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

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

Leon, or, to make it simple:
Q: why the test1 gives a correct result (31415) and the test4 gives
an error (|significand| > 16777215).
The test1 is the same as the test4, the only difference is the test4
has minus10000 as the first multiplicand.
Pito

----- PŮVODNÍ ZPRÁVA -----
Od: "pito" <pi...@vo...>
Komu: leo...@gm..., amf...@li...
Předmět: Re: [Amforth-devel] Float testing - small/large numbers
Datum: 29.8.2010 - 20:01:01

> 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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