Re: st: rounding the minimum of a negative number

[annoporci <annoporci@gmail.com>]

Thanks Nick, will follow up on the reference.

P.S. yes I did accidentally omit a minus sign.


On Thu, 10 Jan 2013 22:32:01 +0800, Nick Cox <njcoxstata@gmail.com> wrote:

> You say that
>
> > if Stata had
> > displayed something like
> >
> > -1.980000009999
> >
> > for 1.9810, I would not have been puzzled.
>
> I would have been puzzled to see that. Minus sign aside,
> 1.980000009999 does not round to 1.9810 under any rule.
>
> We can match ignorance. I have never used Python, but it does not make
> any sense to talk about the "true" decimal value of 0.1 or even of the
> binary representation that any language uses for 0.1.
>
> The short answer to your last question is, I believe, no.
>
> Otherwise do read William Gould's blog entry.
>
> Nick
>
> On Thu, Jan 10, 2013 at 2:19 PM, annoporci <annoporci@gmail.com> wrote:
> > Thanks Nick for these precisions.
> >
> >
> > > If you want _display_ to a fixed number of decimal places, that is
> > > ultimately a question of formatting and not a problem of numerics.
> >
> >
> > Yes. I guess another way of expressing my puzzlement is that Stata does
> > not display, by default, to a greater number of decimal places.
> >
> > I don't know anything about this, but in Python, for instance, according
> > to the documentation: "On a typical machine running Python, there are 53
> > bits of precision available for a Python float." And, to quote more:
> >
> > If Python were to print the true decimal value of the binary  
> > approximation
> > stored for 0.1, it would have to display:
> >
> > 0.1000000000000000055511151231257827021181583404541015625
> >
> > So that's still quite a few zeros after the first 1. And if Stata had
> > displayed something like
> >
> > -1.980000009999
> >
> > for 1.9810, I would not have been puzzled.
> >
> > I do have one last question and then I'll consider the matter closed:
> >
> > Would I get a more accurate approximation of "-1.981" with Stata if I
> > input "-1.981000000001" than if I input "-1.981" ? in the sense that it
> > would "force" Stata to store the zeros after 981? (or am I
> > misunderstanding the whole issue?)
> >
> > Thanks Nick,
> >
> > --
> > Patrick Toche.
> >
> > References:
> > http://docs.python.org/2/tutorial/floatingpoint.html
> >
> >
> >
> >
> >
> > On Thu, 10 Jan 2013 20:15:45 +0800, Nick Cox <njcoxstata@gmail.com>  
> > wrote:
> >
> > > I don't think that is a clear specification of what Stata is doing (it
> > > doesn't "make up its own digits") or of what it should, in your view,
> > > do instead.
> > >
> > > If you want _display_ to a fixed number of decimal places, that is
> > > ultimately a question of formatting and not a problem of numerics.
> > > That is,
> > >
> > > display %3.2 f  1 + 98/100
> > >
> > > will ensure that you see "1.98" and this last step is in essence
> > > string manipulation with numeric characters. But all that is done by
> > > (e.g.)
> > >
> > > scalar foo = 1.98
> > >
> > > is putting a binary approximation of 1.98 in a scalar. Adding bits
> > > will change the accuracy of the approximation (only).
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--
Patrick Toche.
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