Un-dimensioned arrays

Hated Moron · Post by **Hated Moron** » Sun 17 Sep 2023, 02:47

There are three ways in BBC BASIC of creating an un-dimensioned array, that is an array which exists (its name is present in the global namespace) but it has never been DIMensioned, so there is no storage allocated to it. If you need to, you can determine this state by reading its pointer.

The first is by making it LOCAL:

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DEF PROCtest
LOCAL array()
REM Here the array exists but it has not been DIMensioned
PRINT PTR(array()) : REM prints 1

The second is by making it PRIVATE (assuming it was not DIMmed in a previous call to the same function):

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DEF PROCtest
PRIVATE array()
REM Here the array exists but it has not been DIMensioned
PRINT PTR(array()) : REM prints 0

The third is by passing it to a function as a RETURNed parameter (assuming it was not DIMmed previously):

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DEF PROCtest(RETURN array())
REM Here the array exists but it has not been DIMensioned
PRINT PTR(array()) : REM prints 0

So to detect an array in this state, check whether its pointer is zero (global or PRIVATE) or one (LOCAL).