BBC BASIC Programmers' Reference

Sub-structure arrays

by Richard Russell, December 2020

Sub-structure arrays (that is, a structure member that is itself an array of structures) are not supported in any current version of BBC BASIC. For example one might prefer to have code something like this, but unfortunately it doesn't work:

      DIM ss{a, b%, c$}
      DIM mystruct{ss{(9)}=ss{}} : REM Doesn't work
 
      FOR I% = 0 TO 9
        mystruct.ss{(I%)}.a = SQR(I%) : REM Doesn't work
      NEXT
 
      FOR I% = 0 TO 9
        PRINT mystruct.ss{(I%)}.a : REM Doesn't work
      NEXT

Fortunately there is a reasonably neat workaround available:

      DIM ss{a, b%, c$}
      DIM mystruct{ss&(9,DIM(ss{})-1)}
 
      FOR I% = 0 TO 9
        PTR(ss{}) = ^mystruct.ss&(I%,0)
        ss.a = SQR(I%)
      NEXT
 
      FOR I% = 0 TO 9
        PTR(ss{}) = ^mystruct.ss&(I%,0)
        PRINT ss.a
      NEXT

This works by replacing the sub-structure array ss{()} by a two-dimensional byte array ss&() of the same size. Then, when the contents of one of the sub-structure array elements needs to be accessed, the pointer to the ss{} structure is set equal to the memory address of the matching element of the ss&() array.

This takes advantage of an extension to the PTR() pseudo-variable and only works, as shown, in BBC BASIC for Windows version 6.14a or later, BBC BASIC for SDL 2.0 version 1.15a or later, or BBC BASIC Console Mode editions version 0.26 or later.