[futurebasic] Re: [FB] Rounding Rule

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From: Stu Cram <stu@...>
Date: Mon, 5 Dec 2005 17:11:29 -0600
Rounding numbers was one of my pet peeves over the years, especially 
when transferring a program to new processors or systems  
(360-->UNIVAC-->PET-->APPLE II-->Mac Plus-->...OS-X, etc.) This was 
especially troublesome with two results that were calculated by 
different methods but should be the same answer mathematically. Most 
realize that there often are small differences in the answers due to 
internal binary representations so these answers turn out 'not' equal.

Re rounding problems and its effects...

My approach was to have my own rounding function to be used on the 
results of ALL calculations. It's an old method - just add 1/2 of the 
next decimal position to the value and then chop (truncate) the 
unneeded decimal places. Then use PRINT USING to display the results as 
required. (or use LET x$ - USING...) if desired.

In my gymnastics work, the official rule is to keep just 3 decimal 
places and IGNORE any extra ones. So I just change my rounding function 
and the rest of the program logic remains the same.

The routine works something like this (from memory)...

LOCAL FN Round#( num#, places% )   '  ex.  18.5627, 3
   DIM result#, incr#, pwr#
   pwr# = 10 ^ places                 ' <--- 1000
   incr# = .5 / pwr#                  ' <--- .0005
   result# = num# + incr#             ' <--- 18563.2
   result# = result# * pwr#           ' <--- 18563
   result# = FIX( result# )	         ' <--- 18563
   result# = result# / pwr            ' <--- 18.563
END FN = result#

Most will realize the above instructions could easily be combined or 

Here's the rest of a short test program and its sample results. 
(FB4.2,console mode, OSX 10.3.9).

def tab = 12
print 18.5627, fn round#( 18.5627, 3 )
print 18.5621, fn round#( 18.5621, 3 )
print 18.5625, fn round#( 18.5625, 3 )
print 18.562499, fn round#( 18.562499, 3 )

  18.5627     18.563
  18.5621     18.562
  18.5625     18.563
  18.562499   18.562

And now a word from the sponsor...

Just a bit of my philosophy - FB (and other languages) are just tools 
to help achieve a goal. If the built-in functions don't do exactly what 
you want, then create your own.

This problem was aggravated 3-4 years ago when FB3 changed the way its 
INT() function worked. It used to chop off the decimal places in FBI 
and FBII (and in MS BASIC, FORTRAN, and ...) but now it rounds to the 
nearest integer instead. That caused a bit of updating in some of my 
program once the problem was discovered. MORAL - read the manuals 
(twice even) when new versions of a program come out. Hopefully changes 
will be clearly marked but ... Just need more time.. ;-)

My $.02 worth
- Stu