[Info-vax] VMS Software Q1 '23 Update

[Arne Vajhøj]

On 1/28/2023 6:19 PM, Johnny Billquist wrote:
> On 2023-01-28 23:35, Arne Vajhøj wrote:
>> On 1/28/2023 3:01 PM, Steven Schweda wrote:
>>>> Consider the case where the numbers represent money.
>>>
>>>     Who cares what the numbers represent?  For example, if you round to
>>> the nearest integer, then 1.49999 and 1.50001 differ by only 0.00002,
>>> but one rounds to 1, and the other to 2, so the lame test says that
>>> they're "unequal".  And 1.50001 and 2.49999 differ by 0.99998, but they
>>> both round to 2, so the lame test says that they're "equal".  Any test
>>> which treats a difference of 0.00002 as significant, and a difference of
>>> 0.99998 as negligible, is, I claim, (obviously) a bad test, regardless
>>> of what the numbers might represent.
>>
>> There are some specific rules for money usually about
>> always rounding 2 digits.
>>
>> 1494 dollars divided by 1000 = 1.494 get rounded to 1.49
>>                                diff 0.002          not equal
>> 1496 dollars divided by 1000 = 1.496 get rounded to 1.50
>>
>> 1496 dollars divided by 1000 = 1.496 get rounded to 1.50
>>                                diff 0.008            equal
>> 1504 dollars divided by 1000 = 1.504 get rounded to 1.50

> So by your own example, two values differing by 0.002 could be 
> considered not equal, but two values differing by 0.008 could be 
> considered equal. Don't you see that this leads to a very bad algorithm 
> for comparing for approximate equal?
> 
> More generally, 0.002 of difference might be considered equal or not 
> equal purely based on what the actual values are. For some, they will be 
> equal, and for some others not. The delta that is required for two 
> numbers to be considered unequal varies based on what the actual numbers 
> are.
> 
> At least in my book, that is a clear indication that it's a very bad 
> method.

I am not sure that I get your point.

This is how rounding of money works.

Software that operate on money needs to implement
the rules.

Same reason that good rounding supports different
rounding mode: UP (round .5 up), EVEN (round .5 to
even) etc.. Different places has different rules and
the software need to implement those.

Arne