[R] Selective column loads with scan()

[Peter Dalgaard BSA]

Prof Brian Ripley <ripley at stats.ox.ac.uk> writes:

> > Try 
> > 
> >   scan(file.name, what = list(, , ""), flush = TRUE)

[ really: scan(file.name, what = list(NULL, NULL, ""), flush = TRUE)

> Unfortunately that gives you the first column.  With S you need the NULLs
> in there (and then it is a known trick), but they are not accepted in R.
...> 
> I don't know a good way to do this in R, but then I would not try it.
> Either use `cut' to extract the columns(s) needed from the file, or
> use a database and a connection such as RODBC to do the extraction in the
> database.

Doesn't look like rocket science to make the NULL entries work in R,
but probably a tad late for 1.2.1 (next Monday)

Basically, I think it would amount to (in src/main/scan.c): 

 * The allocation code in scanFrame should stick in a NULL for ans[i]
   rather than error out if what[i] is NULL. Also watch for this when
   the n == blksize && colsread == 0 condition is handled.

 * fillBuffer should treat NULL as STRSXP

 * extractItem should ignore NULL types (does already, it seems)

On the other hand, it might make better sense to have a mask= argument
controlling which columns that are kept/discarded. Or select=.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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