st: Re: saving memory local vs usual variable

[Kit Baum <baum@bc.edu>]

< >
Eva is correct. Two objects that differ by 10^-16 are not identical.  
The point I was making that in practice one can usually consider them  
to be so. From a computer science standpoint, as I indicated earlier,  
scalars hold their contents in binary and avoid conversion to and from  
string form (which is what lives in a macro). In practice one should  
never test something that is held as a real number for equality, as  
many Stata FAQs and Stata Tips suggest. But my original point is that  
storing something as a macro---for instance, a regression coefficient,  
held internally with ~15-digit accuracy---will be just about as  
accurate as storing it as a scalar. Yes, the macro version will be an  
approximate representation, but most of us don't sweat getting the  
15th digit right. As was once said (by Norbert Weiner to John von  
Neumann, I recall) "Economics is a one or two digit science".

Kit Baum, Boston College Economics and DIW Berlin
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html


On Oct 8, 2008, at 02:33 , Eva wrote:

> thank you for your reply. I have in front of me my old NetCourse 152
> (Stata 8!) which says:
> "Scalars give us a way of obtaining a perfect binary copy of the
> binary number in ...".
>
> Later on, it says: "In short, you must use scalars if you plan to
> obtain a value from the data and then, later, test the data for exact
> equality with that value. In all other cases, use scalars when you
> need greater accuracy."
>
> And here is an example showing that the local macro is not the same as
> the scalar. Am I misunderstanding something completely?
>
> Thanks,
> Eva
>
>
> . sysuse auto, clear
> (1978 Automobile Data)
>
> . replace foreign = 1.10101 if foreign==0
> foreign was byte now float
> (52 real changes made)
>
> . replace foreign = 1.20101 if foreign==1
> (22 real changes made)
>
> . sum foreign
>
>    Variable |       Obs        Mean    Std. Dev.       Min        Max
> - ------------- 
> +--------------------------------------------------------
>     foreign |        74     1.13074    .0460189    1.10101    1.20101
>
> . local min = r(min)
>
> . scalar Min = r(min)
>
> . count if foreign==`min'
>    0
>
> . count if foreign==Min
>   52
>
> . di `min' - Min
> - -2.220e-16

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