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:

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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.

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.