A few questions about arrays in Atari BASIC

[telengard]

This is a modified snippet of some Atari BASIC code I'm looking at and I don't understand 2 aspects of it

 

DIM B$(1536)

B$="B"
B$(1536)="B"
B$(2)=B$

FOR J=0 TO 1280 STEP 256
    B$(45+J,45+J)="O"
NEXT J

 

The first thing I don't get is the use of B$ without an index, does this just default to B$(0)?  same goes for the line that does B$(2)=B$, is that just shorthand for B$(2)=B$(0)?

 

The second thing is the line that indexes a 1d array as a 2d array, B$(45+J,45+J)="O".   What is that doing?

Edited January 27, 2023 by telengard

[thorfdbg]

2 hours ago, telengard said:

This is a modified snippet of some Atari BASIC code I'm looking at and I don't understand 2 aspects of it

 

DIM B$(1536)

B$="B"
B$(1536)="B"
B$(2)=B$

FOR J=0 TO 1280 STEP 256
    B$(45+J,45+J)="O"
NEXT J

The first thing I don't get is the use of B$ without an index, does this just default to B$(0)?  same goes for the line that does B$(2)=B$, is that just shorthand for B$(2)=B$(0)?

 

The second thing is the line that indexes a 1d array as a 2d array, B$(45+J,45+J)="O".   What is that doing?

No. Atari only has statically sized strings, so the DIM just reserves a string of a maximum capacity of 1536 bytes. B$="B" just sets this string to the value "B".  String indices in Atari basic create or address parts of the string, similar to what LEFT$, MID$ and RIGHT$ do for other Basic dialects. B$(X) is the substring starting at offset X, thus B$(1536)="B" sets the last character of the entire string to "B", and thus ensures that the string is actually 1536 characters long, leaving the characters at indices 2 to 1535 undefined. Finally, B$(2)=B$ copies the string to the substring starting at offset 2, thus moves the character at offset 1 to the offset 2, then the character at offset 2 to offset 3 and so on, until the entire string is copied. Thus, it fills the entire string with "B"s.

 

In short, B$(X) is the substring starting at X, and B$(X,Y) the substring from offset X to Y, which both works at left and right side of an assignment. The drawback is that Atari Basic does not have string arrays.

 

Edited January 27, 2023 by thorfdbg

[telengard]

9 minutes ago, thorfdbg said:

No. Atari only has statically sized strings, so the DIM just reserves a string of a maximum capacity of 1536 bytes. B$="B" just sets this string to the value "B".  String indices in Atari basic create or address parts of the string, similar to what LEFT$, MID$ and RIGHT$ do for other Basic dialects. B$(X) is the substring starting at offset X, thus B$(1536)="B" sets the last character of the entire string to "B", and thus ensures that the string is actually 1536 characters long, leaving the characters at indices 2 to 1535 undefined. Finally, B$(2)=B$ copies the string to the substring starting at offset 2, thus moves the character at offset 1 to the offset 2, then the character at offset 2 to offset 3 and so on, until the entire string is copied. Thus, it fills the entire string with "B"s.

 

In short, B$(X) is the substring starting at X, and B$(X,Y) the substring from offset X to Y, which both works at left and right side of an assignment. The drawback is that Atari Basic does not have strings.

 

Thank you!  So it's not an array, just a non default sized string.  Makes sense.  If I'm understanding correctly, the x,y indexing having the same value for x,y is just setting a particular index to the value?

[thorfdbg]

In the above, I am using X and Y only as placeholders. They could be avariables, or arbitrary (numeric) expressions.

[vitoco]

 

1 hour ago, telengard said:

The first thing I don't get is the use of B$ without an index, does this just default to B$(0)?

There is no position 0, so B$(1) is the same as B$, i.e. the substring staring at position 1 of B$ up to its full current lenght.

 

36 minutes ago, telengard said:

If I'm understanding correctly, the x,y indexing having the same value for x,y is just setting a particular index to the value?

Using both indexes when assigning a value prevents the string to be resized to that index plus the length of the assigneed value.

 

DIM A$(100)
A$="12346789"
A$(5,5)="X"
PRINT A$
A$(5)="Y"
PRINT A$

 

The first result is 1234X6789, while the second is just 1234Y and the final length of the string is only 5 chars.

 

[+MrFish]

1 hour ago, thorfdbg said:

The drawback is that Atari Basic does not have strings.

?

 

[thorfdbg]

String arrays. My bad.

[Rybags]

String arrays start at element 1, not 0.

 

Value assignment works differently if you use A$(x,y) vs A$(X)

If you use A$(x,y) = "b" the string length will be preserved if it was longer than y.

If you use A$(x) = "b" the string length will terminate at the end of "b"  - the string won't be allowed to grow bigger than it's DIM value though.

 

For initial filling you can use a shortcut, no need to populate the end character to be filled.  e.g.

DIM A$(10)

A$="0" : A$(10)="" : A$(2)=A$