArray[0] is a and MyArray[1] is b, correct?

Yes, that's right. The number in square brackets is the offset to the starting address, so an offset of 0 gives you the starting address (a). The assembly equivalent is

 

  ; MyArray[0]
  LDX 0
  LDA MyArray,X
  ; MyArray[1]
  LDX 1
  LDA MyArray,X

Michael

[SeaGtGruff]

I don't know for sure, but I thought arrays were read-only with bB:

If you dim an array to ROM, it will be a read-only array. But if you dim an array to RAM, it will be a read/write (variable) array.

 

Note that if you dim an array to expansion RAM that requires separate read and write addresses, you'll need to either dim the array twice-- once for writing, and once for reading-- or you'll need to modify the index as needed if the expansion RAM is small enough.

 

For example:

 

  rem dim a Superchip array for writing and reading
  dim w_MyArray=$F000
  dim r_MyArray=$F080
  w_MyArray[12]=rand
  w_MyArray[12]=r_MyArray[12]+1

  rem modify the index instead of dimming twice
  dim MyArray=$F000
  rem use index 0 to 127 for writing
  rem use index 128 to 255 for reading
  MyArray[12]=rand
  MyArray[12]=MyArray[140]+1

In the second example, note that 140=12+128, so it does the same thing as the first example (add 1 to the element with index 12). However, the second example/method is harder for the reader to understand, and it will work only for Superchip expansion RAM (since the Superchip has 128 bytes of RAM, so there are a total of 256 write and read addresses).

 

Michael