[R] skip lines on a connection

[Vadim Ogranovich]

Andy,

It is surprising that scan() attempts to read anything at all: note that
I set nmax=0, which AFAIK means read no lines.

Thank you for a reference to replicate(). I didn't know about it.

Thanks,
Vadim

-----Original Message-----
From: Liaw, Andy [mailto:andy_liaw at merck.com] 
Sent: Saturday, May 01, 2004 5:28 PM
To: Vadim Ogranovich; r-help at stat.math.ethz.ch
Subject: RE: [R] skip lines on a connection


Your scan() call doesn't work because default argument what=0; i.e., it
expects numeric data.  You probably can just use what="".

The other alternative is to just loop readLines() n times, reading one
line at a time.  It probably won't be too bad in terms of time, and
surely will save on memory usage.

(Try using replicate().)

HTH,
Andy

> From: Vadim Ogranovich
> 
> Unfortunately, seek only works in terms of bytes not lines and I only 
> know how many lines I need to skip, but not bytes.
> 
> 
> -----Original Message-----
> From: Gabor Grothendieck [mailto:ggrothendieck at myway.com]
> Sent: Saturday, May 01, 2004 3:44 PM
> To: r-help at stat.math.ethz.ch
> Subject: Re: [R] skip lines on a connection
> 
> 
> 
> 
> ?seek
> 
> Vadim Ogranovich <vograno <at> evafunds.com> writes:
> 
> :
> : Hi,
> : 
> : I am looking for an efficient way of skipping big chunks of 
> lines on a
> : connection (not necessarily at the beginning of the file). 
> One way is
> to
> : use read lines, e.g. readLines(1e6), but a) this incurs the overhead
> of
> : construction of the return char vector and b) has a (fairly remote)
> : potential to blow up the memory.
> : 
> : Another way would be to use scan(), e.g. 
> : 
> : scan(con, skip=1e6, nmax=0)
> : 
> : but somehow this doesn't work:
> : 
> : > scan(con, skip=10, nmax=0)
> : Error in scan(con, skip = 10, nmax = 0) : 
> :  "scan" expected a real, got "A;12;0;"
> : 
> : I can stick to readLines, but am curious if there is a better way.
> : 
> : I use R-1.8.1 on RH-7.3.
> : 
> : Thanks,
> : Vadim
> 
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