[R] memDecompress and zlib compressed base64 encoded string

[Prof Brian Ripley]

On Thu, 14 Jan 2010, Johannes Graumann wrote:

> Hi,
>
> I have zlib compressed strings (example is attached)

What is that file? Not gzip compression:

gannet% file compressed.txt
compressed.txt: ASCII text, with very long lines

since gzip uses a magic header that 'file' knows about.  And even if 
the header was stripped, such files are 8-bit and yours is ASCII.
Try
> x <- 'Johannes Graumann'
> xx <- charToRaw(x)
> xxx <- memCompress(xx, "g")
> rawToChar(xxx)
[1] "x\x9c\xf3\xca\xcfH\xcc\xcbK-Vp/J,\xcd\0052\001:\n\006\x90"

to see what a real gzipped string looks like.

> and would like to decompress them using memDecompress ...
>
> I try this:
>> connection <- file("compressed.txt","r")
>> compressed <- readLines(connection)

You have not told us the 'at a minimum' information requested in the 
posting guide.  But you should not expect that to read a binary file, 
especially not in a MBCS locale.  We have readBin for that purpose.

>> memDecompress(as.raw(compressed),type="g")

I don't think you know what as.raw does: it does not convert bytes in 
a character string to raw (for which you need charToRaw).

It is always a good idea to look at each stage of your computation:

> as.raw(compressed)
  [1] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00
[26] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00


> Error in memDecompress(as.raw(compressed), type = "g") :
>  internal error -3 in memDecompress(2)
> In addition: Warning messages:
> 1: In memDecompress(as.raw(compressed), type = "g") :
>  NAs introduced by coercion
> 2: In memDecompress(as.raw(compressed), type = "g") :
>  out-of-range values treated as 0 in coercion to raw
>
> Can anyone nudge me into the right direction regarding this?

> Thanks, Joh

-- 
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)
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