R devel - Jul 2021 - On read.csv and write.csv

Stephen,

the "unhelpful" column are the row names. They are considered an
important part of a data frame and therefore the default (row.names = TRUE) is
to not lose them (as there is no way back once you do). If you don't want to
preserve the row names you can simply set row.names=FALSE.

Cheers,
Simon

PS: this is likely a question for R-help rather than R-devel


> On 1/07/2021, at 9:15 AM, Stephen Ellison <S.Ellison at LGCGroup.com>
wrote:
> 
> Apologies if this is a well-worn question; I haven?t found it so far but
there's a lot of r-dev and I may have missed it in the archives. In the mean
time:
> 
> I've managed to avoid writing csv files with R for a couple of decades
but we're swopping data with a collaborator and I've tripped over an
inconsistency between read.csv and write.csv that seems less than helpful.
> The default line number behaviour for read.csv is to assume that, when the
number of items in the first row is one less than the number in the second, that
the first column contains row names. write.csv, however, includes an empty
string ("") as the first header entry over row names when writing. On
rereading, the original row names are then treated as data with unknown name,
replaced by "X".
> 
> That means that, unlike read.table and write.table,  something written with
write.csv is not read back correctly by read.csv .
> 
> Is that intentional?
> And whether it is intentional or not, is it wise?
> 
> Example:
> 
> ( D1 <- data.frame(A=letters[1:5], N=1:5, Y=rnorm(5) ) )
> write.csv(D1, "temp.csv")
> 
> ( D1w <- read.csv("temp.csv") )
> 
> # Note the unnecessary new X column ...
> #Tidy up
> unlink("temp.csv")
> 
> This differs from the parent .table defaults; write.table doesn?t add the
extra "" column label, so the object read back with read.table does
not contain an unwanted extra column.
> 
> Wouldn?t it be more sensible if write.csv() and read.csv() were consistent
in the same sense as read.table and write.table?
> Or at least if there were a switch (as.read.csv=TRUE ?) to tell write.csv
to omit the initial "", or vice versa?
> 
> Currently using R version 4.1.0 on Windows, but this reproduces at least as
far back as 3.6
> 
> Steve E
> 
> 
> *******************************************************************
> This email and any attachments are confidential. Any u...{{dropped:13}}

> the "unhelpful" column are the row names. They are considered an
> important part of a data frame and therefore the default (row.names >
TRUE) is to not lose them (as there is no way back once you do). If you
don't
> want to preserve the row names you can simply set row.names=FALSE.
Please run the reproducible example provided. 
When you do, you will see that write.csv writes an unnecessary empty header
field ("") over the row names column. This makes the number of header
fields equal to the number of columns _including_ row names. That causes the
original row names to be read as data by read.csv, following the rule that the
number of header fields determines whether row names are present. read.csv 
accordingly assumes that the former row names are unnamed data, calls the
unnamed row names column "X" (or X.1 etc if X exists) and then adds
new, default, row names _instead of the original row names written by
write.csv_.
That's not helpful.

By contrast read.table correctly reads the first entry in each row as a row name
when the number of header fields is one less than the number of data columns.
write.table includes row names as row names _without a header field_, so a file
written with write.table is correctly formatted for read.table to interpret the
first data field as a row name.
I think it would be more sensible if write.csv did the same as write.table when
row.names=TRUE - as it is, by default.





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