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

[Duncan Murdoch]

On 11-01-13 6:26 AM, Rainer M Krug wrote:
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> 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.

If you want to save a dataframe to disk and read it back unchanged, you 
should use save() and load().

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
>
> - --
> Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
> Biology, UCT), Dipl. Phys. (Germany)
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