[R] Unexpected behaviour of write.csv - read.csv

[Rainer M Krug]

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On 01/13/2011 07:06 PM, Prof Brian Ripley wrote:
> On Thu, 13 Jan 2011, Duncan Murdoch wrote:
> 
>> On 11-01-13 6:26 AM, Rainer M Krug wrote:
> Hi
> 
> Assuming the following:
> 
>>>>> x<- data.frame(a=1:10, b=runif(10))
>>>>> str(x)
> 'data.frame':    10 obs. of  2 variables:
>   $ a: int  1 2 3 4 5 6 7 8 9 10
>   $ b: num  0.692 0.325 0.634 0.16 0.873 ...
>>>>> write.csv(x, "x.csv")
>>>>> x2<- read.csv("x.csv")
>>>>> str(x2)
> 'data.frame':    10 obs. of  3 variables:
>   $ X: int  1 2 3 4 5 6 7 8 9 10
>   $ a: int  1 2 3 4 5 6 7 8 9 10
>   $ b: num  0.692 0.325 0.634 0.16 0.873 ...
>>>>>
> 
> Using the two functions write.csv and read.csv, I would assume, that the
> resulting data.frame x2 be identical with x, but it has an additional
> column X, which contains the row names of x.
> 
> I know read.table and write.table which work as expected, but I would
> like to use a csv for data exchange reasons.
> 
> I know that I can use
> write.csv(x, "x.csv", row.names=FALSE)
> 
> and it would work, but shouldn't that be the default behaviour?
>>>
>>> I don't think so.  The CSV format is an export format which holds less
>>> information than a dataframe.  By exporting the dataframe to CSV and
>>> importing the result, you are discarding information and you should
>>> expect to get something different.
> 
>> You need to read it with read.csv("x.csv", row.names=1)

Thanks - that makes sense.

> 
>> Nothing in the csv format lets R know that the first column is the row
>> names (in the format used by read.table, having a header that is one
>> column short does).  Now R could guess that a .csv file with an empty
>> string for the first column name is meant to be the row names, but that
>> would be merely a guess based on one (barely documented for
>> spreadsheets) convention.

OK - accepted - assuming things which are only barely documented is the
first step towards incompatibilities - and that is the last thing would
like to have.

Just for clarification, it might be useful to state this in the help
page - or did I miss it there? - as this is an important point and
difference between write.table and write.csv.

> 
>>> If you want to save a dataframe to disk and read it back unchanged,
>>> you should use save() and load().
> 
>> Or one of the other serialization options such as serialize() and
>> .saveRDS().  R's own admin uses .saveRDS() for such purposes.

They look exactly like what I was looking for, but it says in the help page:

####################################
Details:
     Since these are internal, the file format is subject to change
     without notice.  The current format is that of ‘serialize’,
     compressed as if by ‘gzip’ if ‘compress = FALSE’.
####################################

This sounds frightening - unless, that the existing version is kept and
can be used even if the default version changes.

Cheers,

Rainer

> 
> 
>>>
>>> Duncan Murdoch
>>>
>>>
> And if this is not compliant with csv files, shouldn't the function
> read.csv convert the first column into the row names?
> 
> Cheers,
> 
> Rainer
> 
>>>
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>>
>> ______________________________________________
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- -- 
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
Biology, UCT), Dipl. Phys. (Germany)

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