Q11880: Negative Array Subscripts Checked with Debug, /D

Article: Q11880
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 28-DEC-1989

Question:

I have two questions on subscripts in QuickBASIC, as follows:

1. What is supposed to happen when I use negative array subscripts in
   QuickBASIC?

2. Does QuickBASIC check the validity of subscripts?

Response:

The following are responses to your questions:

1. Negative subscripts are not supported in Versions 3.00 and earlier,
   so when you use negative subscripts, the results are unpredictable.
   Versions 4.00 and later support the "TO" optional syntax, which allows
   for subscripts in the range of -32768 to 32767. However, invalid
   subscripts, such as negative values in Versions 3.00 and later, and
   subscripts out of range in all versions, are not checked for unless
   you have compiled with /D.

   For example, compile the following program with BC.EXE without /D:

      DIM A(10), B(10)
      A(9) = 1
      A(10) = 2
      PRINT B(-1)  'the value printed is 2, which is value of A(10)
      PRINT B(-2)  'the value printed is 1, which is the value of A(9)
      END

      Since BASIC may move items around in memory, the above results
      might not occur if there were other operations between the
      assignment statements and the PRINT statements.

2. QuickBASIC checks subscript range validity when the /D switch is
   on, or when the Debug option is selected in the editors in Versions
   2.x and 3.00.

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