Q31785: "Overflow" Error in Intermediate Integer Calculation

Article: Q31785
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 17-JAN-1991

Microsoft BASIC compilers make certain compile-time assumptions about
the numerical types of intermediate results in mathematical
expressions. These assumptions can sometimes lead to an "overflow"
error at run time.

The information below applies to QuickBASIC versions 4.00, 4.00b, and
4.50 for MS-DOS, to Microsoft BASIC Compiler versions 6.00 and 6.00b
for MS-DOS and MS OS/2, and to Microsoft BASIC Professional
Development System (PDS) versions 7.00 and 7.10 for MS-DOS and MS
OS/2.

As an example of this behavior, if you run the following program in
QuickBASIC, you might expect the output to be 36,000; however, an
"overflow" error displays instead:

   x& = 6 * 6000
   PRINT x&

QuickBASIC sees that the "6" and the 6000 are both short integers
(that is, integers between -32,768 and +32,767), so it assumes that
the intermediate multiplication result should also be a short integer.
However, an "Overflow" error results because 36,000 is not in the
range of short integers.

To force the intermediate expression to be calculated as a long
integer, make the constants into long integers in the first
calculations performed in that expression. For example, the following
two statements work without any error:

   x& = 1& * 6 * 6000   ' 1. & suffix makes a constant a long integer.
   x& = CLNG(6) * 6000  ' 2. CLNG forces type to be long integer.

Using 1& in the multiplication of 1& * 6 creates an intermediate long
integer variable that forces the subsequent multiplication (* 6000) to
be done using long integer.

Note that 6 * 6000 * 1& fails with an "overflow" because the first
multiplication is done as short integer. The calculation 6 * (6000 *
1&) avoids this "Overflow" error by changing the order of calculation
so that a long integer intermediate variable is created before
multiplying by the 6.

Integer Data Type Notation Standards for BASIC
----------------------------------------------

Note that the number 6 is the same as 6%, and 6000 is the same as
6000% in BASIC's short integer notation. Appending the percent (%)
symbol to a constant makes it explicitly a short integer. Appending
the ampersand symbol (&) to a constant makes it explicitly a long
integer. Constants whose types are not explicitly declared will
default as shown in Chapter 2, "Data Types," of the BASIC language
reference manual for QuickBASIC versions 4.00 and 4.00b, and Microsoft
BASIC Compiler version 6.00 and 6.00b for MS-DOS and MS OS/2. This
information is also found in Appendix B of the "Microsoft BASIC 7.0:
Programmer's Guide" manual for BASIC PDS 7.00 and 7.10.

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