Q33712: "Subscript Out of Range" DIM SHARED Dynamic Array in SUBmodule

Article: Q33712
Product(s): See article
Version(s): 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 28-FEB-1990

A "Subscript out of range" message displays at run time when a dynamic
array is dimensioned using REM $DYNAMIC and DIM SHARED statements at
the module level of a support (nonmain) module (composed of just
subprogram or FUNCTION procedures). In contrast, if the DIM SHARED
array is a statically dimensioned array instead of a dynamically
dimensioned array, the program runs without the error message. This
difference between dynamic and static arrays is not a problem with
QuickBASIC, but occurs because a DIM for a dynamic array is an
executable statement (processed only if encountered in the flow of
control at run time), whereas a DIM for a static array is a
nonexecutable statement (always processed at compile time).

This information applies to QuickBASIC Versions 4.00b and 4.50 for
MS-DOS, to Microsoft BASIC Compiler Versions 6.00 and 6.00b for
MS-DOS, and to Microsoft BASIC Professional Development System (PDS)
Version 7.00 for MS-DOS and OS/2.

A DIM for a dynamic array is only executed at run time if the
program's control flows through that DIM statement. Since no
executable statements are ever executed at the module-level of a
support (nonmain) module, the DIM statements for dynamic arrays at the
module level of a support module are ignored. "Subscript out of Range"
displays for the first time an undimensioned array is referenced in
the subprogram.

In contrast, static arrays are allocated at compile time; and the DIM
statement for a static array at the module-level of a support module
is always recognized. A DIM statement for a static array is a
compile-time, nonexecutable statement.

When compiling to an EXE file, be sure to produce debug code (that is,
compile with the /D switch) to catch array problems.

To work around the compiler's ignoring of DIM SHARED for dynamic
arrays at the module level of support modules, dimension the dynamic
array in the main module (source file), and pass the array to the
second module (separate source file) using the COMMON SHARED statement
in both modules.

The following code fails with a "Subscript out of range" error when an
element of the array is assigned to a value:

Module #1
---------

   DECLARE SUB test()
   CALL test

Module #2
---------

   REM $DYNAMIC            'will work if this is commented out
   DIM SHARED a(10,30,30)
   SUB test STATIC
   a(1,1,1) = 4           'Subscript out of range error
   PRINT a(1,1,1)
   END SUB

The following code successfully shares the dynamic array with the
procedure in the support module:

Module #1
---------

   DECLARE SUB test()
   REM $DYNAMIC
   COMMON SHARED a()
   DIM a(10,30,30)
   CALL test

Module #2
---------

   COMMON SHARED A()
   SUB test STATIC
   a(1,1,1) = 4
   PRINT a(1,1,1)
   END SUB

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