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

[Uwe Ligges]

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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