Newbie question on precision

Hi all,
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 problem here is that, as far as R in concerned, once you have entered
  x=0.99999999999999999
then x is *exactly* 1. Namely, your x is 1 - 1e-17, and the two things
you need to take note of are:

  .Machine$double.eps
  # [1] 2.220446e-16
  .Machine$double.neg.eps
  # [1] 1.110223e-16

and the descriptions in ?.Machine which say:

  "double.eps: the smallest positive floating-point number 'x'
   such that '1 + x != 1'."

  "double.neg.eps: a small positive floating-point number 'x'
   such that '1 - x != 1'. "

Both of these (in particular "double.neg.eps") are greater
than 1e-17, so your x was stored as 0.

I believe there may be packages which can work to greater precision,
but I have to leave it to others to describe them (if any).

Ted.

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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

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