Dynamic allocation of BBC BASIC variables from within a BASIC program

I’m just teaching myself a bit of Aarch64, messing round with Xen and the like;

A question that arises is :

How can someone ‘set’ a variable from within a BASIC program?

An example program would be:
10 testvariable%=65535
20 A$=“testvariable%”
25 A$=A$+" = 13"
30 EVAL “A$” : REM (meaning it does ‘testvariable%=13’)
40 PRINT testvariable% (which should equal 13, not 65535

Is there a trick within RISC OS BBC BASIC that allows a programmer to do something like this? I would be extremely grateful if someone knows how to do this!

(otherwise I will need to spend time messing with the BASIC stack, LVAR, and so on – and BASIC is written in extremely efficient ARM code, almost as if the creator of the ARM CPU could see the data flowing down each individual line as it went through the CPU :)

Line 30 doesn’t work and wouldn’t be expected to, but is there a way to send a ‘command’ to BASIC somehow – to set a variable dynamically at runtime?

You need to be looking at the SWI for setting and reading values.

That kind of thing is covered in the Programmers Reference Manuals and the specific SWI’s you’re after to deal with your question are OS_SetVarVal and OS_ReadVarVal

Lots of stuff under OS SWI Calls

Input for runtime variable change can be pulled from various sources, commonest is of course the keyboard. Checkout OS_Byte key scan is OS_Byte 121.

Good examples of SWI use are in various of the BASIC programs that are part of the standard OS install. My best advice is do it different to me. :)

and BASIC is written in extremely efficient ARM code, almost as if the creator of the ARM CPU could see the data flowing down each individual line as it went through the CPU

Almost like the people that designed the first ARM and BBC BASIC and the OS tied it all together and were a very small, tight-knit group.

The ususal way is a variation on:

PROC_LET(A$,13)
...
DEFPROC_LET(var$,val)
 IF EVAL("FN_LET("+var$+")")
ENDPROC
DEFFN_LET(RETURN var):var=val:=TRUE

That works for integers or floats and there is the equivalent for strings.

There are other ways, but ‘trickier’. ;-)

It’s the EVAL I was steering away from. I seem to recall recent discussions about EVAL and compiling…

You cannot do this with ABC because EVAL cannot be compiled. This is another, problematic, difference between the ABC dialect of BASIC and ARM BASIC.

BTW System variable != BASIC variable. ;-)

BTW System variable != BASIC variable. ;-)

Ah, I was pointing at the method I used to do a Multi-user front end for PopStar in BASIC.

Thank you! I will try these (the EVAL way looks good for what I want for a little project I am working on!)

That’s helped me out too. I don’t suppose you could explain how the magic works?

I don’t suppose you could explain how the magic works?

Think it through. PROC_LET("testvariable%", 13) will result in the code calling

IF EVAL("FN_LET(testvariable%)")

Since FN_LET() simply sets and returns the parameter to the locally-global value of val, you end up with the global variable testvariable% holding the value 13.

To be honest, I would suggest that if you’re doing stuff like this in production code, you’re almost certainly doing it wrong. There will be a much better solution involving conventional data structures just waiting to be implemented… If the aim is to process external data, consider what happens if the data happens to match a real variable that your program uses. If it isn’t, use an array with sensible indices.

There will be a much better solution involving conventional data structures just waiting to be implemented…

I bow to your judgement, but plead that these will be much more complicated for ‘everyday use’. ;-)

BASIC is not replete with data structures, so it is your arrays or Davis’s indirection, or I might revert to extended CASE structures. Another alternative is a home-grown list structure, but it seems a shame to do that in BASIC when variable lists are there for the asking.

I think it interesting that the EVAL FN method was suggested and promoted by Sophie herself. It is, perhaps, a shame that it was not part of BASIC, then. The necessary assembly is extremely simple and I hung it off EVAL as a statement for Basalt:

There are other ways, but ‘trickier’. ;-)

Assembly is one of those. The other is to execute the code fragment varname$+"="+value$, but that is not for the faint-hearted. ;-)

It is so much easier in PHP – $$var=$val;