[R] Scanning data files line-by-line

[Prof Brian Ripley]

On Wed, 30 Apr 2003, R A F wrote:

> Thanks very much.  I guess the answer leads to more questions:
> 
> (a) What if I don't know the number of lines?  So I would like to use
>     a while loop until readLines hits an EOF character.  Would that
>     be possible?

Yes. After you reach the end of the file you will get character(0) since

Value:

     A character vector of length the number of lines read.

and zero lines would have been read.

> (b) When readLines is used, a string is returned.  

Not quite: a character vector is returned.

> I'd like to split
>     the string into fields, and Andy Liaw suggested strsplit, but the
>     number of spaces between fields is variable.  So for example, one
>     line could be 1 space 2 space space 3 and the next line could be
>     4 space space 5 space 6, so I could not do a strsplit using " ".
> 
>     Really what I know is the variable type of each field -- for
>     example, each line is double, string, then double, etc.  How
>     would one use this information to split the string given by
>     readLines?

You could use scan on the line: it works on textConnections.

> Thanks very much again!
> 
> >From: Prof Brian Ripley <ripley at stats.ox.ac.uk>
> >To: R A F <raf1729 at hotmail.com>
> >CC: R-help at stat.math.ethz.ch
> >Subject: Re: [R] Scanning data files line-by-line
> >Date: Wed, 30 Apr 2003 14:13:26 +0100 (BST)
> >
> >It's open() you need, as in
> >
> >con <- file("file")
> >open(con)
> >for(i in 1:10) print(readLines(con, n=1))
> >close(con)
> >
> >In C you would need to (f)open a file to read it line-by-line, just as
> >here.
> >
> >The first two lines can be collapsed to
> >
> >con <- file("file", "r")
> 
> 
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-- 
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 272866 (PA)
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