st: Thanks (was Precision issues with double storage type)

[Hiroshi Maeda <hmaeda1@uic.edu>]

Dear Maarten, Keith, and Austin,

Thank you for your advices. I now understand storage and precision 
issues much better. I almost never deal with fractions and my grasp of 
precision issues turns out to be not as good as I thought.

Hiroshi Maeda

P.S. Recently there was a thread about perceived cultural differences 
beneath the "harshness" of some postings. Here is another occasion of 
"cultural differences": I would have liked to address Maarten, Keith, 
and Austin with their respective titles and last names (e.g., Prof. 
Buis, as opposed to Maarten). But I could not tell Keith and Austin's 
titles. Hence I resorted to the good old American way of 
indiscriminately calling anyone by their first name regardless of their 
stature :-) This may sound almost medieval, but some of us want to be 
polite to those who offer help. Please note that I have no intention of 
starting a new thread concerning cultural differences by my comment. If 
you feel compelled to comment on my medieval way, please do so privately.



> Date: Tue, 7 Aug 2007 08:57:25 -0400
> From: "Austin Nichols" <austinnichols@gmail.com>
> Subject: Re: st: Precision issues with double storage type (??)
>
> Hiroshi Maeda <hmaeda1@uic.edu>:
> Maarten has already indicated how to avoid the precision problems you
> are encountering, and where to read more about them.  Keith has
> indicated that double precision does not guarantee that sums and
> differences will work out exactly (if fractions cannot be represented
> perfectly in binary numbers).  Here is another way of expressing your
> example that demonstrates (among other things) that
> float(x)-float(y)!= float(x-y) in general:
>
> clear
> set type double
> input id dep pay ref
>   1   61.42   21.30   40.12
>   2   69.00   68.49    0.51
> end
> set type float
> foreach v in dep pay ref {
>  g f`v'=`v'
>  }
> d
> g c1=1 if float(fdep)==float(fpay)+float(fref)
> g c2=1 if float(fdep)==float(fpay+fref)
> li, noo
> di %22.16f dep[1], %22.16f pay[1], %22.16f dep[1]-pay[1]
> di %22.16f ref[1], %22.16f dep[1]-pay[1]
> di %22.16f 40.12, %22.16f dep[1]-pay[1]
>
>
> On 8/6/07, Hiroshi Maeda <hmaeda1@uic.edu> wrote:
> > I am puzzled by the discrepancies between what I expect Stata to do and
> > what it actually does, and I suspect storage/precision issues have
> > something to do with it. Before you castigate me for beating this dead
> > horse yet once again, please note that I have read relevant manual
> > entries ([U] 13.10 Precision and problems therein) and statalist
> > postings and that I encounter this problem even when numeric variables
> > with fractions are stored in -double-
> *
> *   For searches and help try:
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> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/

*
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