Re: [Rd] R 2.5.0 refuses to print enough digits to recover exact floating point values

From: hadley wickham <h.wickham_at_gmail.com>
Date: Wed, 23 May 2007 11:51:14 +0200

On 5/22/07, Uwe Ligges <ligges_at_statistik.uni-dortmund.de> wrote:
>
>
> Zack Weinberg wrote:
> > I have noticed that in R 2.5.0, no method of textual output will print
> > a "double" mode quantity with more than 15 digits after the decimal
> > point. From the help page (?print.default) it appears that this is
> > intentional, since digits after the fifteenth may be uncertain.
> > However, fifteen digits after the decimal point are not enough to
> > represent all the values that an IEEE-double can take. (You need one
> > more.) This means it is now impossible to write out data in textual
> > format (e.g. in order to manipulate it with another program) and read
> > back in exactly the same values. Some analyses are sensitive to this
> > sort of extra rounding, especially if it happens repeatedly.
> >
> > I'd really appreciate some way of forcing R to print enough digits to
> > represent every possible IEEE double value. I would also argue that
> > this should be the default behavior of dump(), write.table() and
> > friends, and save(...,ascii=TRUE), to prevent data loss.
>
> Example:
>
> formatC(exp(1), digits=100, width=-1)

formatC(exp(1), digits=1000000, width=-1)

R version 2.5.0 (2007-04-23)
i386-apple-darwin8.9.1

Hadley



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