[R] double precision

[Roger Bivand]

On Tue, 5 Dec 2006, Yoni Schamroth wrote:

> Hi,
> 
> I am attempting to query a data frame from a mysql database.
> One of the variables is a unique identification number ("numeric") 18 digits
> long.
> I am struggling to retrieve this variable exactly without any rounding.

Read it as a character - a double is a double:

> x <- 6527600583317876352
> y <- 6527600583317876380
> all.equal(x,y)
[1] TRUE
> storage.mode(x)
[1] "double"

and why they are equal is a FAQ (only ~16 digits in a double). Integer is
4-byte. Since they are IDs, not to be used for math, leave them as
character strings - which they are, like telephone numbers.


> 
> The function I am using is sqlQuery(), with an ODBC connection.
> Querying directly results in the double being rounded towards the end (eg
> 6527600583317876352 instead of 6527600583317876380)
> 
> Is there anyway of preserving the "numeric" class of the variable and it
> precision without converting it to a factor? Is the double stored as a
> 64-bit integer?
> 
> I have attempted to use the 'dec' as well as 'as.is = TRUE', both
> automatically convert it to a factor which I am trying to avoid. 
> 
> Any advice or assistance would be greatly appreciated.
> 
> Thanks
> 
> Yoni Schamroth
> Director of Research
> MediaBoost LTD
> Tel. +972-3-5333033 (ext. 105)
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> yoni.schamroth at compile-inc.com
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no