[Rd] corruption of data with serialize(ascii=TRUE)

[Prof Brian Ripley]

It is known (happens with save() too and did in earlier save formats). 
Nothing particularly clever is done (the format is "%.16g\n") and 
similarly as.character/parse are not inverses.

Perhaps more relevant is

> b/x -1
  [1]  0.000000e+00 -1.110223e-16  2.220446e-16  0.000000e+00  0.000000e+00
  [6]  2.220446e-16  4.440892e-16  0.000000e+00  2.220446e-16  0.000000e+00

so the error (on my system) is about what you would expect from 
floating-point computations.

There is a comment in serialize.c

 	    /* 16: full precision; 17 gives 999, 000 &c */

which suggests that the format is optimized for size not maximal possible 
accuracy.

Really all you have said is `floating point operations are subject to 
rounding error'.


On Wed, 8 Feb 2006, Roger D. Peng wrote:

> I noticed the following peculiarity with `serialize()' when `ascii = TRUE' is
> used.  In today's (svn r37299) R-devel, I get
>
> > set.seed(10)
> > x <- rnorm(10)
> >
> > a <- serialize(x, con = NULL, ascii = TRUE)
> > b <- unserialize(a)
> >
> > identical(x, b)  ## FALSE
> [1] FALSE
> > x - b
>  [1] -3.469447e-18  2.775558e-17 -4.440892e-16  0.000000e+00  5.551115e-17
>  [6] -5.551115e-17 -4.440892e-16  0.000000e+00  2.220446e-16 -5.551115e-17
>
>
> I expected `x' and `b' to be identical, which is what I get when `ascii = FALSE':
>
> > a <- serialize(x, con = NULL, ascii = FALSE)
> > b <- unserialize(a)
> >
> > identical(x, b)  ## TRUE
> [1] TRUE
>
>
> The same phenomenon occurs with `.saveRDS(ascii = TRUE)',
>
> > .saveRDS(x, file = "asdf", ascii = TRUE)
> > d <- .readRDS("asdf")
> >
> > identical(x, d)  ## FALSE
> [1] FALSE
> >
>
> Has anyone noticed this before?  I didn't see anything in the docs for
> `serialize()' that would indicate this behavior should be expected.
>
> I'm on Linux Fedora Core 4.
>
> -roger
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595