BBC BASIC Programmers' Reference

by Richard Russell, May 2014

Liberty BASIC does not have the REDIM PRESERVE statement, available in some other BASIC dialects, which is used to change the dimensions of an array without destroying the data it contains. Fortunately in LB Booster it is possible to simulate this statement by means of a user-defined subroutine:

SUB RedimPreserve BYREF A(), newsize
    !N% = 5+10*(newsize+1)
    !O% = 5+10*(DIM(A(),1)+1)
    !SYS "GlobalAlloc", 64, N% TO A%
    !IF A%=0 ERROR 9, "No room for array"
    !IF N%>O% SWAP N%,O%
    !SYS "RtlMoveMemory", A%, !^A(), N%
    !A%!1 = newsize+1
    !IF !^A()<LOMEM OR !^A()>HIMEM SYS "GlobalFree", !^A()
    !!^A() = A%
END SUB

SUB RedimPreserve$ BYREF A$(), newsize
    !N% = 5+8*(newsize+1)
    !O% = 5+8*(DIM(A$(),1)+1)
    !SYS "GlobalAlloc", 64, N% TO A%
    !IF A%=0 ERROR 9, "No room for array"
    !IF N%>O% SWAP N%,O%
    !SYS "RtlMoveMemory", A%, !^A$(), N%
    !A%!1 = newsize+1
    !IF !^A$()<LOMEM OR !^A$()>HIMEM SYS "GlobalFree", !^A$()
    !!^A$() = A%
END SUB

The first routine listed is suitable for use with a 1-dimensional numeric array and the second with a 1-dimensional string array. Note that the routines consist of BBC BASIC code, hence the presence of an exclamation mark (!) at the start of each line to force the LBB translator to pass the code unmodified.

Here is an example program:

    DIM array(100)
    FOR i = 0 TO 100
      array(i) = SQR(i)
    NEXT
    CALL RedimPreserve array(), 200
    FOR i = 101 TO 200
      array(i) = SQR(i)
    NEXT
    FOR i = 0 TO 200
      IF array(i) <> SQR(i) THEN PRINT "Error!" : END
    NEXT
    PRINT "Test completed"
    END

SUB RedimPreserve BYREF A(), newsize
    !N% = 5+10*(newsize+1)
    !O% = 5+10*(DIM(A(),1)+1)
    !SYS "GlobalAlloc", 64, N% TO A%
    !IF A%=0 ERROR 9, "No room for array"
    !IF N%>O% SWAP N%,O%
    !SYS "RtlMoveMemory", A%, !^A(), N%
    !A%!1 = newsize+1
    !IF !^A()<LOMEM OR !^A()>HIMEM SYS "GlobalFree", !^A()
    !!^A() = A%
END SUB