MS BASIC, access a DIMed variable with no index?

Question

I'm working my way through my retrobasic interpreter, feeding it an expanding diet of 101 BASIC Games. Here's an odd one...

40 DIM F(6,6),H(6,6),A(4),B(4),C(6),L(3)
...
90 A=INT(6*RND(1)+1)

This kills my interpreter because the DIM says this is an array, but on line 90 we try to set its value without a subscript and it expects two.

What is the right thing here:

1. are DIM A(4) and A two different variables?
2. when there's no subscript, do you assume 1,1...?

Anyone know what a real BASIC does here?

Answer 1

They're two different variables, at least in the version of MS BASIC on the C64:

DIM A(4)

READY.
PRINT A, A(1)
 0         0

READY.
A = 1

READY.
PRINT A, A(1)
 1         0

Basically A and A(...) refer to two different variables, just like A$ and A% are also different variables than A.

Answer 2

It depends on your BASIC. Some Basic distinguish between arrays and simple variables, some don't.

MS-BASIC, Sinclair's ZX81 and many other separate them.

Similar for simple variables, some BASIC distinguish between simple varables of different type. So A, A$, A! and A% are different variables.

The workings come from the way variables are searched. The interpreter always scans a whole variable name until the first stop character. If it's one of the type characters it will be part of then name - this includes opening parentheses as array delimiters.

Answer 3

I propose that Dartmouth's implementation of BASIC should be regarded as definitive, since they invented the language.

In Dartmouth BASIC, A and A() were distinct variables.

The single letter denoting a list or table name may also be used to denote a simple variable without confusion.

(In this context, "list" is a 1-dimensional array, "table" is a 2-dimensional array).

See this 1964 Dartmouth College Computation Center document, page 36.

It's true that the OP is specifically asking about Microsoft BASIC, but since he also asks in the final line "Anyone know what a real BASIC does here?", I think Dartmouth BASIC is relevant. Dartmouth BASIC is the real and original BASIC; any other implementation claiming language compatibility is obliged to follow suit in this respect.

Answer 4

To further confuse the issue, both ANSI/ECMA BASIC standards, in their minimal and full implementations, do not allow simple scalars an arrays to have the same name. This from ECMA-55: Minimal BASIC, page 9:

The same letter shall not be the name of both a simple variable and an array, nor the name of both a one-dimensional and a two- dimensional array.

(emphasis mine).

This is surprising, as:

1. Kemeny, one of the authors of Dartmouth BASIC, was on the ANSI standard committee;

2. Microsoft BASIC-80 v5 — which claims ANSI Minimal BASIC compatibility, including passing its test suite — allows arrays and scalars to share the same name.

For example, running this code through the strict bas55 Minimal BASIC interpreter:

10 DIM A(3)
20 LET A(1)=5
30 LET A=1
40 PRINT A,A(1)
50 END

results in the following errors:

30: error: type mismatch for variable A
 info: it was previously used or DIM as a one-dimension array
 LET A=1
     ^
40: error: type mismatch for variable A
 info: it was previously used or DIM as a one-dimension array
 PRINT A,A(1)
       ^

Every other interpreter I have runs the program without complaint.