Q57926: "Subscript out of Range" Long-Integer Array in IF/THEN in 4.50

Article: Q57926
Product(s): See article
Version(s): 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist4.50 SR# S900117-6 | mspl13_basic
Last Modified: 31-JAN-1990

The QuickBASIC Version 4.50 program shown below produces a "Subscript
out of range" error if compiled with the BC /D (debug) switch. This
appears to be caused by a compiler optimization problem for the
particular combination of using a long-integer variable to subscript a
long-integer array in an IF ... THEN statement.

If the program is run from the QB.EXE environment or is not compiled
with the BC /D switch, the error does not occur.

Microsoft has confirmed this to be a problem in Microsoft QuickBASIC
Version 4.50. We are researching this problem and will post new
information here as it becomes available.

This problem does not occur in versions of QuickBASIC prior to
Version 4.50, in Microsoft BASIC Compiler Versions 6.00 and 6.00b for
MS-DOS and MS OS/2, or in Microsoft BASIC Professional Development
System (PDS) Version 7.00 for MS-DOS and MS OS/2.

The following program exhibits the behavior:

   DIM a(10) AS LONG
   a(1) = 5
   k& = 5
   i& = 1
   IF a(i&) = k&  THEN
      a(i&) = -k&       'this statement causes the error to occur
   END IF

Compile and link the program as follows:

   BC /D TEST;
   LINK TEST;

The "Subscript out of range" error does not occur if the IF ... THEN
syntax is removed or if the variables i and k are of any type other
than long integers.

To work around the problem, replace the problem-assignment statement,
a(i&) = -k&, with either of the following equivalent assignments:

   1. a(i&) = k&
      a(i&) = -a(i&)

   2. a(i&) = -a(i&)

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.