Acorn Arcade forums: Programming: RETURN non-existent variable
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RETURN non-existent variable |
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swirlythingy (20:03 15/2/2013)
nunfetishist (11:33 16/2/2013)
swirlythingy (21:49 16/2/2013)
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Martin Bazley |
Message #121923, posted by swirlythingy at 20:03, 15/2/2013 |
Posts: 460
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In BASIC, I frequently use constructs such as the following:
a%=FNa(b%,c%)
(...)
DEF FNa(RETURN b%,RETURN c%)
b%=2:c%=3
=1
This initialises the variables b% and c%, regardless of whether they actually existed prior to FNa being called.
I'm not sure how far this tolerance in passing a reference to something which doesn't exist yet extends, though. What happens if FNa tries to read b% and c% prior to writing to them? Limited experimentation suggests that RETURNed parameters are automatically initialised to 0 if they didn't exist when the procedure/function was entered, but is this defined behaviour and/or should I rely on it?
The BBC BASIC reference manual doesn't appear to cover such subtleties. |
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Rob Kendrick |
Message #121924, posted by nunfetishist at 11:33, 16/2/2013, in reply to message #121923 |
 Today's phish is trout a la creme.
Posts: 524
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The BBC BASIC reference manual doesn't appear to cover such subtleties. This is why you should be using a more modern and more carefully described language 
On answering your question, I always assumed that such variables are initialised as zero and it was safe to assume that: but if it isn't, it's the caller's fault, and not the function's, IME (in case you're writing a library).
Also, I think if this was not the case, the runtime variable creation hack using EVAL and RETURN variables wouldn't work. |
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Martin Bazley |
Message #121928, posted by swirlythingy at 21:49, 16/2/2013, in reply to message #121924 |
Posts: 460
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On answering your question, I always assumed that such variables are initialised as zero and it was safe to assume that: but if it isn't, it's the caller's fault, and not the function's, IME (in case you're writing a library). In this case I'm writing both the caller and the function. The problem stems from the wide variety of circumstances in which I am attempting to use the same procedure. Depending on a switch, one of its parameters is either an input (and the program should error if it is not initialised) or an output (the program is able to derive the value of that parameter for itself and pass it back to the caller). Determining whether or not it has been "initialised", however, is more tricky - and in some other circumstances, it is perfectly valid for the same parameter to be write only!
Also, I think if this was not the case, the runtime variable creation hack using EVAL and RETURN variables wouldn't work. No, that's safe, because you never read the value of the variable in the EVAL statement, you only assign it. |
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Acorn Arcade forums: Programming: RETURN non-existent variable |
