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

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Thu 09 Feb 2006 - 07:18:52 GMT

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@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

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Received on Thu Feb 09 23:08:03 2006

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