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Q35282: "Subscript Out of Range" Using SGN Function Compiled BC /d

Article: Q35282
Product(s): See article
Version(s): 4.00 4.00b
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist4.00 buglist4.00b fixlist4.50 B_BasicCom | mspl13_basic
Last Modified: 8-DEC-1989

An incorrect "Subscript out of range" error can occur at run time when
a program using the SGN function in a formula is compiled with the
debug (BC /d) option. To correct the problem, assign the SGN function
to a temporary variable and use the temporary variable in the formula.

The program below demonstrates the problem on machines with or without
a coprocessor:

a(1) = (b(1) + 1) * SGN(c)

Microsoft has confirmed this to be a problem in Microsoft QuickBASIC
Versions 4.00 and 4.00b and in Microsoft BASIC Compiler Versions 6.00
and 6.00b for MS-DOS and MS OS/2 (buglist6.00, buglist6.00b). This
problem was corrected in QuickBASIC Version 4.50 and in Microsoft
BASIC Compiler Version 7.00 (fixlist7.00).

The problem happens more frequently on machines with coprocessors,
although the problem may occur on machines without them. The problem
seems to occur when a program has the following characteristics:

1. The program is compiled to an executable .EXE program.

2. The program is compiled using the debug option (/d).

3. The program contains an assignment statement with the following
   features:

   a. The assignment is made to a real number array element.

   b. The SGN expression is used in a formula that contains a
      reference to an array element and another operation of some
      kind.

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