From ligges at statistik.uni-dortmund.de  Tue May 22 12:10:39 2007
From: ligges at statistik.uni-dortmund.de (Uwe Ligges)
Date: Tue, 22 May 2007 12:10:39 +0200
Subject: [Rd] R 2.5.0 refuses to print enough digits to recover exact
 floating point values
In-Reply-To: <eb97335b0705220013m6f3e137cp147d429c46717e24@mail.gmail.com>
References: <eb97335b0705220013m6f3e137cp147d429c46717e24@mail.gmail.com>
Message-ID: <4652C19F.1040509@statistik.uni-dortmund.de>



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)

Uwe Ligges


> [Tangentially, support for C99 hexadecimal notation for floating point
> values in input and output would also be nice, but wouldn't help much
> for interoperability, since very few programs understand it.]
> 
> zw
> 
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[Rd] R 2.5.0 refuses to print enough digits to recover exact floating point values
Uwe Ligges