Re: [Amforth-devel] Float testing - small/large numbers

[pito <pi...@vo...>]

The test cases are designed specifically to se how it handles large
and small numbers. The cases 1-3 are the same as 4-6, the later have
only minus sign with those 10000 numbers. All the results shall be
by design the integer number 31415 (visualised by "10000 x Pi" - as
we do not have float format yet). The Pi is simply multiplied or/and
divided by large numbers (as we do not have float input yet, it is
done by several multiplication/division by 10000).
The results are:
> test1
31415  ok
> test2
31415  ok
> test3
|significand| > 16777215 

Here the sign is chabged only:
> test4
|significand| > 16777215 
> test5
31438  ok
> test6
|significand| > 16777215 
> 

My current understanding is the division is cousing issues. The lost
of significant figures shall not be the case - see the test 1:

tmp = pi / (10000^5)
pi = tmp * 10000^6   (correct result 31415).

The test 2:

tmp = pi / 10000 / 10000 
pi = tmp * 10000^3   (correct result 31415).

Te test 3:

tmp = pi / 10000 / 10000 /10000
pi = tmp * 10000^4   (error).

The test4-6 are same as the above one, but with minus sign at 10000.
So the result shall be 31415 (when ignoring the sign).

As you may see from previous test the f/ is sensitive to signs of
the operands and gives unpecise results - see  for example test5
result.

Pito.




----- PŮVODNÍ ZPRÁVA -----
Od: "Leon N Maurer" <leo...@gm...>
Komu: amf...@li...
Předmět: Re: [Amforth-devel] Float testing - small/large numbers
Datum: 29.8.2010 - 19:17:31

> I don't have my device handy so I can't test your
> code. Could you be a little 
> more clear about which ones failed, what the
> results are supposed to be, and 
> what the results actually are? (When you write
> "the result shall be -31415", 
> is that what it's supposed to be, what it actualy
> is, or something else?)
> 
> It looks like you're dividing and then multiplying
> by very large powers of 10. 
> I wouldn't be surprised if you loose some
> significant figures by doing this.
> -Leon
> 
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