[Rd] On read.csv and write.csv

[Gabriel Becker]

Hi Stephen,

Personally, I don't have super strong feelings about this, but
https://datatracker.ietf.org/doc/html/rfc4180#section-2 does say that the
optional header line should have the same number of fields as the data
records, so in as much as that is the "CSV specification", R's read.csv
behavior is supporting an extension, whereas its write.csv is
outputting "standard" compliant csv.

It is possible that one or a few of the mentioned multitude of independent
specs do specify header can have one less, I don't know, but if so,
according to the ietf, its not overly common.

I can't even speak to whether that is why the behavior is as it is, but I
figured it was worth mentioning.

~G

On Wed, Jun 30, 2021 at 2:15 PM Stephen Ellison <S.Ellison using 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
>
>
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