RFC: type conversion in read.table

Prof Brian D Ripley writes:

On Fri, 31 Aug 2001, Kurt Hornik wrote:

Prof Brian Ripley wrote:

Currently read.table is rather limited in its type conversion.
The algorithm is

0) Read as character
1) Try to convert to numeric. If that works, quit
2) Convert to factor unless !as.is.

I am thinking about adding more flexibility and more classes by the
following two changes.

A) Anticipating the arrival of classes for all R objects, add an
argument say `colClasses' that allows the user to specify the desired
class for every column.  This could default to "auto", or NA if people
think "auto" might be a relevant class name one day.

The effect would be equivalent to running

data[[i]] <- as(data[[i]], colClasses[i])

instead of

data[[i]] <- type.convert(data[[i]], as.is = as.is[i], dec = dec)

except that standard classes such as "numeric", "factor", "logical",
"character" would be dispatched directly, and argument "dec" would be
consulted where appropriate.

colClasses = "character" would suppress all conversions, which cannot
currently be done.

Just a small remark.  I would prefer `NA' to "auto" (or "unknown").  May
be too late to change this now :-)

Anything can be changed up to 1.4.0 release.  In particular, the
present code will have to be changed unless as() is in base by then.

I would also be happier if we did not refer to the variables explicitly
as `columns'.  (This sounds a bit stupid from the person who wrote
write.table and introduced arguments `row.names' and `col.names'.
Although, at least one of these was modelled after an existing
function).  E.g. something like

read.table(......, caseNames, varNames, varClasses, .....)

would be nice ...

The problem is that what is being referred to *is* columns and not
variables.  If you have row names on the file, the numbering is
different.  So it matters to use sufficiently precise terminology.