[R] Selective column loads with scan()

[Prof Brian Ripley]

On Sun, 7 Jan 2001, Uwe Ligges wrote:

> On Sun, 7 Jan 2001, Samir Mishra wrote:
> 
> > Help!
> > 
> > I have a very large data file with about 100 columns. Instead of loading 
> > all of the columns using read.table() (my PC won't be able to handle the
> > resulting data frame), I'd like to read in one column of the datafile at 
> > a time using scan() in conjunction with the 'what = ' option. I've tried 
> > loading just the third column using -
> > 
> > > scan(file.name, what = list(NULL, NULL, "", rep(NULL, 97)). ... )
> > 
> > along with a couple of other options but I can't get the results I want 
> > (if I avoid an error). For example - 
> > > list(NULL, NULL, "", rep(NULL, 97))
> > [[1]]
> > NULL
> > 
> > [[2]]
> > NULL
> > 
> > [[3]]
> > [1] ""
> > 
> > [[4]]
> > NULL
> > 
> > is not what i'm looking for. 
> > 
> > Is there a better way to selectively read in individual columns from a tab
> > separated data file?
> > 
> > I've done this before, and that makes this exercise even more frustrating.
> 
> 
> Try 
> 
>   scan(file.name, what = list(, , ""), 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.

BTW,

> list(, , "")
[[1]]:
[1] ""

> list(NULL, NULL, "")
[[1]]:
NULL

[[2]]:
NULL

[[3]]:
[1] ""

are different.

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.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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