Q42469: FOR…NEXT with Floating-Point Counter May Not Execute Fully

Article: Q42469
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890301-38 B_BasicCom | mspl13_basic
Last Modified: 15-DEC-1989

Because of the nature of the IEEE floating-point format, using a
floating-point counter (single or double precision) in a FOR NEXT loop
can result in the loop being executed one fewer time than anticipated.
Using an integer or long integer instead of floating point for the
loop counter is the only way to guarantee that all iterations will
execute.

This information applies to Microsoft 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 PDS Version 7.00
for MS-DOS and MS OS/2.

The following is an example:

   DEFSNG A-Z
   FOR x = 0 to .8 step .2
     PRINT x" ";
   NEXT x

This will generate the following results:

   0 .2 .4 .600000000000001

Note that due to the rounding error in the fourth value, the loop
counter will be greater than .8 on the next iteration, and the loop
will execute only four times instead of the expected five times.

This IEEE rounding behavior occurs in Microsoft QuickBASIC Versions
4.00, 4.00b, and 4.50, in the BASIC compiler Versions 6.00 and 6.00b,
and in BASIC PDS 7.00.

In most cases, the rounding differences will be minimal when the
variables and numbers involved are double precision. Changing the
first line of the example program to the following will allow the loop
to properly execute five times:

   DEFDBL A-Z

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