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

From: Roger D. Peng <rpeng_at_jhsph.edu>
Date: Thu 09 Feb 2006 - 13:10:55 GMT

Okay, I just wasn't sure of the source of the changes. In retrospect, character and other vectors did serialize/unserialize to the original objects.

-roger

Prof Brian Ripley wrote:
> 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
>>

>
-- 
Roger D. Peng  |  http://www.biostat.jhsph.edu/~rpeng/

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Received on Fri Feb 10 00:14:29 2006

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