2009/9/26 "Jens Oehlschlägel" <[email protected]>:
> Hi,
>
> Is there any official way to determine the colClasses of a data.frame?
> Why has POSIXct such a strange class structure?
> Why is colClasses "ordered" not allowed (and doesn't work)?
>
> Background
> ==========
> I am writing a chunked csv reader that provides the functionality of
> read.table for large files (in the next version of package ff). In chunked
> reading, one wants to learn the colClasses from the data.frame returned for
> the first chunk and submit this as argument colClasses= to the following
> chunks (following calls to read.table).
>
> for most column types
> colClasses <- sapply(data.frame, class)
> works fine. However, two column types have more than one class:
>
> "ordered" has c("ordered", "factor") - currently we can't tell read.table
> that a column is an ordered factor
Possibly more complex than one would wish but it is possible to do this:
Lines <- "A
B
D
C"
setOldClass("ordered")
setAs("character", "ordered", function(from) ordered(from))
DF <- read.table(textConnection(Lines), colClasses = "ordered")
str(DF)
> "POSIXct" has c("POSIXt","POSIXct") - here the LESS specific class "POSIXt"
> is in the first position and would win in class-dispatch over the MORE
> specific class "POSIXct". Why?
>
Its a historical error that is too late to correct now. See
discussion in Chambers' recent book.
______________________________________________
[email protected] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
