Conditional NOT

I’ve run into something that’s puzzling me. I have a simple function that does some bounds checking on a heap block before trying to read a value from it:

DEF FNheap_read(heap%,block%,ptr%,RETURN val%)

The function returns 0 if there’s an attempted read from outside the block’s bounds, otherwise it updates val% and returns 1. I’d hoped to then be able to be able to use this kind of formulation:

IF NOT FNheap_read(h%,b%,p%,a%) THEN PROCerror

The problem is that this calls PROCerror even when the parameters are all correct and FNheap_read returns 1. This, on the other hand, works fine:

IF FNheap_read(h%,b%,p%,a%)=0 THEN PROCerror

Have I misunderstood the use of NOT here? Or got something else wrong? I thought those two statements should be equivalent.

That’s because NOT 1 isn’t 0. It’s -2. You need to make the function return either 0 (FALSE) or -1 (TRUE).

Have I misunderstood the use of NOT here?

Yes, you have. BBC BASIC’s “boolean” operators are not boolean: they’re bitwise. Conditional statements like IF treat zero as FALSE and non-zero as TRUE.

Because FALSE is &00000000 (zero) and TRUE is &FFFFFFFF (minus 1), FALSE is, indeed NOT TRUE. However, NOT 1 is &FFFFFFFE, which isn’t FALSE; it’s also not TRUE, but it is definitely non-zero, so IF will be happy to take it as true.

You have to be very careful when doing boolean logic in BBC BASIC. Unless you know that all of the inputs will always evaluate to TRUE or FALSE, then you must reduce them to TRUE or FALSE before applying any combination of AND, OR or NOT. That’s why you’ll see things like

IF (FNfoo <> 0) AND (FNbar <> 0) THEN PROCbat

in BBC BASIC. In C, it’s not unusual to see

if (foo() && bar())
  bat();

because the && operator is properly boolean. However, the BBC BASIC version is actually the equivalent of

if (foo() & bar())
  bat();

(with the bitwise & and not the boolean && as the operator).

Thanks both – that’s very helpful.