"Overflow" Error in Intermediate Integer Calculation (31785)

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As an example of this behavior, if you run the following program in Visual Basic for MS-DOS, you might expect the output to be 36,000; however, an "overflow" error message is displayed instead: Visual Basic for MS-DOS 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 message 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 correctly:

   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 a long integer.

Note that 6 * 6000 * 1& fails with an "overflow" error because the first multiplication is done as a 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 for MS-DOS, versions 4.0 and 4.0b; and Microsoft Basic Compiler for MS-DOS and MS OS/2, version 6.0 and 6.0b. This information is also found in Appendix B of the "Microsoft Basic 7.0: Programmer's Guide" manual for Basic PDS for MS-DOS and MS OS/2, versions 7.0 and 7.1.