Unexpected behaviour of write.csv - read.csv

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)

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.

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.

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