[R] Newbie question on precision

[Magnus Torfason]

On 1/8/2010 1:29 PM, Magnus Torfason wrote:
> Paul Evans wrote:
>> How can I get R to change the default precision value? For example:
>>> x=0.99999999999999999
>>> 1-x
>> [1] 0
>>
>> Is there a way that I can get a non-zero value using some parameter,
>> or some package?
>> many thanks.
>
> The 'gmp' package allows calculation with arbitrary precision rationals
> (and every finite-digit decimal is a rational).
>
> See a recent post of mine, listing some gmp examples:
> http://tolstoy.newcastle.edu.au/R/e9/help/10/01/0579.html

And some more looking uncovered the 'Rmpfr' package, which allows 
arbitrary precision floating point calculations. Example:

 > x = mpfr("0.99999999999999999", precBits=100)
 > x
1 'mpfr' number of precision  100   bits
[1] 0.99999999999999998999999999999944
 > 1-x
1 'mpfr' number of precision  100   bits
[1] 1.0000000000000556923992179579934e-17

It does not store decimals exactly (as a rational would), because the 
exponent is base 2 rather than base 10. However, it has parsing and 
as.string functions that return decimal-formatted strings (whereas the 
bigq class in gmp would require such functions to be written manually). 
I guess for people who are looking for arbitrary precision decimal 
fractions the the choice of these packages becomes a choice of style or 
specific application needs.

Best,
Magnus